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From: Winfield Hill
Subject: Re: Querry, Kick Starting Crystal Oscilator.
Date: 9 Nov 2002 14:54:59 -0800
Organization: Rowland Institute
References: <3DCA905A.59FD928B@mmm.com.DELETETHIS> <3DCC9A71.7DF3C3A9@mfi.net>
X-Newsreader: Direct Read News 4.10
>Roy McCammon wrote:
>> I've got a processor with a 32KHz crystal. The problem
>> is that the crystal oscillator can take a l_o_n_g time
>> to start and stabilize.
>> My thought is to build a 32kHz cmos schmidt trigger RC
>> oscillator and feed that through a small capacitance into
>> the input of the crystal oscillator to get it going.
> http://www.ecsxtal.com/pdf/ecs-3x8.pdf I believe this was the
> crystal we used from ECS crystals and was Digi-Key stock number
> X801-ND. If you search Digi-Key for 32.768 KHz crystal you will
> see they carry quite a wide range in specifications.
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. However it's
likely you cannot simply overide the crystal, because its phase
shift won't track the RC oscillator.]
As for determining a safe processing delay at startup, once the
crystal energy is high enough so the oscillator runs subsequent
logic, it can in principle be used. Waiting an additional few
thousand cycles (as many CPU chips do) before starting serious
computing adds a safety margin that's usually sufficient.
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