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MSP430FR5994: Very fast spikes on Vcc causes UART communication interruption on the MSP430FR5994

user1082833
Intellectual 330 points
Part Number: MSP430FR5994

Hello,

we use the MSP430FR5994 with an LTE-Modem on the same power supply. The whole system works with 3,4V from a stable power supply. Between the modem and the MSP430FR5994 we only use the UART for the communication.

Near to the power pin of the modem we installed, as mentioned in the datasheet, a combination of 3300uF, 100nF, 10nF, 68pF, and 15pF capacitors (between Vcc and GND).

Near to the Vcc pins of the MSP430FR5994 we installed a 4,7uF and a 100nF capacitor.

Most of the time, the system works fine. But sometimes the MSP430FR5994 goes in a non reachable state (by the UART), from witch it could only be released by a hardware reset. This occurs sometimes, but not always, during the modem connection to the internet (short power peaks up to 850mA). On the oscilloscope normally there is nothing special, that could be seen on the Vcc line. But each time when the MSP430FR5994 goes in this unreachable state (maybe a BOR state (?)), there is a very short spike down to 2,8V and then up to 3,6V. After that first spike we could also see 3 further  decreasing after waves spikes, before the Vcc is restabilised to the normal power level of 3,4V. The whole event, including the after waves, has a very short duration of approximatively 20nS.

First question: According to you, could this short spike be the origin of the unreachable state of the MSP430FR5994? Is it a BOR reset? Or maybe something else?

Second Question: What could you advise us to change in the layout to remove the influence of this short spike?

Thank you very much for your help.

  • 0
    TI__Mastermind 49692 points

    Hello,

    Thank you for your detailed post.

    user1082833 said:
    First question: According to you, could this short spike be the origin of the unreachable state of the MSP430FR5994? Is it a BOR reset? Or maybe something else?

    Yes, this is possible. Under the table in Section 8.3 in the datasheet, Footnote 2 says the following:

    Fast supply voltage changes can trigger a BOR reset even within the recommended supply voltage range. To avoid unwanted BOR resets, the supply voltage must change by less than 0.05 V per microsecond (±0.05 V/µs). Following the data sheet recommendation for capacitor CDVCC should limit the slopes accordingly.

    Footnote 4 gives this guidance:

    For each supply pin pair (DVCC and DVSS, AVCC and AVSS), place a low-ESR ceramic capacitor of 100 nF (minimum) as close as possible (within a few millimeters) to the respective pin pairs.

    You can confirm if this scenario is getting caused by a BOR by checking the SYSRSTIV register after start-up.

    user1082833 said:
    Second Question: What could you advise us to change in the layout to remove the influence of this short spike?

    You could try adding a current-limiting resistor or inductor between the power supply and the MSP430. Also, increasing the capacitance on the supply pins mentioned above should help too.

    Regards,

    James

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