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Pullup resistor on ~RESET pin

Intellectual 345 points

Replies: 8

Views: 5424

All of the documentation I've seen says to put a 47K ohm pullup resistor on the ~RESET pin. If I don't wan to use the ~RESET feature (i.e. never pull it down) can I just connect it directly to Vcc? I realize this won't work with the debugger because it does use this feature, but how about when the product is finished?

I'm using MSP430F and MSP43G micros,  but others may be interested in an answer beyond them.

  • Should be fine.  On MSP430F5xxx series chips, I don't think it's necessary (built-in pullup resistor on those models).  Not sure if that applies to every single one of the F5xxx chips so check the datasheet to be sure.

  • At power-on, RST is low, as VCC equals GND when power is off. And RST in a high-impedance input. So it needs to be pulled high, to overcome line capacitance etc.

    Eric is right, on most 5x/6x family devices, RST has an internal programmable pull-up resistor. On the MSP54xx (non-A) devices, it is off by default (and therefore makes not much sense), but AFAIK all other 5x/6x family devices have it on by default and an external one isn’t required. Unless you have problems with noise or crosstalk and the internal one is too weak.

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  • In reply to Jens-Michael Gross:

    The question wasn't whether RST needs to be pulled up, but how. The documentation says 47K resistor, but maybe that's only so it can be pulled down. If I don't want to ever pull it down, do I need a 47K resistor or can I do it with a 0 ohm resistor? Eric says 0 is enough. Do you want to make it unanimous (i.e. both of you)?

  • In reply to Bazerko Bob:

    You can pull it up directly to Vcc, but my recommendation is to use a 0-Ohm jumper resistor and not a direct Vcc tie. This gives you an option to change value down the road for unforeseen circumstances.

    ~Brian


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  • In reply to Brian Boorman:

    Using a 47K Resistor slows down the state of line with some nanoseconds.
    This can be good as to filter out nanosecond short brownouts on the Vcc line for unnecessary resets.

    And as the programming is done open-drain style, 0 Ohm or anything under 500 Ohm will be harmful.

  • In reply to Bazerko Bob:

    Bazerko Bob
    The question wasn't whether RST needs to be pulled up, but how.

    Once know why, you should be able to figure out how.
    I can give you a ‘formula’ or a 10-page table of what to do when. Well, I won’t write the table. And your question was rather generic, covering a wide range of different MSPs, with different answers.
    Brian’s suggestion to use a 0-Ohm resistor in any case is a good one: It is no fun trying to patch a resistor into a thin trace.

    _____________________________________

    Time to say goodbye - I don't have the time anymore to read and answer forum posts. See my bio for details.

    Before posting bug reports or ask for help, do at least quick scan over this article. It applies to any kind of problem reporting. On any forum. And/or look here.
    I'm sorry that  I can no longer provide help  in the forum or by private conversation.

  • In reply to Jens-Michael Gross:

    >The question wasn't whether RST needs to be pulled up, but how.

    The RST pin is falling edge detection, it needs to be connected to a stable voltage in the range of 70-100% of Vcc.
    If you call it pull-up or connected to Vcc, it just the same result: it is feed the voltage level of Vcc

    If you are gone put a 0 ohm, you might as well put 47k (or 10K or 22k)
    A 47k resistor does some filtering and protects the pin from overvoltage spikes.

    If you later put a rest button (or jumper) or programming port then it's definitely needed.

  • OK, you convinced me. I'll put the resistor in. Thanks to all who responded.

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