NTSC MiniMig

[Dennis]

have to say that I am not 100% upto speed on the guts of either the Amiga hardware OR MiniMig but I thought that I would have a go and perhaps Dennis would read and help correct a few things.

You're quite right.  :-)
the mclk has to be changed to the NTSC color frequency (and the DCM reprogrammed) and you'll have to make the frames shorter by changing the verbeam compares. But, if I look in the hardware reference manual (page 18, copper), it states that on NTSC machines, the horizontal lines are not all 227 color clocks long. (on Minimig that means 454 bus clocks) Every other line is a long line (228 clocks) from the system's point of view. The display/tv however sees only 227.5 clock lines. So, you will have to do some stuff with the horbeam compares and horizontal sync generator too. That should be all though.

Dennis

[Dennis]

If the only physical change is to swap the crystal. Maybe a change should be incorperated such that both clock sources (pal=4.433619 ntsc=3.579545) are available to the fpga..?

It should be possible to generate both clocks from a single 20Mhz or 50Mhz source. (20Mhz would be better) Someone on the old minimig thread suggested a possible setting of the DCM to do just that; generate all PAL clocks from a single 50Mhz clock using the DCM. If I recall, he was only a couple of ppm off, which should be good enough. You could de the same for NTSC, hence all could be done in software/verilog.

Dennis