Despite good progress made by shoving bits in my software development tool of choice, finally the CHRE firmware resisted the plan to show at least some real color graphics before the end of RetroChallenge. So, I waved the white flag, took some sleep and got back to the issues today.
When sending video data from the MCU straight to the DAC it was possible to produce some nice looking pictures:
A closer look to the color gradient…
Ok, that’s boring, but nice. This graphic shows a resolution of 320 by 480 at 16 out of 256 colors. Apparently, we have a rock-solid timing. Edges are well defined and no distortions are visible.
Unfortunately as soon as I try the same out of the video memory nothing works as expected anymore. To make a long story short, it turned out that I somehow managed to fry Port C of the ATmega1284. Its output drivers are still working, but it draws about 6 mA on each pin in high impedance state. That pulls every bit of data down that the SRAM tries to send to the DAC.
I will continue to work on this project as time permits. Please follow me on Twitter or check back regularly.
By the way, if you are interested in some other PET 2001 work regarding graphics, make sure to visit the awesome RetroChallenge 2017/04 project of Norbert Landsteiner, who implemented a “Space Commander” clone based on character graphics! Hats off to Norbert!