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A project log for YATAC78 - The WWW TTL Computer

Retro computer built from 1978-era TTL logic chips. Internet capable with built in web browser and server

Alastair HewittAlastair Hewitt 10/15/2019 at 03:500 Comments

Quick update after a long weekend. The final version of the CPU has been built and tested. It's not the prettiest thing in the world!

There's not much to demo until the GPU is installed. For now, the most exciting thing it has done is generate a 1 Hz pulse. That may sound simple, but this was using a version of the planned RTC code (accurate to 8.5 ppm). It requires 12 bits to divide down the 8 MHz process clock and would normally only use three bytes of the zero page. The version tested used both the zero page and the full RAM address space of bank 0. The ALU operations were also expanded to test the full 2-cycle ALU addition/subtraction instead of just doing increment/decrement.

A couple of notes on the picture: The 70ns NOR flash was having a hard time meeting the 50ns access cycle of the 16 MHz machine clock, so a couple of slower oscillators are being used for testing (the actual OTP ROM is 55ns and should be fine) . There are patch wires on the ROM address and data busses that can be moved to add/remove bus drivers. The current design exceeds the recommended fanout on the data bus, but it doesn't appear to be an issue. In fact the circuit is a lot more stable without them.

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