ubb-vga, dual screen (was Re: ubb-vga, with DMA)

Werner Almesberger werner at almesberger.net
Mon May 2 06:39:24 EDT 2011

After reducing the burst size of the Ben's LCD controller and making
the timer handling more resilient to disturbances, one can also leave
the Ben's LCD running while displaying a VGA image:


Having the LCD on adds a bit more noise, but it's not too horrible.

I think 1024x768 will be the highest resolution I can usefully
generate: it seems that most modern monitors expect signals with a
refresh rate of at least 50 Hz. The pixel clock is limited by the
MMC controller to 56 MHz. As the picture shows, this is sufficient
for 1024x768 at 50 Hz, but too slow to reach the next higher step,

It may make sense to go to 1280x480, though. This would look like
640x480 to the monitor, but it would double the pixel clock, which
would in turn half the amplitude of horizontal noise.

All my recent development is with the old XEN-1510 monitor. I tried
to output images and more modern and larger monitors and they
didn't like my signals. I made a few improvements since, but I
don't know yet if they're enough to make things work.

- Werner

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