Inside the Apple-1's unusual MOS clock driver chip

Apple's first product was the Apple-1 computer, introduced in 1976. This early microcomputer used an unusual type of storage for its display: shift register memory. Instead of storing data in RAM (random-access memory), it was stored in a 1024-position shift register. You put a bit into the shift register and 1024 clock cycles later, the bit pops out the other end. Since a shift-register memory didn't require addressing circuitry, it could be manufactured more cheaply than a random-access memory chip.1 The downside, of course, is that you had to use bits as they became available, rather than access arbitrary memory locations. The behavior of shift-register memory was a good match for video circuitry, though, since characters are displayed on the screen in a fixed, repeating order (left to right and top to bottom).2

The Apple-1 was sold as a bare board, so users needed to make a case for it, or mount it in a briefcase as shown here. Note the cassette drive used for mass storage. Photo cropped from Binarysequence, CC BY-SA 4.0.

Shift-register memory chips required clock pulses with high current and unusual voltages: from +5 volts to -11 volts. These pulses were provided by a special chip, the DS0025 Two-Phase MOS Clock Driver. This chip, introduced in 1969, was the first monolithic (i.e. integrated circuit) clock driver. In this blog post, I look inside the chip and explain how it was implemented.

Die of the DS0025 clock driver. Click this image (or any other) for a larger version.

The photo above shows the silicon die under the microscope. This chip is very simple, containing four large NPN transistors, four diodes, and four resistors. The silicon appears blue-gray in this image, while the metal layer on top appears speckled white. Around the outside of the die, are six dark rectangles, the pads where golden bond wires connected the die to the chip's external pins.

The die was encased in an epoxy package. To expose the die, Eric (@TubeTimeUS) tediously sanded through the plastic package until the die was visible. Some bits of epoxy remained, caught in the bond wires, so I cleaned up the die with a few drops of boiling sulfuric acid.

The Apple-1's display

The Apple-1 displayed 24 lines of forty characters on a television monitor. Like most computers at the time, the Apple-1 stored characters rather than pixels to reduce memory requirements. A character-generation ROM converted each character into a 5×7 matrix of pixels as it was displayed. To reduce memory even more, the display didn't store full bytes, but 6-bit characters, supporting upper-case letters, numbers, and some symbols.

The six-bit display characters were held in six 1024-bit shift registers. A seventh shift register tracked the cursor position.3 The diagram below shows the shift registers and the clock driver on the Apple-1 circuit board. These chips are in 8-pin packages, so two chips fit into the space of a regular TTL chip.

Apple-1 circuit board, showing the 1024-bit shift register chips and the clock driver chip. Original image from Achim Baqué, CC BY-SA 4.0.

Transistors

Next, I'll discuss the components of the chip. Because the chip generates high-current pulses, it uses large NPN transistors, with a different construction from most integrated circuit transistors. Each transistor consists of 24 emitters, paralleled in two groups. (You can consider it one large transistor, 2 transistors, or 24 small transistors.) The transistor is structured vertically with the collector (made of N-doped silicon) underneath, a thin P-type base in between, and the N-type emitters embedded in the top, forming the N-P-N layers of the transistor. The doped silicon regions are faintly visible with black lines around their boundaries.

Half of a transistor, with 12 emitters.

In the photo above, you can see the metal wiring for the transistor's collector, base, and emitter. The collector wiring is on the outside, with base wiring in between. The collector and emitter wiring is tapered: at one end, the wiring needs to support the full current load, while at the other end it only handles 1/12 of the current. The tapered approach saves space, since it is thicker only where it needs to be thick.

Resistors

The resistors are formed from silicon doped to have higher resistance. The doped silicon rectangle is faintly visible in the die photo. At each end of the resistor, a contact connects the silicon to the metal layer on top. The 1000Ω resistor on the left is longer than the 250Ω resistor on the right, giving it more resistance.

Two resistors as they appear on the die.

"Tunnels"

The chip has a single layer of metal wiring, which poses a problem if two signals need to cross. The solution is to put one signal in the silicon layer so it can pass under the metal layer. In essence, a low-valued resistor is used to pass under the metal layer. The image below shows how a tunnel appears on the die.

The conductive silicon strip at the top connects the metal regions on either side. The conductive strip at the bottom doesn't fulfill a wiring need, but ensures that both paths encounter the same resistance.

One problem is that the silicon has relatively high resistance compared to metal, so the tunnel adds resistance. The chip is carefully designed so both "sub-transistors" encounter the same resistance, to avoid one transistor turning on before the other. You can see that the input path in the upper left has a tunnel to pass under the metal wiring, while the path in the lower right has a tunnel of identical dimensions that doesn't go under any metal. While the second tunnel appears pointless, it assures that both paths have the same resistance.

The chip's circuit

The shift register requires a two-phase clock, that is two clock signals in alternation that step the bits through the circuit. To support this, the clock driver chip has two identical driver circuits. The schematic below shows one of the circuits. When the input goes high, it turns on transistor Q1, pulling its collector low. This pulls the output low through diode CR2. When the input drops, Q1 turns off. This lets R2 provide a current to the base of Q2, turning it on, and pulling the output high. Thus, the circuit is essentially an inverter, but one that can provide up to 1.5 amps of output.4

Schematic of the DS0025, from the application note.

The image below shows the various components of the schematic as they appear on the die. Most of the chip is occupied by the large power transistors. Although the chip is mounted in an 8-pin package, only six pins are used; the corresponding pads are labeled below. The chip consists of two identical mirror-image drivers; one is labeled. There are a few blackened regions in the transistors; we suspect this is where the chip failed.

Die with the components labeled. Note: diodes are labeled CR (crystal rectifier) in the schematic but D here.

Conclusion

This chip provides an interesting view of computer technology in the 1970s. The Apple-1 used shift-register memory, a technology that rapidly became obsolete as RAM prices dropped. Shift-register memory required a specialized clock driver integrated circuit, a chip that contained just four large transistors. With billions of transistors in modern integrated circuits, it's hard to imagine that it was once worthwhile to build a chip that was this simple. The Apple II, introduced just a year later in 1977, used RAM chips for all its storage, making shift-register memory a thing of the past.

I announce my latest blog posts on Twitter, so follow me @kenshirriff. I also have an RSS feed. Thanks to @TubeTimeUS for supplying the chip. I've written about the Intel 1405 shift register memory if you want to know more about this type of storage.

Notes and references

1. Looking in an old Byte magazine from 1976, a 1-kilobit shift register chip cost $9 ($34 in current dollars), while a 4-kilobit DRAM chip cost $20 ($75 in current dollars). Thus, it appears that even by the time the Apple-1 was released, DRAMs had become cheaper than shift registers. (This also illustrates the amazing drop in memory prices since the 1970s, as described by Moore's Law.)

   The Apple-1 used 4-kilobit RAM for data and program storage. It's possible, though, to build a computer that uses shift-register storage for its main memory. The Datapoint 2200 is one example. If memory is accessed sequentially, shift-register storage is efficient since the bits are provided sequentially. However, if you access memory out of sequence, the processor has to wait while the memory cycles around, until the desired bits become available. In a way, shift-register memory is a throwback to very early computers such as EDSAC(1949), which used mercury delay lines for main storage. ↩

2. The IBM 2260 video display terminal (1965) used a technique similar to shift registers: it stored data in a sonic delay line, sending torsional pulses through a 50-foot nickel wire. But unlike the Apple-1, this delay line stored pixels, not characters. For more about this system, see my blog post. ↩

3. The display circuitry has some additional complexity. Characters can't be taken directly from the display shift register: since each character is made up of eight scan lines; a line of character must be processed eight times. To handle this, a second shift register (six 40-bit registers) buffers a line of characters and feeds each character into a display ROM. Another 1024-bit shift register keeps track of the cursor position. For more details, see this post. The Apple-1 schematic is in the Operation Manual. ↩

4. The large current is required because of the design of the shift-register memory. The clock line snakes through the chip, providing a clock signal to each stage of the shift register. As a result, the clock line has a fairly high capacitance, about 150 picofarads. This clock line must be switched between +5 volts and -11 volts at a 1 megahertz rate. The combination of large capacitance and large voltage swing with the fast rate requires a high current. ↩

A look inside the chips that powered the landmark Polaroid SX-70 instant camera

The revolutionary Polaroid SX-701 camera (1972) was a marvel of engineering: the world's first instant SLR camera. This iconic camera was the brainchild of Dr. Edwin Land, a genius who co-founded Polaroid, invented polarized sunglasses, helped design the optics for the U-2 spy plane, and created a theory of color vision. The camera used self-developing film2 with square photos that came into view over a few minutes.3 The film was a complex sandwich of 11 layers of chemicals to develop a negative image and then form the visible color image. But the film was just one of the camera's innovations.

Edwin Land and the Polaroid SX-70 camera were featured on the cover of Life magazine, October 27, 1972.

The camera required complex new optics to support the intricate light path shown below. The components included a flat Fresnel mirror, aspherical lenses, and a moving mirror. These optics could focus from infinity down to a closeup of 10 inches. The optics are even more amazing when you consider that the camera folded flat, 3 cm thick and able to fit in a jacket pocket.

Diagram from Life magazine, Oct 1972, showing the light path through the camera.

But I'm going to focus on the camera's electronics, powered by a custom flat battery pack. When the shutter button is pressed, the camera carried out several tasks with precision timing. First, a solenoid closes the shutter, blocking the entry of light into the camera. Next, the motor is turned on, causing the camera's internal mirror to flip up to uncover the film. The solenoid is then de-energized, causing the shutter to open. The film exposure time depends on the light level; at the appropriate time, the solenoid closes the shutter again to stop exposure. Finally, the motor runs again to eject the film and reset the mirror.

I'm helping the openSX70 project by reverse-engineering the chips on the exposure control board. This board contains three surface-mount ICs: a chip to read the light intensity from a photodiode, a timer chip to control how long the shutter blades are open, and a power control IC to drive the motor and solenoids.4

The exposure control circuit board, manufactured by Texas Instruments. Photo from openSX70.

The development of this board was contentious, with Fairchild and Texas Instruments battling to supply the electronics for millions of cameras.5 The exposure control board went through three designs as Fairchild and Texas Instruments struggled to meet Polaroid's price target of just $5.75. First, Texas Instruments built a control board from a ceramic substrate with laser-trimmed resistors. The expensive components put the board way over budget at $100 so Polaroid used these boards only in prototype cameras. Fairchild's design was accepted by Polaroid even though its cost of $20 still exceeded the target. Fairchild's board was used from 1972 to 1973, but Texas Instruments fought back with an all-new design that cost only $4.10. This TI board was the long-term winner, and is the one I am examining in this post.

The optical chip

The exposure control board automatically adjusts the exposure time based on the amount of ambient light. Ambient light is measured by the optical chip, a package that combines a photodiode and a silicon die in one small package. The silicon die is protected by epoxy, but the larger photodiode is exposed so external light can fall on it.

The optical chip contains a photodiode and a silicon die in one package.

To measure the ambient light, the chip implements the integrator circuit below. The photodiode generates a small current that depends on the light level. This current is integrated over time using a capacitor until a threshold is reached. By opening the shutter during this interval, the film is exposed for the desired amount of time. (The film's exposure depends on the total amount of light received, which is the same value that the integration calculates.) The op-amp die outputs the voltage across the capacitor without draining the capacitor in the process.

The optical chip is essentially an integrator.

The photo below shows the silicon die under a microscope. The chip, made by Texas Instruments, is dominated by the zig-zags forming two interlocking JFET transistors. A JFET is a special type of transistor, used before MOSFETs became popular. These transistors have very low input currents, so they won't drain the capacitor as it charges. The interlocking layout ensures that both transistors are at the same temperature, so the circuit will stay accurate even if the chip heats up unevenly. The chip also contains NPN and PNP transistors, resistors, and a capacitor (the large pink square labeled 28710).

The optical chip. Photo from siliconPr0n, (CC BY 4.0).

By reverse-engineering the die, I created the schematic below. It is an op-amp, measuring the difference between two inputs (one tied to ground). The two JFET (Q12/Q13) transistors are configured as a standard differential pair circuit. A fixed current (from the current mirror Q8/Q6) is fed into the transistors, and whichever transistor has the higher input will pass almost all the current. The result is amplified by Q5 for the output. In addition to the op-amp circuitry, the chip contains a reset circuit to discharge the capacitor before use (Q1/Q2). (The internal capacitor C1 stabilizes the op-amp; the integration capacitor is external.)

Schematic of the optical chip. Click for a larger version.

The power driver chip

Next, the power chip drives solenoids and the motor to activate the camera's mechanisms. The high-current power transistors (the golden triangular shapes) take up most of this chip. Smaller transistors below form the control circuitry.

The power driver chip. Photo from siliconPr0n, (CC BY 4.0).

My reverse-engineered schematic shows that the chip has three parts. First, a simple inverter. This probably interfaces the logic chip to the motor control board.

Schematic of the inverter in the power driver chip. Click for a larger version.

Second, a high-current driver. This uses the large power transistor at the left of the die. This probably drives a solenoid.

Schematic of the driver in the power driver chip.

Finally, a high-current driver with a separate circuit for the solenoid hold current. (That is, the solenoid is pulled into position with a high current, and then held in that position with a lower current.) This uses the large power transistors at the right of the die. There's a single large transistor underneath the main "triangle" of transistors; that transistor is for the hold current. This circuit uses separate power and ground pads from the rest of the chip.

Schematic of the driver/hold circuit in the power driver chip.

The driver circuits are more complex than I'd expect, using current sources and current mirrors. Maybe this design minimizes standby current use.

The logic chip

The camera is controlled by a complex logic chip that controlled the timing of the various mechanisms, running the motor and solenoids. It had to handle four different use cases: ejecting the protective cover sheet when a film package was inserted, taking a photo, taking a photo with the flash, and ejecting an empty film package.

This chip was constructed from Integrated Injection Logic (I²L), an obscure 1970s logic family featuring high density and low power. Because the camera ran off a battery in the film pack, minimizing power consumption was a critical factor. At the time, I²L was a good choice for dense, low-power circuitry, although it was soon overtaken by CMOS. Texas Instruments did a lot of development with I²L, including digital watch chips and the 76477 sound chip so it's not surprising that they chose I²L for the camera chip.

The logic chip. Photo from siliconPr0n, (CC BY 4.0).

I2L gates can be packed together at high density, as shown below. Each vertical gray rectangle is two transistors (one above the horizontal centerline and one below), corresponding to two gates. The chip has very little wasted space, especially compared to TTL logic, which was commonly used at the time but required multiple transistors and bulky resistors for each gate.

Closeup of the logic chip.

I²L is a bit tricky to understand since an I²L gate has one input and multiple outputs. How can that work? The schematic below shows an I²L gate, with one input and three outputs. Normally the current from the injector (ICC) turns on the output transistor, pulling the output low. But if the input is low, the output transistor turns off and the output will be high. Thus, the gate inverts the input. (You can think of the injector as a pull-up resistor on the input.)

Implementation of an I²L gate. Note that it has a single input and multiple outputs. Icc is the injected current. From "Integrated Injection Logic: A Bipolar LSI Technique".

Since the circuit above has a single input, it may seem to be just an inverter. But by wiring several signals together at the input, you get an AND gate "for free": if any signal is low, it will pull the wire low, and otherwise the signal is high. This is called "wired-AND". The wired-AND input to the I²L inverter results in a NAND gate.

One problem arises with wired-AND: if you connect an output to more than one wired-AND, everything gets shorted together. The solution is to have multiple outputs from the inverter. Thus, each I²L NAND gate has a single input and multiple identical outputs. In the diagram below, the outputs from various gates (A and B below) are connected together and fed to the input of an I²L gate, creating a NAND gate.

Diagram of a NAND gate implemented in Integrated Injection Logic (I²L). From "Integrated Injection Logic: A Bipolar LSI Technique".

The transistors in I²L have multiple collectors, which may seem strange, but the diagram below shows how they are constructed. Each collector has an N region (purple) with a P region (tan) below for the base, and another N region (green) at the bottom, forming an NPN transistor. The multiple collectors are built by creating multiple N regions. Physically, the injector PNP transistor is just a P region for the emitter, reusing the emitter and base's N and P regions; this makes the injector more compact than a "full" transistor.

Die photo and cross-section diagram of an I²L gate. The transistor base, collectors, and emitters are labeled along with the current injection.

I haven't reverse-engineered this chip yet. I believe that it contains an oscillator and a chain of flip flops for timing, as well as a comparator for the light level and some miscellaneous control logic.

Conclusion

While the electronics of the SX-70 camera aren't impressive by modern standards, they were cutting edge at the time. They made the SX-70 easy to operate by handling the exposure and timing automatically. Texas Instruments split the electronics across three chips: a precision JFET op-amp with a photodiode, a high-current power driver chip, and a complex logic chip using dense, low-power I²L logic.

Unfortunately, innovative technology wasn't enough for Polaroid. The company declined after competition from Kodak, the expensive failure of the Polavision instant home movie system, and the rise of digital cameras. Polaroid declared bankruptcy in 2001 and the company was broken up. The SX-70 has seen a resurgence in popularity, with film and cameras sold by polaroid.com, which acquired the Polaroid name in 2017.

Follow me on Twitter @kenshirriff for more posts. I also have an RSS feed. Thanks to Joaquín De Prada and Peter Kooiman of openSX70 for providing the chips and John McMaster for decapping them.

The openSX70 project is building extensions to the SX-70 camera.

For more about the SX-70, see the interesting and quirky 10-minute movie below, which markets the SX-70, explains how to use it, and discusses the internal operation. This movie was made in 1972 by the famous designers Ray and Charles Eames.

Notes and references

1. The name SX-70 comes from its inventor Dr. Edwin Land. He numbered all his "special experiments" in a notebook and his instant picture experiment was number 70. Although the camera was 30 years after special experiment 70, he felt that it embodied the system he had envisioned in the mid-1940s. ↩

2. Land introduced instant photography in 1947, and then color instant film in 1963, based on a peel-apart technology. The SX-70 eliminated the problems of the peel-apart instant photos. As Polaroid said, “No pulling the picture packet out of the camera, no timing the development process, no peeling apart of the negative and positive results, no waste material to dispose of, no coating of the print, no print mount to attach, no chance for double exposure, no chance to forget to remove the film cover sheet and spoil a picture, no exposure settings to make, no flash settings to remember, no batteries to replace.”  ↩

3. Although Polaroid photos develop in a minute or so without any user intervention, shaking the photo was a common custom. "Shaking the Polaroid" was the theme of a 1998 Polaroid ad. Outkast's 2003 song Hey Ya! featured the refrain "Shake it like a Polaroid picture". ↩

4. I haven't investigated all the chips in the SX-70. The motor control module contains a linear control IC and power transistors. The camera also takes a flashbar with five flashbulbs. The flash circuit has another chip that checks the bulbs to find an unused bulb. ↩

5. "The Battle for the SX-70 Camera", IEEE Spectrum, May 1989 discusses in detail the battle between Fairchild and Texas Instruments to win the Polaroid contract.

   The internal circuitry of the camera is described in "Behind the Lens of the SX-70", IEEE Spectrum, Dec 1973. This circuitry, however, is for the earlier Fairchild version and the implementation is considerably different from the Texas Instruments circuitry that I examined. The Fairchild implementation is also described in "Camera Electronics, A New Approach", WESCON 1973. ↩

Yamaha DX7 chip reverse-engineering, part V: the output circuitry

The Yamaha DX7 digital synthesizer (1983) was the classic synthesizer in 1980s pop music. It uses a technique called FM synthesis to produce complex, harmonically-rich sounds. In this blog post, I look inside its custom sound chip and explain how the chip's output circuitry works. You might expect it's just a digital output fed into a digital-to-analog converter, but there's much more to it than just that.

Die photo of the YM21280 chip with the main functional blocks labeled. Click this photo (or any other) for a larger version.

The composite die photo above shows the DX7's OPS sound synthesis chip under the microscope, revealing its complex silicon circuitry. Unlike modern chips, this chip has just one layer of metal, visible as the whitish lines on top. Around the edges are the 64 bond wires attached to pads; these connect the silicon die to the chip's 64 pins. The three blocks in red are the focus of this post. The output buffers hold the 16-bit digital values for the 16 notes. The output is controlled by a counter and PLA (Programmable Logic Array). The synthesizer's digital-to-analog conversion uses a sample-and-hold circuit, controlled by the "S/H ctrl" block.

I've discussed the chip's other functional blocks in earlier posts1, so I'll just give a brief summary here. Each of the 96 oscillators has a phase accumulator used to generate the frequency. The oscillators are combined using the operator computation circuitry in the middle of the chip, under the control of the algorithm ROM. The signal synthesis uses sine and exponential functions, implemented with lookup tables in ROMs.

The Yamaha DX7 synthesizer with its 61-key keyboard and digital controls. Photo by rockheim (CC BY-NC-SA 2.0).

The synthesizer's output circuitry

Before I dive into the details of the chip, I'll explain the synthesizer's output circuit. The heart of the synthesizer is the OPS (Operator S) sound chip that digitally generates the notes. It provides digital values to the digital-to-analog converter (D/A). The resulting analog signal goes through a low-pass filter (LPF). The volume is controlled by a foot pedal and the synthesizer's volume control. Finally, the signal is amplified for the line and headphone outputs.

Block diagram of the output circuit. Based on the DX7/9 Service Manual.

The digital-to-analog conversion is more complex than you might expect. The process starts with the digital-to-analog converter (DAC) chip2 that takes a 12-bit digital value from the sound chip and converts it to an analog value in the range 0 to 15 volts.3 The multiplexer allows the overall synthesizer volume to be controlled by MIDI, but with just 8 levels.

Schematic of the volume control and DAC circuit. Based on the DX7 schematics.

The DAC provides 12 bits of resolution, but an additional circuit (below) provides approximately two more bits. This scaler circuit divides the analog signal by 1, 2, 4, or 8, using a resistor network and IC switch. The scaler is controlled by the sound chip through the scale factor signals SF0-SF3. The scaler adds more dynamic range to the digital value; the result is similar to a floating-point value with a sign bit, 11-bit mantissa, and two-bit exponent.4

The scaler divides the voltage by 1, 2, 4, or 8. Based on the DX7 schematics.

Next, the signal goes to a sample-and-hold circuit that samples the analog voltage at a point in time and holds it in a capacitor, kind of like an analog memory. An op-amp buffers the capacitor's voltage so it can be "read" without draining the capacitor. There are two hold circuits, used in alternation, so the last two samples are stored and summed to form the circuit's output.5 The SH1 and SH2 control signals load the analog value into a capacitor, using IC52 as a switch. Finally, the output from the sample-and-hold circuit is filtered,6 the volume is adjusted,7 and the signal is amplified for the output (circuitry not shown).

The sample-and-hold circuit. IC52 looks complicated because it uses pairs of switches in parallel. Based on the DX7 schematics.

To summarize, the sound chip interacts with the output circuitry in three ways. The 12-bit digital value (DA1-DA12) is most important as it specifies the output value for each voice. The scale factor signals (SF0-SF3) are also a key contributor to the signal. The sound chip also provides the sample-and-hold control signals (SH1 and SH2).

Time-division multiplexing

The DX7 has 16 voices, so it can play 16 notes at once. Each note is produced by an "algorithm" that combines 6 oscillators in a particular way, so there are 96 oscillators in total. An oscillator can modulate the frequency of another oscillator to generate complex sounds with FM synthesis.

The chip performs all its processing sequentially, one oscillator at a time, rather than computing the notes in parallel. Internally, the chip has one "operator" calculation circuit to combine oscillators. As shown below, the chip starts by processing operator 6 for note 1, then operator 6 for note 2, and so forth through note 16. Then it processes operator 5 for notes 1 through 16. Finally it processes operator 1 for notes 1 through 16, generating the output sound values. It takes a bit over 20 µs to compute all 16 notes in a complete processing cycle.

Timing diagram of sound production. This time interval corresponds to 49.096 kHz. From the DX7/9 Service Manual.

You might expect the chip to combine the 16 notes into a single digital output. However, the sound chip outputs the 16 notes sequentially, using a technique called time-division multiplexing. Each time interval (~20µs) is divided into 16 intervals and one note is output from the chip per interval. (Note that these intervals don't line up with the intervals in the diagram above.) Thus, digital values are output at 786 kilohertz, 16 times the underlying frequency, and the DAC chip converts them to analog at this rate.

As an example, consider two notes that are sine waves with different frequencies. The digital output would look like the image below. You might think that this signal is unusable since it jumps around wildly from point to point.

Output data with two multiplexed sine waves. (Theoretical, not actual DX7 data.)

However, applying a low-pass filter smooths out the waveform (essentially summing nearby points). The result is the waveform below, which shows the sum of the two sine waves.8 The point is that time-division multiplexing data may look strange, but the analog circuitry's filtering creates a "normal" waveform.

Output data after filtering with a 16 kHz low-pass filter.

Output buffer

Inside the chip, the output buffer stores values for the 16 notes as they are generated, and outputs them in sequence. Rather than RAM, the chip uses shift registers for storage. The shift registers are arranged in a loop of 16 stages, one stage for each note. On each clock cycle, the values in the shift register move to the next stage. The output value is fed back into the shift register's input so the value is retained. Alternatively, a new value can be stored in the shift register. Shift registers provided an efficient way to store data, but they cannot be accessed arbitrarily; instead, data must be processed as it becomes available.

The schematic below shows how one stage of the shift register is implemented. The chip uses a two-phase clock. In the first phase, clock ϕ1 goes high, turning on the first transistor. The input signal goes through the inverter, through the transistor, and the voltage is stored in the capacitor (kind of like DRAM). In the second phase, clock ϕ2 goes high, turning on the second transistor. The value stored in the capacitor goes through the second inverter, through the second transistor, and to the output, where it enters the next shift register stage.

Schematic of one stage of the shift register.

The die photo below shows the output buffer, with the 16 shift-register loops arranged in columns. These hold the 16-bit sound values (four scale factor bits and 16 data bits.) Each shift register is 16 stages long to hold the 16 notes. In the next sections, I'll discuss the bit shifters, the logic, and the output latches.

Closeup of the die, showing the output buffer circuitry.

The scale factor: pseudo floating point

The DX7 uses a 12-bit digital-to-analog converter chip, but the scaling circuit (discussed earlier) will scale the voltage by 1, 2, 4, or 8, which adds more resolution. This isn't quite equivalent to 14-bit resolution; it's more like a floating-point number with a sign, 11-bit mantissa, and 2-bit exponent. This provides more resolution for low signals and reduces signal noise.

Inside the chip, scaling is implemented with a shifter that shifts the data bits by 0 to 3 bit positions. (This is unrelated to the shift registers that hold data.) The shifter (below) is implemented as eleven chevron-shaped logic gates; each gate selects one of four potential bits for each mantissa position.

A data sample is shifted 0 to 4 bits by this shifter circuit.

The operator circuitry generates data as 15 bits (2's complement, so one of the bits provides the sign). The output from the chip is 12 bits, so three bits must be discarded. Normally these are the low-order bits, but by using the shifter, high-order zero bits can be discarded instead, and the external scaler counteracts this. The result is more bits of precision in the output.

The shifter is controlled by the logic circuitry to the left of the buffer, which controls the amount of shift based on the number of leading zeros. (For a negative number, leading 1's.) With 5 leading zeros, the number is shifted left by 3 positions. With 4 leading zeros, the number is shifted by 2 positions. With 3 leading zeros, the number is shifted by 1 position. With 2 or fewer leading zeros, the number is unshifted.

Note that the circuit leaves two leading zeros when it shifts, so it's "wasting" two potential bits of precision. I assume this is because the scaler won't be perfectly linear (due to the resistor imperfections9), so you want to avoid switching scale levels for large signals (which don't really need the extra bits).10

The output latches

As mentioned earlier, the 16 notes are output individually, spaced across the interval. This timing doesn't line up with the timing of the output buffer, which shifts to a new note every clock cycle. To fix the timing, two 16-bit latches sit between the output buffer and the output pins. While one latch outputs the current note, the other latch grabs the next note as it is shifted out of the shift register. At the appropriate time, the latches swap roles; the second latch outputs the note while the first latch waits for the next note.

The timing for the latches is fairly tricky to make sure the note data is loaded into the right latch at the right time. These latches are controlled by the chip's master counter, which is the subject of the next section.

To summarize, the sound chip runs at 4.7 MHz. Data values are produced at this rate (but intermittently) and stored into the output buffer. The output latches provide data values to the DAC chip at 786 kHz for an overall audio rate of 49096 Hertz.

Keeping track of 96 clock cycles: the chip's counter and timing PLA

One complete cycle of the sound chip takes 96 clock cycles: processing all 16 notes through the 6 operators that form an algorithm. Because data can only be accessed when it exits a shift register, everything must be timed so the right data is available at the right time. A critical part of the chip is the counter that keeps track of the current note number and operator number to keep everything synchronized.

On the right of the die photo below is the counter, consisting of seven toggle flip flops: four to count the note number (0-15) and three to count the algorithm number (0-5). On the left is the PLA that defines what happens for particular time slices. (A Programmable Logic Array (PLA) is similar to a ROM, but implements arbitrary logic.) The PLA has 39 columns, each one implementing an AND gate triggered by a particular counter output, corresponding to a particular operator and note. Below the PLA is some logic; mostly buffers with a few gates.

The chip's main counter, along with the control PLA.

Of the PLA's 39 columns, the 32 columns on the left control the data output latches,11 two columns control loading data values into the output buffer, one generates the chip's sync output signal, three reset the operator count, and the last increments the operator count.12

Sample-and-hold

The chip outputs two signals to control the sample-and-hold circuitry, SH1 and SH2. These signals are activated in alternation to take an analog sample of each digital output.

The sound chip on the DX7 schematic has three missing pins, indicated in red.

The sound chip has three unused pins next to the SH1 and SH2 pins; the DX7 schematic doesn't show pins 6-8. I traced the chip's internal circuitry and found that these pins, in conjunction with the sample-and-hold pins, count out the 16 samples. It appears that the chip is designed to sample-and-hold all 16 notes individually, so the synthesizer could have had separate outputs for all 16 notes.13

Moreover, the chip has data buffers to hold separate algorithm algorithms for the 16 notes. This would let the chip drive 16 independent voices, each with a separate algorithm. My conclusion is that the sound chip supports much more flexibility than is used in the DX7 synthesizer.

Conclusion

The DX7 generates sounds digitally and then converts the digital values to the analog output. This process turns out to be more complicated than one would expect, with circuitry inside the chip interacting with synthesizer circuitry to scale and adjust the signal. My hope is that my analysis of this process will help DX7 emulators to achieve more accuracy. Looking at the chip's internal circuitry reveals the floating-point format of the output data as well as the function of the three unused pins.

I plan to continue investigating the DX7's circuitry, so follow me on Twitter @kenshirriff for updates. I also have an RSS feed. Thanks to Jacques Mattheij and Anthony Richardson for providing the chip and discussion.14

Notes and references

1. My previous posts on the DX7: DX7 reverse-engineering, The exponential ROM, The log-sine ROM, and How algorithms are implemented. ↩

2. The DAC chip is the BA9221, a 12-bit D/A converter that produces an output current based on a 2's-complement input value. A datasheet is here. The DAC receives an input voltage reference. This voltage reference can be one of 8 values selected by a multiplexer. This allows the overall volume to be set via MIDI, but only with 3-bit resolution (see the DX7 ROM code here). The volume is an exponential function (so linear in decibels) except that 0 is off. Also see this DAC discussion and this StackExchange discussion.) ↩

3. The output from the DAC is centered around 7.5 volts. In other words. a digital value 0 corresponds to 7.5 volts, with positive digital value above 7.5 volts and negative digital values below 7.5 volts. I would have expected the signals to be centered around 0 volts, which is what the DAC datasheet shows. I think strictly positive voltages were used because they work better with the TC4066 and TC4066 integrated circuits switches (IC 41 for scaling and IC 52 for sample-and-hold respectively). The DX7 converts the signals to zero-centered voltages for the low-pass filter. ↩

4. The amplitude scaler is built from an R-2R resistor ladder, similar to a DAC circuit. However, the attenuator only has 4 useful values, not the 16 levels you might expect with four control lines, because only one control line can be activated at a time. Combinations of control lines do not yield useful outputs. For example, if the top switch is on, you get the maximum output regardless of the other switches. Other switch combinations are non-monotonic. Thus, the scaler only provides two additional bits of resolution, not four. ↩

5. The benefit of keeping two samples is not clear to me. One theory is that this reduces intermodulation distortion between the voices, the effect of one signal on another. With one "solid" sample and one changing sample, the effect of the changing sample will be reduced. Another, more speculative, possibility is that the circuitry was originally designed for stereo, holding one sample for each channel. ↩

6. The filter is a sharp low-pass filter around 16 kHz using a Sallen-Key topology. ↩

7. The volume is controlled by an external volume pedal and a volume control on the synthesizer. The signal also passes through a relay, which cuts the output when the synthesizer is being reset (presumably to avoid random noise).

   The external volume pedal has an interesting circuit. The pedal is essentially a variable resistor, so you might expect the output signal to pass through it. Instead, the pedal is connected to a photocoupler with an LED and a cadmium sulfide photocell inside. The output signal passes through the photocell and is attenuated as controlled by the LED. I think the motivation behind using a cadmium sulfide photocell instead of a phototransistor is that the photocell is completely resistive, so there is no nonlinear distortion of the signal. ↩

8. Time-division multiplexing and filtering isn't perfect, and will contribute some artifacts to the output. In particular, there will be some aliasing, where high frequencies turn into lower frequencies. The low-pass filter will eliminate most of the high frequencies—I believe the DX7's filter is at 16 kHz—but it's not perfect and will add its own color to the sound. Two notes could also interact differently based on their relative positions in the time slice. These artifacts probably contribute to the DX7's characteristic sound. You could consider the artifacts desirable if you're trying to duplicate the DX7 sound. ↩

9. The scale resistors are marked on the schematic with Ⓑ, which probably indicates they are higher-precision resistors. Assuming they are 1% resistors, a 1% error in a large signal would be much more error than the benefit of additional bits of precision. For smaller signals, the additional bits reduce the quantization noise, which is probably more important than the nonlinearity error from scaling. ↩

10. There's an interesting timing issue for the scale calculation. The scaling logic requires about 3 clock cycles to determine the scale factor, so the straightforward implementation would shift a voice based on the amplitude of an earlier voice. The solution is that the 5 bits for scale calculation are pulled out of the operator shift register six stages (3 clock cycles) earlier. Thus, these shift registers have two output; the "early" output gives the scale factor circuitry time to work. ↩

11. The output buffer has two latches, used by alternating notes. Each latch has one control line to latch a data value and one control line to output the latched value, so there are four control lines in total. Curiously, it appears that the notes aren't output sequentially; the order is 1, 13, 5, 11, 3, 15, 7, 10, 2, 14, 6, 12, 4, 16, 8, 9. I don't know if there's a motivation for this; it's also possible that I'm misinterpreting the circuit. ↩

12. A few notes on the PLA outputs in case anyone looks at them more closely. Because signals get delayed through multiple shift registers and clock cycles, things don't happen on the cycle you'd expect. For instance, SYNC is generated 6 cycles before the end. Likewise, loading of the output buffer is triggered midway through operator 6, about 26 cycles later than operator 1 started generating outputs. Most PLA columns are triggered for a specific voice and operator value. The exception is the last column, which increments the operator regardless of the operator value. Curiously, there are three counter reset lines. One resets near the end of operator 1 (as you'd expect). The other two reset near the end of the two invalid operator values (there are 6 operators but 8 possible bit values). Presumably this keeps the synth from starting up in a bad state. Below the PLA are some gates. These are mostly buffering and clock synchronization. ↩

13. Someone with a DX7 could probe the three unused pins and verify that they count out the notes. ↩

14. For more information on the DX7 internals, see DX7 Technical Analysis, DX7 Hardware, OPLx decapsulated, and the video Emulating the DX7 the hard way. ↩

Silicon die teardown: a look inside an early 555 timer chip

If you've played around with electronic circuits, you probably know the 555 timer integrated circuit,1 said to be the world's best-selling integrated circuit with billions sold. Designed by analog IC wizard Hans Camenzind2, the 555 has been called one of the greatest chips of all time.

An 8-pin 555 timer with a Signetics logo. It doesn't have a 555 label, but instead is labeled "52B 01003" with a 7304 date code, indicating week 4 of 1973. Photo courtesy of Eric Schlaepfer.

Eric Schlaepfer (@TubeTimeUS) recently came across the chip above, with a mysterious part number. He tediously sanded through the epoxy package to reveal the die (below) and determined that the chip is a 555 timer. Signetics released the 555 timer in mid-1972 4 and the chip below has a January 1973 date code (7304), so it must be one of the first 555 timers. Curiously, it is not labeled 555, so perhaps it is a prototype or internal version.3 I took detailed die photos, which I discuss in this blog post.

The 555 timer with the package sanded down to expose the silicon die, the tiny square in the middle.

A brief explanation of the 555 timer

The 555 timer has hundreds of applications, operating as anything from a timer or latch to a voltage-controlled oscillator or modulator. The diagram below illustrates how the 555 timer operates as a simple oscillator. Inside the 555 chip, three resistors form a divider generating references voltages of 1/3 and 2/3 of the supply voltage. The external capacitor will charge and discharge between these limits, producing an oscillation. In more detail, the capacitor will slowly charge (A) through the external resistors until its voltage hits the 2/3 reference. At that point (B), the upper (threshold) comparator switches the flip flop off and the output off. This turns on the discharge transistor, slowly discharging the capacitor (C). When the voltage on the capacitor hits the 1/3 reference (D), the lower (trigger) comparator turns on, setting the flip flop and the output, and the cycle repeats. The values of the resistors and capacitor control the timing, from microseconds to hours.5

Diagram showing how the 555 timer can operate as an oscillator. The external capacitor charges and discharges through the external resistors, under the control of the 555 timer.

To summarize, the key components of the 555 timer are the comparators to detect the upper and lower voltage limits, the three-resistor divider to set these limits, and the flip flop to keep track of whether the circuit is charging or discharging. The 555 timer has two other pins (reset and control voltage) that I haven't covered above; they can be used for more complex circuits.

The structure of the integrated circuit

I created the photo below from a composite of microscope images. On top of the silicon, a thin layer of metal connects different parts of the chip. This metal is clearly visible in the photo as light-colored traces. Under the metal, a thin, glassy silicon dioxide layer provides insulation between the metal and the silicon, except where contact holes in the silicon dioxide allow the metal to connect to the silicon. At the edge of the chip, thin wires connect the metal pads to the chip's external pins.

Die photo of the 555 timer. Click this image (or any other) for a larger version.

The different types of silicon on the chip are harder to see. Regions of the chip are treated (doped) with impurities to change the electrical properties of the silicon. N-type silicon has an excess of electrons (negative), while P-type silicon lacks electrons (positive). In the photo, these regions show up as a slightly different color surrounded by a thin black border. These regions are the building blocks of the chip, forming transistors and resistors.

NPN transistors inside the IC

Transistors are the key components in a chip. The 555 timer uses NPN and PNP bipolar transistors. If you've studied electronics, you've probably seen a diagram of an NPN transistor like the one below, showing the collector (C), base (B), and emitter (E) of the transistor, The transistor is illustrated as a sandwich of P silicon in between two symmetric layers of N silicon; the N-P-N layers make an NPN transistor. It turns out that transistors on a chip look nothing like this, and the base often isn't even in the middle!

Schematic symbol for an NPN transistor, along with an oversimplified diagram of its internal structure.

The photo below shows a closeup of one of the transistors in the 555 as it appears on the chip. The slightly different tints in the silicon indicate regions that have been doped to form N and P regions. The whitish areas are the metal layer of the chip on top of the silicon - these form the wires connecting to the collector, emitter, and base.

Structure of an NPN transistor on the die.

Underneath the photo is a cross-section drawing illustrating how the transistor is constructed. There's a lot more than just the N-P-N sandwich you see in books, but if you look carefully at the vertical cross-section below the 'E', you can find the N-P-N that forms the transistor. The emitter (E) wire is connected to N+ silicon. Below that is a P layer connected to the base contact (B). And below that is an N+ layer connected (indirectly) to the collector (C).6 The transistor is surrounded by a P+ ring that isolates it from neighboring components.

PNP transistors inside the IC

You might expect PNP transistors to be similar to NPN transistors, just swapping the roles of N and P silicon. But for a variety of reasons, PNP transistors have an entirely different construction. They consist of a small circular emitter (P), surrounded by a ring-shaped base (N), which is surrounded by the collector (P). This forms a P-N-P sandwich horizontally (laterally), unlike the vertical structure of the NPN transistors.

The diagram below shows one of the PNP transistors in the 555, along with a cross-section showing the silicon structure. Note that although the metal contact for the base is on the edge of the transistor, it is electrically connected through the N and N+ regions to its active ring in between the collector and emitter.

A PNP transistor in the 555 timer chip. Connections for the collector (C), emitter (E) and base (B) are labeled, along with N and P doped silicon. The base forms a ring around the emitter, and the collector forms a ring around the base.

The output transistors in the 555 are much larger than the other transistors and have a different structure in order to produce the high-current output. The photo below shows one of the output transistors. Note the multiple interlocking "fingers" of the emitter and base, surrounded by the large collector.

A large, high-current NPN output transistor in the 555 timer chip. The collector (C), base (B) and emitter (E) are labeled.

How resistors are implemented in silicon

Resistors are a key component of analog chips. Unfortunately, resistors in ICs are large and inaccurate; the resistances can vary by 50% from chip to chip. Thus, analog ICs are designed so only the ratio of resistors matters, not the absolute values, since the ratios remain nearly constant.

A resistor inside the 555 timer. The resistor is a strip of P silicon between two metal contacts.

The photo above shows a 10KΩ resistor in the 555, formed from a strip of P silicon (pinkish gray), contacting metal wiring at either end. Other metal wires cross the resistor. The resistor has a spiral shape to fit its length in the available space. The resistor below is a 100KΩ pinch resistor. A layer of N silicon on top of the pinch resistor makes the conductive region much thinner (i.e. pinches it), forming a much higher but less accurate resistance.

A pinch resistor inside the 555 timer. The resistor is a strip of P silicon between two metal contacts. An N layer on top pinches the resistor and increases the resistance. This resistor is crossed by a vertical metal line.

IC component: The current mirror

There are some subcircuits that are very common in analog ICs, but may seem mysterious at first. The current mirror is one of these. If you've looked at analog IC block diagrams, you may have seen the symbols below, indicating a current source, and wondered what a current source is and why you'd use one. The idea is you start with one known current and then you can "clone" multiple copies of the current with a simple transistor circuit, the current mirror.

Schematic symbols for a current source.

The following circuit shows how a current mirror is implemented with two identical transistors.7 A reference current passes through the transistor on the right. (In this case, the current is set by the resistor.) Since both transistors have the same emitter voltage and base voltage, they source the same current, so the current on the right matches the reference current on the left.8

Current mirror circuit. The current on the right copies the current on the left.

A common use of a current mirror is to replace resistors. As explained earlier, resistors inside ICs are both inconveniently large and inaccurate. It saves space to use a current mirror instead of a resistor whenever possible. Also, the currents produced by a current mirror are nearly identical, unlike the currents produced by two resistors.

Three transistors form a current mirror in the 555 timer chip. They all share the same base and two transistors share emitters.

The three transistors above form a current mirror with two outputs. Note the three transistors share the base connection, tied to the collector on the right, and the emitters on the right are tied together. On the schematic, the two transistors on the right are drawn as a single two-collector transistor, Q19.

IC component: The differential pair

The second important circuit to understand is the differential pair, the most common two-transistor subcircuit used in analog ICs. 9 You may have wondered how a comparator compares two voltages, or an op amp subtracts two voltages. This is the job of the differential pair.

Schematic of a simple differential pair circuit. The current source sends a fixed current I through the differential pair. If the two inputs are equal, the current is split equally.

The schematic above shows a simple differential pair. The current source at the bottom provides a fixed current I, which is split between the two input transistors. If the input voltages are equal, the current will be split equally into the two branches (I1 and I2). If one of the input voltages is a bit higher than the other, the corresponding transistor will conduct exponentially more current, so one branch gets more current and the other branch gets less. A small input difference is enough to direct most of the current into the "winning" branch, flipping the comparator on or off. The 555 chip uses one differential pair for the threshold comparator and another for the trigger comparator.10

The 555 schematic interactive explorer

The 555 die photo and schematic11 below are interactive. Click on a component in the die or schematic, and a brief explanation of the component will be displayed. (For a thorough discussion of how the 555 timer works, see 555 Principles of Operation.)

For a quick overview, the large output transistors and discharge transistor are the most obvious features on the die. The threshold comparator consists of Q1 through Q8. The trigger comparator consists of Q10 through Q13, along with current mirror Q9. Q16 and Q17 form the flip flop. The three 5KΩ resistors forming the voltage divider are in the middle of the chip.12 Urban legend says that the 555 is named after these three 5K resistors, but according to its designer 555 is just an arbitrary number in the 500 chip series.

Click the die or schematic for details...

Conclusion

I hope you've found this look inside the 555 timer chip interesting. Next time you're building a 555 project, you'll know exactly what's inside the chip. I've written about the 555 timer before; this post is pretty much the same as that one but with a different die. I've also written about a CMOS version. Thanks to Eric Schlaepfer13 for providing the die; see his Twitter thread for background on this chip.

I announce my latest blog posts on Twitter, so follow me @kenshirriff and you won't miss an article! I also have an RSS feed.

Notes and references

1. The 555 timer is iconic enough to appear on mugs, bags, caps and t-shirts. Whole books are devoted to 555 timer circuits. ↩

2. The book Designing Analog Chips written by the 555's inventor Hans Camenzind is really interesting, and I recommend it if you want to know how analog chips work. Chapter 11 has an extensive discussion of the 555's history and operation. Page 11-3 claims the 555 has been the best-selling IC every year, although I don't know if that is still true. The free PDF is here or get the book. ↩

3. The die has the part number 1000 and revision "C", so this probably corresponds to the 01003 number on the package. I suspect this chip is the third mask revision of the original 555.

   

   The first 555 die with the part number "1000" highlighted and the revision "A" magnified.

   The die of the first 555 timer version (above) is marked with the number "1000" and revision "A". I compared this image with the die photo that I took and I couldn't see any differences except the revision changed to "C". The mask changes must have been fairly subtle. (This image is at Wikipedia and IEEE Spectrum. The image is captioned as the die shot of the first 555 timer IC manufactured in 1971.) ↩

4. The 555 chip was introduced in mid-1972 according to Signetics Analog Applications page 149. ↩

5. The brilliant part of the 555 timer is that the oscillation frequency depends only on the external resistors and capacitor and is insensitive to the supply voltage. If the supply voltage drops, the 1/3 and 2/3 references drop too, so you might expect the oscillations to be faster. But the lower voltage charges the capacitor more slowly, canceling this out and keeping the frequency constant.

   This voltage insensitivity is so tricky that the chip's designer didn't figure it out until near the end of the 555's design, but it made a big difference. The original design was more complex and required nine pins, which is a terrible size for an IC since there are no packages between 8 and 14 pins. The final, simpler 555 design worked with 8 pins, making the chip's packaging much cheaper. (See page 11-3 of Designing Analog Chips for the full story.) ↩

6. You might have wondered why there is a distinction between the collector and emitter of a transistor, when the typical diagram of a transistor is symmetrical. As you can see from the die photo, the collector and emitter are very different in a real transistor. In addition to the very large size difference, the silicon doping is different. The result is a transistor will have poor gain if the collector and emitter are swapped. ↩

7. For more information about current mirrors, check wikipedia, any analog IC book, or chapter 3 of Designing Analog Chips. ↩

8. The schematic has the unusual symbol below, which indicates a transistor with two collectors. The base is drawn on the same side as the emitter and collectors, which adds to the confusion. On the die, this transistor is implemented with two separate transistors, with the emitters and the bases wired together. Other circuits sometimes use a single transistor that has two physical collectors present.

   

   This symbol indicates a transistor with two collectors.

    ↩

9. Differential pairs are also called long-tailed pairs. According to Analysis and Design of Analog Integrated Circuits the differential pair is "perhaps the most widely used two-transistor subcircuits in monolithic analog circuits." (p214) For more information about differential pairs, see wikipedia, any analog IC book, or chapter 4 of Designing Analog Chips. ↩

10. In the 555, the threshold comparator uses NPN transistors, while the trigger comparator uses PNP transistors. This allows the threshold comparator to work near the supply voltage and the trigger comparator to work near ground. The 555's comparators also use two transistors on each input (Darlington pair) to buffer the inputs. ↩

11. The 555 schematic used in this article is from the Philips datasheet. It is identical to the Signetics schematic p150. ↩

12. Note that the three resistors for the voltage divider are parallel and next to each other. This helps ensure they have the same resistance even if there are electrical variations across the silicon. ↩

13. Evil Mad Scientist sells a very cool discrete 555 timer kit, duplicating the 555 circuit on a larger scale with individual transistors and resistors — it actually works as a 555 replacement. Their 555 footstool is also worth a look.

    

    Large-size 555 timer created by Evil Mad Scientist Lab.

     ↩