• State Resolved
  • Locked Locked
  • Replies 8 replies
  • Subscribers 80 subscribers
  • Views 7329 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Pullup resistor on ~RESET pin

Bob Morris
Intellectual 355 points

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.

  • 0
    Guru 12956 points

    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.

  • 0
    Guru 227245 points

    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.

  • 0 in reply to Jens-Michael Gross
    Intellectual 355 points

    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)?

  • 0 in reply to Bob Morris
    Guru 25210 points

    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.

  • 0 in reply to Brian Boorman
    Guru 12050 points

    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.

  • 0 in reply to Bob Morris
    Guru 227245 points

    Bazerko Bob said:
    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.

  • 0 in reply to Jens-Michael Gross
    Guru 12050 points

    >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.

  • 0
    Intellectual 355 points

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

**Attention** This is a public forum