So far, when using DMA, all the 640x480 modes suffered from what I call FIFO jitter, where many lines are shifted by 10-20 pixels, similar to the artefacts shown here:
http://downloads.qi-hardware.com/people/werner/ubb/vga/ubb-vga-1024x768-36Hz.jpg I now cooked up a new mode called 640x480/61 that doesn't always do this. It still has a probablility of about 30% of suffering FIFO jitter during an entire session, but if a session starts cleanly, it stays clean. Here is what it looks like on my 1680x1050 Samsung: http://downloads.qi-hardware.com/people/werner/ubb/vga/ubb-vga-hsync-noisy.jpg I also tweaked the clocks a bit to make the MMC controller run faster than the bus clock whenever possible. This may also have helped to improve the chance of getting a stable image. As one can see, there is still considerable jitter in the range of a few pixels. I suspected that this was caused by the low accuracy of my timer polling loop: the timer runs at about 112 MHz but we can only poll it with approximately 10 MHz. To try to compensate for this, I changed the timer function to read the current (112 MHz) timer value at the end of the wait, and to run a delay loop for the difference between this value and a fixed point a bit further ahead. For example, if we waited for time TCNT = 1000, we would detect that the timer has crossed that value at TCNT = 1000+n, with n between 0 and 20. If we set the real deadline to 1000+24, the loop would then run for the difference between 1024 and the value read from the timer register, multiplied with a suitable scale factor. This turned out to work quite well: http://downloads.qi-hardware.com/people/werner/ubb/vga/ubb-vga-hsync-clean.jpg Next, I examined the hsync signal with a scope and plotted the statistical distribution of the signal edges: http://downloads.qi-hardware.com/people/werner/ubb/vga/hsync-noisy.png http://downloads.qi-hardware.com/people/werner/ubb/vga/hsync-clean.png Trigger is on the falling edge. Red means there was often a sample at this point, green a bit less, blue rarely, black never. - Werner _______________________________________________ Qi Hardware Discussion List Mail to list (members only): [email protected] Subscribe or Unsubscribe: http://lists.en.qi-hardware.com/mailman/listinfo/discussion
