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MSP430F249: What happens when the High signal is input to the RESET pin after the MCU is powered on?

Expert 2125 points
Part Number: MSP430F249

Hi, 

1) If a High signal is input to the RESET terminal when the power of the MCU is turned on, the microcomputer may not start even if a Low signal is input to the RESET terminal after that.
Is this because the reset signal is a High signal at startup?

2) Please let me know if there is a relevant document part regarding 1).

3) What kind of event can occur if the High signal is input to the RESET pin instead of the Low signal when the MCU is powered on?

Thanks,

Koki

  • Hello Koki,

    What do you mean by #1?  What do you mean by high signal input?  

    /RST should be pulled high with an external resistor, and can be driven low by and external source to trigger a reset.  Everything having to do with the actual power-on of the MCU, should be handled by the Brownout Reset (BOR) circuitry of the MSP430.  This is explained in section 2.1.1 of the Family User's Guide

    At power-up, the BOR will hold the MCU in reset as the VCC rises until it hits a a threshold and then after a short period of time the internal reset is release and the MCU will start.  the /RST pin will default to /Reset mode and if there is a high signal, the MCU should start running/executing. 

    If the /RST is pulled low, it will trigger a Power-on Reset (POR) which is effectively the same reset as a power cycle. 

    Are you having a specific issue with the /RST pin?  

    Thanks,

    JD

  • Hi, JD

    What do you mean by #1?  What do you mean by high signal input?  

    Is it a problem to apply High to the reset pin before the power supply voltage exceeds VCC (start)?

    Are you having a specific issue with the /RST pin?  

    RESET pin = High (when VCC crosses the VCC (start) level), then Low is input to the RESET pin. However, the MCU may not start.
    Therefore, I suspect that it is a problem to apply the HIgh signal to the reset pin before the VCC crosses the VCC (start) level.

    Thanks,

  • Hello Koki,

    No, applying a high signal to reset via a pull-up resistor tied to VCC at power-up should be completely fine.  /RST should not need to be kept low through power-up or cycled after power-up.  As long as /RST is kept high when power is applied, the MCU should start operating right away.  

    After power-up, pulling /RST low and releasing it back to high will cause a POR reset in the MCU.

    Thanks,

    JD