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MSP430F5510: Requirement for external oscillator when using USB

Oba
Genius 17335 points
Part Number: MSP430F5510

Hello, 

I would like to confirm some requirements for external oscillator specification when using USB.
Now my customer is going to use 4MHz external oscillator.
Then could you tell me the below items? 

  1. Acceptable duty range
  2. Acceptable rising time and falling time
  3. Acceptable frequency tolerance

Regards,
Oba

  • 0
    TI__Genius 17960 points

    Hello,

    about the clocking of the MSP430F5510 in case of a USB implementation I can give you following information.

    1. The internal PLL of the USB module runs with 48MHz. The USB module can use various frequencies from 1.5MHz to 32MHz. In the MSP430F5xx User's Guide  in Table 42.2 you can find the usable frequencies and the respective control bit settings for proper operation.

    1. In terms of the duty cycle the recommendation is to stay within the specification of the XT1/XT2 oscillators in high frequency mode, means 40%-60%.

    2. Not sure, what you means by rising and falling time. A crystal oscillator does not have such parameters, as the analog signal is a sign wave. In terms of the safety factor and stability of operation, the recommendation would be to apply tests as described in the application report . Though the report is addressing 32kHz oscillators, the described tests and design recommendations are very well applicable also for high frequency crystals.

    3. The USB specification, specifies a clock accuracy of +/-2500ppm. Any crystal with a frequency of the mentioned table 42-2 will meet these requirements.

    In addition in terms of clocking the following information might be interesting. For sufficient processing bandwidth a minimum CPU clock is 1.5MHz. Though of course dependent on other activities higher processing power might be better.

    In terms of the possible crystal frequencies an interesting aspect is the automatic detection capability of the USB module of the 4MHz clock source. This is especially interesting in case of firmware updates via USB in the field, or production programming, using the built-in USB BSL. In this case the 4MHz value is the preferred clock frequency.

    Best regards

    Peter

  • 0 in reply to Peter Spevak
    TI__Genius 17335 points

    Hello Peter,

    Thanks for your reply and sorry I'm wrongly push "This resolved my issue" button.

    >2. Not sure, what you means by rising and falling time. A crystal oscillator does not have such parameters, as the analog signal is a sign wave. In terms of the safety factor and stability of operation, the recommendation would be to apply tests as described in the application report

    > Though the report is addressing 32kHz oscillators, the described tests and design recommendations are very well applicable also for high frequency crystals.

    What I want to confirm is not internal oscillator with crystal, but external oscillator, meaning square-wave clock. I would like to know the rise and fall time requirement of it.

    Regards,
    Oba

  • 0 in reply to Oba
    TI__Genius 17960 points

    Hello Oba,

    no problem. We do not have rise and fall time specifications on the external clocking. The customer needs to ensure the duty cycle and accuracy requirements. The rise time is rather related to signal integrity. Usually faster rise times tend to create ringing and reflection effects on the square wave signals. This should of course be avoided, to prevent glitches on the resulting clock.

    The signal should not be extremely slowly rising, as this would potentially create issues with jitter.

    That's unfortunately all we can state on this.

    A comment to be considered too, when considering an external digital clock. With 4MHz you can use the automatic BSL invoke of the USB BSL for initial programming or FW updates. Unfortunately, this works of course only in case of use of the 4MHz crystal, as this is the default setting after POR. For an external digital clock source the clock module settings would need to be reconfigured, which is not included in the USB BSL.

    Best regards

    Peter

  • 0 in reply to Peter Spevak
    TI__Genius 17960 points

    Hello,

    under the assumption your questions have been answered sufficiently, I am closing this thread.

    Best regards

    Peter

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