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External Oscillator on MSP430F123

Intellectual 470 points

Replies: 5

Views: 301

Hi,

Been using the MSP430F123IPW in a product for a number of years. We've been using it with a 7.2 MHz crystal.

I want to run it with an oscillator.

I believe that is impossible because this device does not have a XT2CLK input. The LFXT1CLK is designed for crystals or resonators, not oscillators.

Is this true?

Thanks.

Mike

  • I think you can use XIN for to input an external clock. Both the data-sheet and the user's guide said that.
  • In reply to old_cow_yellow:

    I've done some more research in the last 10 minutes ... and I believe you are right. The user's guide is somewhat contradictory. Section 4.1 seems to indicate you can not do that but 4.2 says you can.

    Thanks.
  • In reply to Mike Unger:

    You're right, the 4.1 overview seems to indicate that only XT2 is for external clock signals. However, even XT2 has no configuration register to switch between crystal or external oscillator. Except for power consumption, this isn't necessary. XTIN is exactly that: an input. It has a low trigger amplitude (so yuor oscillator signal shouldn't have a low-level output more than ~200mV), but other than that, it is unimportant whether the signal comes from a crystal 'output' or an oscillator output. It is just that on MSPs with selectable configuration, the inverter that drives the other side of the crystal (its 'input', even thou the crystal itself is symmetrical) is deactivated and reduces power consumption. On the F123, it will stay active and 'drive' the pin capacitance, producing some EMI and consuming some energy.

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  • In reply to Jens-Michael Gross:

    According to the data-sheet (slas312c.pdf) page 22, when using an external logic-level clock source at XIN, the required voltage level is: logic low should be between 0% and 20% of Vcc, logic high should be between 80% and 100% of Vcc.

  • In reply to old_cow_yellow:

    You're right, I never checked this. And I'm a bit surprised, as the crystal oscillation on XTIN never reached 0.8*Vcc on the MSPs I had under test. Since there is no switch to bypass mode (as in later families) that could switch the input buffer behavior, I wonder how this fits.

    _____________________________________

    Time to say goodbye - I don't have the time anymore to read and answer forum posts. See my bio for details.

    Before posting bug reports or ask for help, do at least quick scan over this article. It applies to any kind of problem reporting. On any forum. And/or look here.
    I'm sorry that  I can no longer provide help  in the forum or by private conversation.

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