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MSP430 RST signal needed at power up?

Till Rosenband
Prodigy 220 points
Other Parts Discussed in Thread: MSP-EXP430FR5739

Several boards (e.g. MSP-EXP430FR5739) have an RC pair (47kOhm to Vcc, 2.2 nF to GND) tied to the RST pin.  Presumably this gives a reset rising edge at power up.  But the processor docs also indicate that the processor generates an internal reset at power up.  So is the C part of the RC pair needed?  And won't the capacitor interfere with the Spy-bi-wire signal that uses the same device pin?  It seems wise to keep the R so the pin doesn't float.  Should I think of the RC pair as a debouncer for when power is applied?

  • 0
    Guru 58965 points
    In theory, the C part is not needed. But it might be helpful to reduce the chance of unwanted reset caused by EMI. If SBW is used, you must keep C small.
  • 0
    Guru 12050 points

    It's good to have the cap there as if your power source have a high ESR,
    reset pin may dip a little during some high power surge and you get a Reset.

    I have seen Reset trigger but not the POR brownout reset, so I had to fix it in software until next pcb reversion gets a 2200 pF cap

    If you actual have a reset button, it will help with switch denounce but don't go over 2200pF (2.2nF) for SBW programming.

    Could put a 1K series resistor between R/C and your sbw programming input as to get harder edges.
    If you put a 10K series resistor , the cap could probably be 0.1uF and SBW will still work.

  • 0
    Guru 34483 points

    Till Rosenband said:

    Several boards (e.g. MSP-EXP430FR5739) have an RC pair (47kOhm to Vcc, 2.2 nF to GND) tied to the RST pin.  Presumably this gives a reset rising edge at power up.  But the processor docs also indicate that the processor generates an internal reset at power up.  So is the C part of the RC pair needed?  And won't the capacitor interfere with the Spy-bi-wire signal that uses the same device pin?  It seems wise to keep the R so the pin doesn't float.  Should I think of the RC pair as a debouncer for when power is applied?

    There is internal resistor (20-50k) on RESET pin of FR devices, and external one is not needed (if environment is not noisy and supply is fine). Standard TI external combination (47k + 1nF) will limit SBW max frequency to around 1 MHz (from 10 MHz that is max specified in datasheet).
  • 0
    Prodigy 220 points
    Thanks for all the responses. I think they're all more or less right, but let me summarize that the MSP430FR57xx Family User's Guide describes the SFRRPCR register which by default is set to pull up RST internally. External pull up is not needed then. The document (sections 1.6 and 1.7) suggests including a capacitor to ground which should not exceed 2.2 nF if SBW is used. Other MSP430 devices probably include their own specs in the processor UG or family UG.

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