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Reset pin

Alan Rouse
Prodigy 60 points


I am working on a product that incorporates an intelligent peripheral (a radio device) containing an MSP430 processor.  After a reset the host exercises I/O pins that are connected to the MSP430 in the peripheral.  It is therefore important that the MSP430 remains in its reset state for at least 20ms after power-up.  The simplest way of achieving this is to fit a 1uF capacitor between the MSP430's reset and 0V.  This will obviously cause a slow risetime on the reset pin.

I cannot find any documentation that relates to this.  The MSP430 datasheet indicates that the reset input does not have a Schmitt trigger action, so I guess there is a possibility of uncertainty as the input passes through the switching threshold.  However, I have seen memtion of using 2.2nF on the pin which will cause the same potential problem.

Does anyone know if there are likely to be any problems using a larger capacitor?

 

 

  • 0
    Guru 227245 points

    You're right, a slowly rising reset signal will introduce problems. It may be that the MSP starts, then resets once more, then starts again etc. It usually happens if you have a switching power supply, so teh voltage raises, then falls a bit, then raises more and falsl a little bit again etc. Without a schmitt trigger, this can toggle the reset pin several times. A continuously rising voltage should be no problem. Only if it has 'ripples' while rising.

    Even if, this is not critical as long as teh MSP has not started executing user code (there are some actions which are performed first before jumping to the reset vector) and is also not critical, if the user code is a standard C init code (whcih spends quite some time initializing the global variables). But it might be a problem if there is low-level assembly code that does something important right at the beginning.
    The recommended combination of 2.2nF and a pullup rises fast enough to be no problem. In fact, the 2.2nF is just to not add too much load to the reset output of the programming tool (FET), so it can keep up with some timing requirements in JTAG/SBW communication.

    The bes tway would be to add a retriggerable monoflop to the reset pin. It will pull the RST pin low once its inputs are low and release it a given time after its input has been raised. The 74HC4538 is a good candidate.

    However, if you only wnat to keep the MSPs I/O pins inactive, you can do so in software. The MSP won't activate any I/O pins unless your program orders it to do. After a reset and until you say something different, all I/Os are high.impedance inputs.

    So you can simply set up a software delay before initializing the MSP ports.

  • 0 in reply to Jens-Michael Gross
    Prodigy 60 points

    Thanks for your prompt reply.

    Unfortunately I don't have any control over the firmware in the MSP430.  It is built into a radio that we buy in and fit onto an interface board.  I can put an external timer on the reset pin, but I was hoping to solve the problem on existing boards without modifying the PCB.

    I have tried fitting a 1uF capacitor on one board and it works fine, but I need to be confident that it would work on all existing boards.  Long term we will modify the artwork.

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