68300

>
> The input frequency range for the MC68332 is 25 - 50 KHz.  So, just go right
> ahead and us a 38.4 KHz crystal and enjoy life.


Yeah, I had double-checked that before I made the suggestion...didn't want to
look like an idiot in public (at least too often).  ;-)


>
>  Now, remember, regardless of what crystal you use, you must limit the
system
> clock frequency to what ever the part is guaranteed for.  For instance, if
> you buy an MC68332CFC20, you cannot run the system clock greater than 20
MHz,
> or 25 MHz  for a CFC25.


As part of a built in self test I run on our incoming boards, I ramp the
PLL up
one notch at a time until it won't lock.  I usually get to about 45-47 MHz, at
room temperature, running from external memory.  It does better when I
blast it
with cold spray, and I'd never do it in production--it's just a fun little
test
I threw in.  Anyone else do any overclocking experiments?


>
>  The reason that many people use the 32.768 KHz crystal is that it is a
power
> of 2 (actually 2 to the 15th power).  Thus, if you have a 2E15 divider
inside
> the part, you get 1 second ticks.  That's why this frequency crystal is many
> times referred to as a "watch" crystal.  Because this crystal is used in
> clocks, it enjoys very high volume, many manufacturers and good
availability.


Agreed.  I've never run a 1 Hz ticker--something like 100 Hz seems more useful
for a low-end 32 bit controller.


>
> The big down side of 32.768 KHz crystals is that they are hard to get in
> automotive grade environments and temperature ranges.


What about startup-time and mechanical fragility?  You've still got those
problems with the 38.4kHz too.  Maybe a better question--what's the
_advantage_
of a low frequency tuning fork rock?


newell