Spehro Pefhany wrote in news: snipped-for-privacy@4ax.com:
I thought some microprocessors like that were meant to be very cheap, designed to allow a realtime clock as well as the stuff for processing timebased information. One of those might be ideal. Just tap the realtime clock circuit for the reference.
Lostgallifreyan wrote in news:Xns99288CAFC46E7zoodlewurdle@140.99.99.130:
Typo, meant 0.3%, even 0.1%, certainly better than 0.58% should be had from a freq/volt converter and multimeter...
Some micros have two oscillators built in, so a reference xtal and a DUT could be connected directly. But I can buy a lot of new guaranteed-accurate crystals for the cost of the engineering to make such an instrument.
Yup. It might make more sense to just use the micro as an oscillator and measure the frequency with a frequency counter! But even that requires some programming (or configuration, at least) on many modern micros.
Best regards, Spehro Pefhany
Typically that's the intended purpose of the alternate oscillator. You have to provide the external crystal and perhaps some additional parts.
You don't necessarily have access to the internal nodes to make it do what you want, it might take some external gates or it might not even be possible to make it work well, depending on the particular processor and peripheral implementation. For example, you might have to live with jitter in the interrupt latency affecting the gate time if the right hardware isn't in place internally.
Best regards, Spehro Pefhany
the yearly (or weekly) average is extremely good, but minute to minute it's not all that hot - a cheap crystal keeps better time.
most people would run it through a step-down transformer first :)
pics are so cheap: why bother?
maybe you could make it so it gets its comparison time from a serial port
send it a pulse on the serial port (0 byte - 9 bit times worth of "+12V" on an rs232) and it responds with the duration of the pulse in crystal cycles.
this way you compare the unknown crystal against the crystal in the serial port.
Bye. Jasen
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