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Measuring approx. 10 MHz frequency

Teuvo Telaranta
Prodigy 20 points

I try to figure out how to measure a frequency approximately 10 MHz using MSP430 -series micro-controller. Of curse, external clock input maximum frequency may be only 8-10 MHz. An external divide-by-two FF could be used to scale down the frequency to approx. 5 MHz, so that's only a minor problem.

The challenge is to configure the system to count number of cycles during a given time period, so that the frequency would be calculated using the processor's oscillator as a reference. Currently I see no such possibility. However, maybe I just can't find it.

Any ideas and/or proposals?

  • 0
    Guru 58965 points

    If you look at the Timer Section of the data-sheet of the chip you are using, you can find that each Timer can use one of the port pins as its clock input pin (called TACLK or TBCLK). And the Port Schematics Section tells you how to set up that pin as Timer clock input pin.

    In most MSP430 chips, you can connect your ~10MHz clock directly to that pin. Use that Timer to count that input for a known amount of time.

  • 0
    Guru 34973 points

    Teuvo Telaranta said:

    I try to figure out how to measure a frequency approximately 10 MHz using MSP430 -series micro-controller. Of curse, external clock input maximum frequency may be only 8-10 MHz. An external divide-by-two FF could be used to scale down the frequency to approx. 5 MHz, so that's only a minor problem.

    There is no need for any external components. Here is my MSP430F550x based zero-parts USB frequency meter that is able to measure 0 Hz - 50 MHz with 0.5 Hz precizion...

    http://forum.43oh.com/topic/3317-msp430f550x-based-frequency-meter

  • 0 in reply to zrno soli
    Guru 46710 points

    zrno soli said:
    There is no need for any external components. Here is my MSP430F550x based zero-parts USB frequency meter that is able to measure 0 Hz - 50 MHz with 0.5 Hz precizion...

    You mean resolution, right? - Because precision depends on reference timebase. For example using 32kHz fork oscillator with typical +/- 20ppm error, absolute measurement precision will be +/- 20Hz when measuring 1MHz. That's why quality frequency counters have TCXO or OCXO and optionally external 10MHz reference input.

  • 0 in reply to Ilmars
    Guru 34973 points

    Ilmars said:

    You mean resolution, right? - Because precision depends on reference timebase. For example using 32kHz fork oscillator with typical +/- 20ppm error, absolute measurement precision will be +/- 20Hz when measuring 1MHz. That's why quality frequency counters have TCXO or OCXO and optionally external 10MHz reference input.

    Yes, resolution is 0.5 Hz, not precision. With 24 MHz MCLK (XT2) there was no problem with measuring 48 MHz (timer input counter is not limited with MCLK, even is noted in datasheet that is), but didn't test higher frequencies so don't know what is max possible input value.

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