My aim here is to replicate the core chip set of the two most popular 16 bit
home computers. I am being quite selfish, I used to write demos
for these machines, and software emulation isn't (quite) good enough to run them.
I also always fancied a portable Atari, and as no one else has done it, it looks like I'm going
to have to do the hard work myself.
I have finished the sound chip used in the Atari, a YM2149 - but as people are quite fussy about the audio,
I am running it in parallel with a real one in a machine, and comparing the volume and timing of the two.
The picture shows an FPGA with some probes onto the real chip. The bit of veroboard to the left is a simple dac / sound amp.
The audio core does the final mix of the three outputs in the real chip and outputs a 12 bit mono digital output.
Notice the subtle use of masking tape ...
The next picture shows a real 68000 processor wired to a old Xilinx FPGA (it needed to be tolerant
to the 5Volt signals the processor uses). The FPGA runs a dynamic test suit which cycles through the entire
instruction set, and this is compared against the soft core which lives (so far) in the same chip.
The following two pictures show two of the custom chips in an Atari ST being analysed. Once I have a rough cut of
the code, I can then run them in sync with the original custom and look for any differences. The Atari chips are
very simple, the Amiga AGA chipset is more complex, but will still easily fit in a small FPGA. The only problem is
the amount of time needed to get it right ...
