Choices for the PLLDIV value...

cbmeeks

I was looking through some video drivers to see how they worked.

In one example (a TMS9918 type driver), the FRQA was set to 2 * ColorBurst and the PLLDIV mode set to VCO/2. So that comes out to 16 * ColorBurst.

However, in another driver (a ULA type driver) the FRQA was set to ColorBurst and the PLLDIV mode set to VCO/1. Which translates to the same value.

So both frequencies are equal from what I understand.

My question is, why use one over the other? Is it just personal preference or do you think there was a reason?

If it helps any, the TMS driver is a 4 color driver whereas the ULA is a 2 color.

Thanks.

Bean

The input to the PLL is spec'd at 4 MHz to 8 MHz so colorburst 3.57MHz is really too low, but I'm sure it works fine.

Bean

cbmeeks

Oh, so you're saying it's probably because that NTSC colorburst is too slow (under 4MHz) and to x2 would put it somewhere within spec (7.16MHz) and fall within that sweet spot?

That does make sense.

Thanks.