Things had been going very well;<\/p>\n\n\n\n Compact flash working, console working, beeper working, 6522 VIA working and display working!<\/p>\n\n\n\n Here is a 1024 x 600 Mandelbrot with 32 iterations, looks fabulous.<\/p>\n\n\n\n Getting the screen working took the longest time, the new driver chip (RA8889) is way more complex than the previous RA8875 one, but was needed for the higher resolution\/larger screen. That probably took me about 2 weeks to get working properly.<\/p>\n\n\n\n Now the problem is that I rather naively assumed that my ATMEGA644P I use for the keyboard scanning would be able to respond to a request to read data from the CPU and put it on the data bus quickly enough to work, oh boy was I wrong \ud83d\ude41 Now I could respin the PCB, but that’s extra cost, wasted time and money in getting this one to where it is. So I’m trying something different, I fortunately routed two pins to the main CPLD for control lines, these happen to be the unused UART pins on the AVR, so I’m trying to fit a UART inside the CPLD (you know I’m mad, right?).<\/p>\n\n\n\n It’s been a challenge, so far I’ve managed to get Serial data in and into a register.<\/p>\n\n\n\n But it’s been really difficult to get this far. Not having a clock to synchronise things to is difficult. I’ve not tried the transmit yet, so there is a long way to go. <\/p>\n\n\n\n If this fails then I will have to respin the PCB, which is something I really don’t want to have to do.<\/p>\n\n\n\n![\"LT6502b](\"https:\/\/www.maddox.pro\/wp-content\/uploads\/2026\/04\/MainPCB_20260407-1024x790.jpg\")
<\/figure>\n\n\n\n![\"LT6502b](\"https:\/\/www.maddox.pro\/wp-content\/uploads\/2026\/04\/Mandelbrot_31itt_1024x600-1024x655.jpeg\")
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The AVR takes 5 clock cycles to respond to an interrupt and 5 to exit, even if I do the clock stretch thing down to 15:1 that still only leaves one or two cycles to get the data, enable the output on that port and put the data on the port… then I have to make the port input again at exactly the right moment.
The reason I did this is that I didn’t want to use the 6551 VIA as it’s only available in DIP format and that takes a lot of space on the PCB. What I should’ve done was use something like a 16C752, hey ho, lesson learned.<\/p>\n\n\n\n![\"\"](\"https:\/\/www.maddox.pro\/wp-content\/uploads\/2026\/04\/UARTCPLD-1024x545.png\")
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