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MSP430FR2355: VLO clock frequency tolerance

Mike_H
Intellectual 305 points

Part Number: MSP430FR2355

I am developing an application that will make heavy use of the VLO clock. Timing is not critical, but I couldn't find any tolerance on the specified 10 kHz frequency. I measured my Launchpad on a calibrated scope and got 9.52 kHz, ie. about a 5% error. I used very simple code that generates a signal by toggling an output pin. The VLO sources Timer B that runs in the Up mode with period set by TB0CCR0. Also: The spec sheet notes that the clock will run even slower when switching from active mode to LPM3 or LPM4 and I confirmed this. What kind of device-to-device variation can I expect for the VLO clock frequency when the CPU is active?

  • 0
    TI__Guru 71216 points

    Hi Mike,

    For reference, here is the spec from the datasheet (in case others are following this thread).

    Yes, this is a good question, and you are correct.  There isn't a spec for the initial "typical" tolerance and there is no way to tell what the variation will be from device to device.  Let me ask our product test team to see if they can provide some information.

  • 0 in reply to Dennis Lehman
    TI__Guru 71216 points

    Hi Mike,

    Apologize for the long delay, but it can take a week or so to get data back from our production site in Shanghai.

    Basically I'm being told there is a test for the VLO in production with 4kHz lower and 20kHz upper limits.

    From the initial characterization data that was used to derive these limits, there is a bell curve with a peak (majority of the samples) around 11.5kHz, and left tail down to 7.5kHz and right tail up to 17kHz.  This data was collected at 30C and 3.0v.

    This doesn't tell you the effects of temperature though, so I'm sure if any of this actually helps you, other than you know you will never see a VLO < 4kHz or > 20kHz at 30C and 3.0v.

  • 0 in reply to Dennis Lehman
    Intellectual 305 points

    Turns out the TI documentation does give a tolerance: On page 68 of the MSP430FR235x datasheet I see the VLO frequency listed at 10 kHz +/- 50%, consistent with the numbers you provided and the VLO frequency tolerance shown in other MSP430 datasheets. Way too loose for my application, so I'm using REFO instead. Thanks for the assistance!

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