>-----Original Message-----
>From: dan.lloyd@GB.ABB.COM [mailto:dan.lloyd@GB.ABB.COM]
>Sent: 26 March 2004 09:00
>To: PICLIST@MITVMA.MIT.EDU
>Subject: Re: [EE]: Elapsed time counters (with calibration) ?
>
>
>Hi,
>
>The thought had crossed my mind but this needs to be as 'cheap
>as [potato] chips'. (<$1)
>
>One device that looked promising was the Dallas DS1318 but it
>is parallel and has, oddly, no crystal calibration capability
>(considering the accuracy provided by the subseconds counter).
>The requirement is for a rtc with accuracy better than 5ppm @
>25deg C so there are two choices:
>
>a. Better than 5ppm xtal.
>b. RTC with cal option + any cheap crystal.
>
>Currently, I use xtals that are about 20ppm (and use software
>cal) but haven't had any information back regarding the cost
>differential between the <5ppm and 20ppm xtals - do you think
>the cost would be prohibitive?
>
>Regards,
>Dan

You could also tune the crystal a small amount using series/parallel caps.
Of course you then have to take into account any temperature coefficients of
the capacitors if you have tight frequncy tempco requirements.

Regards

Mike




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