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Internal Clock versus External Clock

Saran Kiyabu
Prodigy 230 points


Dear Community,

I'm using a CC430 on an EMCC430F6137-900 board and I'm quite new to the TI environment. I was wondering what were the differences between the different clock sources. I know that internal clocks are normally more inaccurate than external crystals, but I was wondering if there was a way to quantify this inaccuracy. Is it on the datasheet? If so, where, and how can I rationalize the number? For example, if I wanted to flash an LED every 1 minute versus 1 millisecond, would the choice of clock matter? Also, is there a way to quantify how much power one source requires over another?

Thank you!

  • 0
    Guru 227245 points

    Saran Kiyabu said:
    I was wondering if there was a way to quantify this inaccuracy. Is it on the datasheet?

    Yes. The CC430 datasheet contains full specification for the REFO/VLO oscillator (tolerance over temperatur and voltage operating range).
    For the DCO, since it is programmable, it is more difficult to get a 'number' for the accuracy. This is because the DCO has only 256 frequency settings in a range from 70kHz to 135MHz. So you usually modulate it (in hardware) between two frequencies with a 32 step pattern, giving a total of 8192 different average frequencies (with the emphasis on 'average').
    Which frequencies these are has a wide tolerance range.

    Compared to this, an external crystal has one specific and quite stable and jitter-free frequency with a tolerance in the ppm range - at the cost of three external components (crystal and two capacitors). It's your choice what you want/need.

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