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Is there a way to output the 32Khz crystal signal to view with an oscilloscope?

Nate Gold1
Prodigy 20 points
Other Parts Discussed in Thread: MSP430G2553

Is there a way to output the 32Khz crystal signal to view with an oscilloscope?

I can put out frequencies using the ACLK slower than 32Khz (up to 8.19 Khz) but is there any way to get the 32Khz output to a pin and keep the accuracy of the crystal that came with the MSP430 starter kit? I've searched the internet and the forums and am not coming up with a solution that works with the test kit I bought from TI. Would I nee to substitute a higher frequency crystal to get a 32Khz frequency?

  • 0
    Guru 58965 points

    Check the data-sheet of the chip you are using. Look at the "Terminal Functions" table to find a pin that has "ACLK" as part of its NAME or has "ACLK signal out" as part of its DESCRIPTION. That is the pin you can use.

    Next, look at the Port Schematics section and find out how to set up that pin as ACLK signal out. It will show the ACLK signal whether it is 32kHz or not.

  • 0 in reply to old_cow_yellow
    Prodigy 20 points

    Thanks for your help. http://www.ti.com/lit/ds/symlink/msp430g2553.pdf

    I set     P1SEL |= BIT0;
        P1DIR |= BIT0;
        P2SEL |= BIT2;
        P2DIR |= BIT2;

    And it put it out to 1.0 just like you said. Thanks for your help.

  • 0
    Guru 227245 points

    ACLK has a fallback mechanism that switches it to e.g. the VLO if it should run on a crystal but the crystal fails. Your 8kHz are probably the VLO. Or you have ACLK with a /4 divider (8.19*4 = 32.76). However, teh crystal frequency is only routed to ACLK (and therefore the the ACLK output) if the crystal is up and running proberly. So if you have problems to start the crystal, this is of not much  help.
    If the MSPs clock system allows, you can switch SMCLK to the crystal. SMCLK has no fallback and yu should see any oscillation of the crystal 1:1 on the SMCLK output - as long as the oscillation amplitude is above the threshold for the input gate (in our case, it wasn't in some cases, so no signal was generated even though the crystal was oscillating)

    For our crystal problems, we had to buy a specific active probe with extremely low input capacitance and high input impedance (0-8pF/1mOhm, ~$800), so we could look at the crystal pins directly.

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