Scott,
A
couple of things here. First of all, 32.768 KHZ crystals are made in such
high volume that they enjoy "economies of scale". That is, they are one of
the least expensive crystals you can get.
You
did bring up two good points about start up time and mechanical
stability.
The
mechanical stability is a real problem if you are designing something for the
automotive market. Heat and vibration are really hard on the 32.768 Khz
tuning fork. What we finally did at Motorola was to go to a 4.1 Mhz (or
so) crystal and then put a "divide by 32768" on the extal input. Then a
high frequency crystal could be used and we still had the 32.768 KHz clock going
to the PLL.
High
frequency crystals are readily available in automotive
grade.
Also,
low frequency crystals have relatively long start up times and it is important
that the series resistor in the crystal circuit be properly sized. This is
a function of the crystal, not the microcontroller.
NOW,
READ THIS.
On
MC68300 and MC68HC16 products, there are two types of Lock Detect
circuits. They require different external XFC filter circuits. On
the MC68332, the only mask set in production is J30C and it requires the
3-component or high-stability XFC filter. Ditto for the MC68331 where J33C
is the only mask set in production.
If you
don't know what I am talking about here, send me another email and I will give
you the full storey. However, this external XFC filter business is very
important. You have to use the right filter. So, if you get with me
and tell me what part you are using, I can tell you which mask sets require
which filter.
Regards,
Charlie
Show quoted textHide quoted text
>
> 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
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