I was able to use the VGA-666 board to display on a Phillips CM8833-II monitor, which was formerly used with an Amiga 500. The monitor takes RGB and HV-sync signals, and supports horizontal sync frequencies in the range of 15kHz, so it can only display 240p/480i type signals. The only predefined mode I could get to display a stable picture was dpi_group=1, dpi_mode=12 but the picture was severely squashed horizontally.
I then found http://forum.arcadecontrols.com/index.p ... msg1545662
which suggested that when driving DPI at low resolutions, the pixel clock has to be an integer division of 19.2MHz and suggested a custom hdmi_timings value for 336x252 resolution at 50Hz. I was able to get a usable picture using these settings:
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hdmi_timings=336 1 10 30 34 252 1 15 3 42 0 0 0 50 0 6400000 1 # 336:252 6.4MHz progressive
By trial and error with different settings from http://www.epanorama.net/faq/vga2rgb/calc.html
I was able to find another custom timings line that gave me 320x240 at 60Hz refresh rate:
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hdmi_timings=320 1 16 30 34 240 1 2 3 22 0 0 0 60 0 6400000 1 #240p
This works on my monitor, but it technically uses a horizontal sync frequency of 16 rather than 15kHz, so it might not work for everyone. I wasn't able to reduce the vertical front porch/sync pulse/back porch values any further without getting lines cut off at the top or bottom of the display.
I wasn't able to get any interlaced (~480i) modes to display a good picture. The interlaced mode from the linked post (500x375 with a 9.6Mhz pixel clock) showed a picture but the flicker was horrible, it looked as if every other frame was blank. I think the interlaced modes may require a pixel clock between 9.6 and 19.2 MHz, which doesn't seem to work over DPI. I wonder if this is a hardware limitation or just a problem with the firmware? This post: viewtopic.php?f=29&t=112735
seems to suggest it's a problem with how the firmware is configuring the clock source for DPI.