It was also a popular hobbyist basys2 spartan 3e bitcoin, easy to experiment with and available at Radio Shack. I reverse-engineered the chip from die photos and found some interesting digital circuitry inside. Perhaps the most interesting is a shift register based white noise generator, useful for drums, gunshots, explosions and other similar sound effects. Functionality blocks inside the 76477 sound chip, indicated on the die.
Die photo courtesy of Sean Riddle. Looking under a microscope, you can see the circuitry that makes up the chip. The yellowish lines above are the metal traces that connect the circuits of the die. The reddish and greenish regions are the silicon of the chip, forming transistors and resistors. The black blobs around the edges show where tiny bond wires connected the die to the integrated circuit pins.
I’ve outlined the analog circuits outlined in purple, while digital circuits are in cyan. The block diagram below shows the 76477 chip’s functional elements and can be compared to the die photo above. The “super low frequency” SLF oscillator generated a triangle wave. Feeding this into the VCO generated a varying pitch, useful for bird chirps, sirens, or the warbling sound of the UFO in Space Invaders. Block diagram of the 76477 sound chip, from the datasheet. Digital inputs: triangles, resistor inputs: red, capacitor inputs: cyan, voltage inputs: violet.
The remainder of this article will dive into how the digital circuitry of the 76477 chip was implemented. Next, the noise generator, control logic and the digital mixer are reverse engineered and explained. Integrated Injection Logic You may be familiar with TTL integrated circuits, such as the popular 7400 family. These chips are built from bipolar transistors—NPN and PNP transistors—and were fast. Minicomputers were built from boards full of TTL chips, taking advantage of its speed. Die photo of a TTL inverter.
The inverter uses four transistors, four resistors and two diodes. 1970s, integrated circuits based on MOS transistors led to the rise of the microprocessor. Unlike TTL, MOS transistors could be densely crammed onto VLSI integrated circuits, but unfortunately, MOS was much slower than TTL. This posed a dilemma in the 1970s: TTL couldn’t implement dense circuitry and MOS was too slow. I2L solved the problems of TTL and MOS with a clever new design.
Each I2L gate was built from a single multi-collector bipolar transistor instead of the multiple transistors of TTL. Even better, the transistors could be arranged in a high-density grid. Finally, the bulky resistors of TTL were replaced by an “injector”, a tiny transistor that injected the necessary current. The diagram below shows six logic gates on the 76477, implemented with I2L logic. There are two columns of gates, with injectors down the middle.
Note the high density of the circuitry, without wasted space. Six I2L gates in the 76477 chip. Each gate is implemented with a single, compact transistor. But an I2L gate has one input and multiple outputs!