> Are the internal crystals on these chips hugely affected by the input
> voltage?

The internal clocks are resistor-capacitor based, and do vary with
temperature and Vcc, but in "normal" circumstances not by as much
as 50-100%. Figures 13-15 and 13-16 in the 629 datasheet show
more like -160kHz + 60kHz for temperature and +/- 80kHz for Vcc

The 629 is 4MHz at 3.5V and 25C

> 50Hz = 20ms per cycle = 16-bit timer loading of 10000. An IRQ
> > using this will give you the start point of each cycle
 
> I don't understand this at all, but i'm guessing it's just not being
> familiar enough with the language yet

My mistake, I meant -20000. 10,000 is a half-cycle, which would
be used for 50% duty cycle timing

Well, Timer1 increments with each instruction cycle. If you load it
with -20,000 it will take 20,000 IC (= 20ms @ 1us IC) to reach
0000. At that point TMR1IF is set and, if enabled, an interrupt will
occur. With TMR1 re-loaded, that will happen again in 20,000 IC
time, giving you the 20ms frame/cycle time

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