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MSP430F148: Resonator Capacitance Effects

Marco Rojas
Intellectual 2850 points
Part Number: MSP430F148


Hey team,

One of my customers (Delta-Q) asked me a question regarding certain capacitances of resonators they'd like to use with the MSP430F148. See below

"Due to circumstances, I must change the current external oscillator, Murata resonator (CSTCC8M00G53-R0), connected to the Texas Instruments MCU (MSP430F148IPM). The resonator is directly related to the MCU's XT2OUT (52) and XT2IN (53) pins. The current 8MHz Murata resonator has a built-in capacitance of 15pF. Some alternative resonators I have done validation testing have a different built-in capacitance of 30pF. The testing so far has shown no issues.

My question is, will the change in capacitance have any effects that I am unaware of, or does that solely depend on how the MCU is set up on our end and what is optimal? Can the MSP430F148IPM handle a 30pF resonator? I have already tried looking at the user guides and found nothing."

Thanks in advance!

Marco

  • 0
    TI__Guru 71216 points

    Hi Marco,

    In my experience, no.  With a crystal, it is important to use a combination of external capacitors to satisfy the crystal's load capacitance, but since the resonator already has built-in capacitance, there shouldn't be any additional ones needed.

    Now, a couple of things to consider when switching to another resonator is to make sure to test the new one provides same frequency accuracy and meets the application's start-up time.  Customer might also want to measure the amplitude and compare to existing one.

  • 0 in reply to Dennis Lehman
    Prodigy 21 points

    Hello Dennis,

    I was the person who originally asked the question. I have tested the start-up times, and everything meets the application start-up time relative to the reference device. Everything looks suitable for the start-up time; what worries me is the drop in voltage in XT2IN for the alternative sources. The original Murata resonator yielded a peak-to-peak voltage of 3.2V, while some alternatives had peak-to-peak voltages of 2.8V, 2.6V, and 2.5V. Specification-wise, the resonators are all the same except for the built-in capacitance change from 15pF to 30pF for some. The value for the resonator that measured 2.8V is also internally 15pF. My concern is that with such voltage drops, will the MCU resonator experience any abnormalities and not function? Will the drop affect the software? So far, testing-wise, I haven’t seen anything out of the ordinary. I need some confirmation regarding the change to complete my validation testing.

    Thank you,

    Haseeb Khan

  • 0 in reply to Haseeb Khan
    TI__Guru 71216 points

    Hi Haseeb,

    Sorry for the late response.  The difference in p-p voltages shouldn't impact the performance, but if you can provide me with your original Murata PN and a few of the replacement candidate PNs, I'll run this past our designers.

  • 0 in reply to Dennis Lehman
    Prodigy 21 points

    The original was the Murata resonator CSTCC8M00G53-R0, and some alternatives would be the ECS Inc. ECS-CTP-8.00-30-TR and Abracon AWSCR-8.00CPLA-C15-T4. 

  • 0 in reply to Haseeb Khan
    TI__Guru 71216 points

    Hi Haseeb,

    Sorry for the delayed response.  There is no specification for oscillator amplitude in the datasheet for either a crystal or resonator.  There are specs if driving the XT pins from an external clock source. If you use those specs then if the resonator voltage swing is > 2.64v, you should be ok.  Ultimately, you will want to test several devices across temperature.

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