0037-2020-06-29.md

29 Jun 2020

test07d modified:

• Simplified char ROM code/logic.
• Added support for hex digits to char ROM.
• Counter is now 6 digits: Lowest 2 count frames 0..63 in hex; next 3 are decimal and tick over every 64 frames; highest is a continuation, but counts in hex. Hence, highest reading can be F9993F before it rolls over to 000000.
• How close can we get to 64Hz with our current clock? Take 25MHz, divide by 64, and then divide by these horizontal values to get close to good vertical values:

  Divide by...	...lines	...rounded Hz
  772	505.99	63.9989Hz
  775	504.03	64.0041Hz
  789	495.09	64.0115Hz
  786	496.98	63.9972Hz

• Should try using a flexible carry-based BCD counter: https://www.element14.com/community/groups/fpga-group/blog/2014/10/12/seven-segment-bcd-counter-using-the-valentfx-logi-edu
• Check out:
  • https://allaboutfpga.com/sequence-detector-using-mealy-and-moore-state-machine-vhdl-codes/
  • https://zipcpu.com/dsp/2017/12/14/logic-pll.html
  • https://zipcpu.com/tutorial/
  • https://hackaday.com/2015/06/15/hackaday-prize-entry-an-open-source-graphics-card/ and https://hackaday.io/project/6309-vga-graphics-over-spi-and-serial-vgatonic
  • https://hackaday.io/project/841-teaching-fpga-parallel-computing?utm_source=hackaday&utm_medium=hacklet&utm_campaign=28
  • https://hackaday.io/project/1943-vgatonic and https://github.com/dqydj/VGAtonic
  • http://pldworld.org/html/technote/intro.cpld.fpga.design.pdf

NOTE: With my little old TFT monitor, I successfully tweaked the timing to use a line timing of 772 "dots" and a frame timing of 506 lines. This makes the monitor sync at ~63.99Hz, and still correctly show the 640x480 image. As a result, the counter remains very closely in sync with my iPhone stopwatch, second by second.

I have since changed the code back to use normal 640x480 @60Hz timing.

Project ideas for next test:

• Move a cursor on the screen in X and Y. Left button increments X, right button increments Y, both have wrap-around.
• Simulate acceleration and deceleration using inertia counters.
• Keyboard or keypad input, or even RS-232, to make things more interesting.
• Interfacing with SRAM for video bitmap image.