topic Re: XTAL -> pull up to 3V3 or not ? in ColdFire/68K Microcontrollers and Processors

XTAL -> pull up to 3V3 or not ?

efk — Tue, 23 Sep 2008 01:38:26 GMT

Hello,

In the M52210DEMO kit, XTAL has a 10K pull up resistor to 3V3.
In the M52211EVB kit, this pull up resistor does not exists.
My future application uses a clock driven by a 48MHz quartz.

So i would like to know if it is advised to include this resistor in final printed circuit boards production, or not ?

Re: XTAL -> pull up to 3V3 or not ?

RichTestardi — Tue, 23 Sep 2008 10:49:08 GMT

See section 6.6.2 and 6.6.4 of the reference manual.

XTAL -- This output is an internal oscillator connection to the external crystal. If CLKMOD0 is driven low during reset, XTAL is sampled to determine clocking mode.

The pull up selects between the external oscillator and the internal oscillator for early boot (and also for EzPort access, I believe).

This is shown in Figure 6.1, which is critical for understanding the clock selections, and I attached the highlited section of that figure to this post.

-- Rich

Re: XTAL -> pull up to 3V3 or not ?

RichTestardi — Tue, 23 Sep 2008 10:54:40 GMT

And I probably should have added that, in your case, I assume you're driving CLKMOD0 high during reset, so I believe XTAL (well, the pull-up, anyway!) is moot.

Message Edited by Rich T on 2008-09-22 09:55 PM

Re: XTAL -> pull up to 3V3 or not ?

efk — Tue, 23 Sep 2008 14:17:17 GMT

Thank you for your answer.

I read carefully section 6.6.4.

My application uses exclusively a 48MHz crystal.
CLKMOD0 has effectively an external pull up resistor.
CLKMOD1 is not available (64 pins version) ; the datasheet says it has an internal pull down resistor.

Anyway, i understood that, whatever state is CLKMOD1, when CLKMOD0 state is "1", XTAL state has no consequence. So i think the pull up resistor to XTAL is not useful. I will remove it.

Regards.