From: Spehro Pefhany
Subject: Re: Querry, Kick Starting Crystal Oscilator.
References: <3DCA905A.59FD928B@mmm.com.DELETETHIS> <3DD26B65.email@example.com>
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Date: Wed, 13 Nov 2002 16:10:55 GMT
NNTP-Posting-Date: Wed, 13 Nov 2002 11:10:55 EST
On Wed, 13 Nov 2002 15:09:50 GMT, the renowned Fred Bloggs
>You should forget about playing games with the 32kHz crystal and go with
>a 4.096MHz crystal driving a /128 prescalar into the processor. Since it
> has been demonstrated that Q roughly sets the start-up time, and if
>you notice that this is actually a number of cycles, then increasing the
>frequency will decrease start-up time proportionately. A 4.096MHz should
>be in full oscillation in 20ms.
Okay so far.
>The increased power dissipation will be
>negligible compared to the processor.
Many modern CMOS processors use tens of uA or less at 32kHz clock.
Some only one or two uA. For example, this 16-bit micro:
> If you survey some typical
>oscillators such as at http://www.ecsxtal.com/surfosc.htm , then you
>will see that the ~4MHz oscillators have a start-up time in the 4-30ms
>range. The 4.096MHz crystal is readily available in several styles.
Those packaged oscillators draw a huge amount of current, over 10mA
for some.. you could run thousands of CPUs with that much juice. ;-)
If current consumption is not a factor, then this solution is the
obvious one, but then why not just run the whole micro at 4MHz?
Can you make a 4MHz oscillator that runs reliably on just a few uA?
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