68300

-----Original Message-----
From: Melear Charles-rdph40 [mailto:charles.melear@...]
Sent: Thursday, May 23, 2002 3:05 PM
To: '68300@yahoogroups.com'
Subject: RE: [68300] Favorite clock frequency?

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

-----Original Message-----
From: Scott Newell [mailto:newell@...]
Sent: Thursday, May 23, 2002 12:43 PM
To: 68300@yahoogroups.com
Subject: RE: [68300] Favorite clock frequency?

>
> 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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