FPGA computer in VHDL

I designed the video timings to match the Atari series because that was what I ultimately wanted to recreate. Their video chip was far more advanced than the 6845 and 6847 or any other video chip at the time. It could mix graphic and text modes in the "display list".

The video clock was based on the 14.318 MHz colour crystal (four times NTSC colour carrier).  This gives squarer pixels than the BBC Micro's 16 MHz timing chain. The ideal 14.75 MHz gives perfect square pixels but 14.318 is convenient for NTSC video and 16 MHz divides easily to the clocks used by the BBC micro chip set. The floppy drive controller needed exactly 8 MHz for example.

14.318 MHz is only 3% lower than ideal, while 16 MHz is 8.5% higher. With hindsight I might have used the commonly available 14.7456 MHz frequency used for baud rate generation. This is only 0.03% low.

I did not get round to making an ANTIC compatible model but I did add someone else's VHDL POKEY which worked as expected.

After the 6847 video controller, the rest of the Acorn Atom is trivial to recreate.

Modelling new machines on old ones provide existing software and avoids the chore of writing it. The Acorn Atom BASIC is small enough to compile into FPGA memory, allowing the writing of programs without external ROM. My first Atom BASIC program was to program BASIC an external ROM. This avoided the need for a device programmer.

The Atom can be modified to run BBC BASIC (with hardware limitations) but I did not get round to implementing that. There are VHDL models of complete BBC micros.

If cloning an 8-bit computer in FPGA, one might as well clone the best one around.

I feel that the Atari 6502 series are the best, for the following reasons.

• Full-custom ASICs designed and built for the job.
• Sprites and collision detection
• Fewer socket contacts to go faulty
• Huge software base
• Accurate video timing
• Versatile video controller

The big hurdle in cloning these machines is that many things have to be up and running at the same time for the software to work. The video, the sound, the keyboard and so on.

So I decided to clone the much simpler Acorn Atom, but fix the many defects of the 6847 chip by basing the video timings on the Atari machines. I did so, and added extra features such as

• Programmable character set, and lower-case text.
• Cross-hairs (in hardware)
• PS/2 keyboard and mouse interface
• USB serial interface
• Sprites
• Colour
• Palette
• Video and audio DACs
• POKEY and AY-3-8910 sound

In 2001, I started by toggling an LED on a development board. By 2005 I had cloned the Acorn Atom and had run out ideas for ways to improve it. Analogue TV started to go extinct and I was working 12 hour days to buy a home. I'd learned VHDL to my satisfaction and didn't really need an Atari clone.