microcontrollers: how to detect absolute clock frequency

Hi,
To do that, you need another frequency reference than your quartz...
Yvan

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Strange concept? can you explain why or what the system is to help give
us a few pointers at possible solutions? generally, without another
timing source the answer is NO, but there may be other timing sources
you could use that may not be obvious.

Al

 

hi,

I need to detect the absolute frequency of the quartz
that feeds my micro. In words, the code must guess the clock
frequency without knowing it.

Is it possible?

thanks
Enrico Migliore

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In a nutshell: no.

The only "measurement" of clock frequency a micro can make without
_another_ clock source to compare against, would be the 1-bit result
of the "go/no-go" test: if code in the micro is currently running, and
has been so for a couple of iterations of a big loop, that means the
clock frequency must be somewhere within the allowed range, or close
to it.

I.e. you can verify that the clock is not zero, and it's not way too
high for the micro. Beyond that, a random number would be as good as
any other as any other result.

 

ti serve per la seriale o cosa??

 

I need to detect the absolute frequency of the quartz

If your part is powered by AC connection, then you can use this as a
reference frequency to phase lock against your quartz. Of course, you'll
have to make some assumptions here -- 50Hz vs 60Hz, and whether the system
might be running off of a UPS etc..

 

Enrico,
you are using the M16C - right ?

Connect the 32Khz XTAL to XCIN and XCOUT (pins 10/11) and you have got a
reference.
Other than with a reference there is no way to determine the clock speed.

If you have a derivative with a ring oscillator you can try some bad tricks
switching clock modes, but then again...

If you only need a ballpark figure you can also measure the current that the
microcontroller (and only the controller) is using and correlate it to the
power consumption table supplied in the M16C datasheet.

Well, I'd vote for the 32KHz XTAL. Simplest way to go.



regards
/jan

 

I need to detect the absolute frequency of the quartz

You could connect a resister and cap to a port pin.

 

Why ?

Answer depends on the precision you need to work to.
ie sense 11MHz or 22MHz is clearly different from
detect of 11.05921MHz vs 11.05920MHz.

-jg

 

Hi,

the (external) reference which is absolutely needed could be a serial
interface that sends characters like 0xAA or 0x55 and you measure
these known lenght charaters with a timer that runs off your
microcontroller clock.
Many 8-bit micros have internal oscillators know with specified
frequency. This is an absolute, thought not highly accurate time
reference. (E.g. microchip PICxx, Philips LPC9xx, Cygnal) just to name
a few.

Cheers, Schwob

 

You clearly need a reference more accurate than the quartz crystal. The
clock off GPS is supposed to be pretty good, as is the time from 60kHz
time signals .. take it over a long period to smooth out decoding
delays. Or a photosensor and leave it outside and measure over a period
of days. Or get the MTBF from the manufacturer, and run it until it
breaks down..

Paul Burke

 

To what precision and how much time do we have to get the result? There
could be a number of ways to do this, but all depend on your precision
requirements and how quickly you need the result. Can you enlighten us more
on your requirements?

-ZO