It was also a popular hobbyist chip, easy verilog shift 1 bitcoin 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.