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From: Fred Bloggs
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Subject: Re: Querry, Kick Starting Crystal Oscilator.
References: <3DCA905A.59FD928B@mmm.com.DELETETHIS> <3DCC9A71.7DF3C3A9@mfi.net> <3DCFC346.A032E097@mmm.com.DELETETHIS>
Date: Mon, 11 Nov 2002 16:32:46 GMT
NNTP-Posting-Date: Mon, 11 Nov 2002 08:32:46 PST
Organization: EarthLink Inc. -- http://www.EarthLink.net
Roy McCammon wrote:
> Winfield Hill wrote:
>> Like most crystals, those have a Q of 60,000 to 90,000 which
>> means they'll take to 2 to 3 seconds to reach 50% of the full
>> oscillation energy. That's the way it goes; the energy cannot
>> simply be kick-started into the crystal. [For time-critical
>> applications workarounds include using a separate RC oscillator
>> to run the CPU until the crystal is up to speed.
> Care to kick this circuit around?
> | slow |
> | starting |
> | 32KHz |---------C-----+-----R-----.
> | crystal | | |
> | osc | | |
> '----------' '----|>o----+-- to CPU
> Schmidt inverter
The start delay is clearly a function of the available drive power from
the inverter as well as crystal R. Intersil has a nice application note
on the HA7210 which shows how to kick-start the typical 32.768KHz
crystal into full oscillation in under 600ms. The HA7210 can be placed
in standby at minuscule overhead current for instant start if necessary.
Your way around the slow start-up clearly consists of using circuitry
external to the processor. If a low frequency oscillator in standby is
not a possibility, then it appears that a much higher frequency crystal
oscillator followed by digital divider which drives the processor clock
will have to be the answer.
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