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MSP430 RESET Circuit issue

Pallav Gandhi
Intellectual 900 points
Other Parts Discussed in Thread: MSP430G2755

hii,

We are facing issue with msp430G2755 reset circuit. We have used RC circuit which has R=47K and C=2.2nF. We often find that capacitor C gets shorted in long run on site.

Q1. Is there something we are doing wrong?

Q2. In certain posts in the forum, it is said that the RC circuit is not required? Is this recommended.

It will be great if someone can guide on this issue.

Thanks in advance.

  • 0
    TI__Mastermind 22685 points
    Hi Pallav,

    The RC circuit is required for conditioning the RST line when it is used in SBWTDIO mode (programming or communicating with the device via JTAG/SBW). If you don't need this circuit (program via BSL or similar), then you can remove the capacitor and you are not using this pin as an NMI input or similar. However, I still recommend leaving the external pullup to ensure you don't inadvertently put the device in reset. Some devices have an internal pullup on RST, but we still recommend including the external pullup of 47k.

    The only thing you are doing wrong that i can see at this point is that we recommend C=1.1nF (the spec says 2.2nF is max) to ensure there is no issue with SBW communications. However, if you are not having any problems programming the device, then I don't think this is an issue.

    You say the capacitor will get shorted, can you tell how this happens? Is the capacitor damaged by an ESD event or something?

    Mike
  • 0 in reply to Mike Pridgen
    Guru 46710 points

    Mike Pridgen said:
    The RC circuit is required for conditioning the RST line when it is used in SBWTDIO mode (programming or communicating with the device via JTAG/SBW). If you don't need this circuit (program via BSL or similar), then you can remove the capacitor

    Are you absolutely sure about that?

  • 0 in reply to Ilmars
    Guru 58965 points
    As far as I know, the R is essential as there is no internal pull-up for nRST in F2xx/G2xx.

    The C is for suppressing RF interference to nRST. It is not essential if there is no RF noise; and you may use capacitors bigger then 2.2nF if that pin is not used for SBW.
  • 0 in reply to old_cow_yellow
    Guru 74080 points
    Another side effect of the RC combination is a delayed start of the MSP compared to the rising supply voltage.
  • 0 in reply to Ilmars
    TI__Mastermind 22685 points

    Ilmars said:
    Are you absolutely sure about that?

    Yes, and OCY and Dennis are correct as well.  The biggest restrictions on the RC circuit on RST are for signal conditioning the line when used as SBWTDIO for device programming (since it is a bidirectional data line and you need sharp enough edges to not interfere with communication).  For this, we recommend a 47k pullup up and a 2.2nF capacitor (max) to ground (we recommend 1.1nF for enabling high speed SBW communication).

    If you are not doing SBW communication, then the requirements relax significantly.  The external pullup resistor is still recommended to ensure you don't accidentally trigger a reset or NMI event.  The capacitor is then only required for noise suppression (if needed) or the slight delayed start that Dennis mentions.

    Mike

  • 0
    Intellectual 900 points

    Thank you all, for your responses. We are using the microcontroller in AC scenario, so yes there might be ESD issues. The other thing we have found is that in some cases the external pull up resistor 47k gets open, and so controller never starts. We have no idea why this is happening. 

    Regarding programming the controller, yes we are using SBW programming interface. So can you point out why the 47k pull up gets open up?

    Regards,

    Pallav Gandhi

  • 0 in reply to Pallav Gandhi
    Guru 74080 points

    Pallav Gandhi said:
    So can you point out why the 47k pull up gets open up?

    What do you mean by this?

  • 0 in reply to Mike Pridgen
    Guru 46710 points

    By advising to remove capacitor on RST pin you are effectively suggesting that msp430 can live without reset circuit, without reset. Is it so? Please confirm.

  • 0 in reply to Ilmars
    Intellectual 900 points
    No. Actually when we are using nornal RC reset ckt with R=47k and C=2.2nF in that normal condition after a long run resister gets open i.e. 47k becomes open due to which RST which was pulled high for normal condition becomes open.

    And our controller is in Reset state, so it won't function normally.
  • 0 in reply to Pallav Gandhi
    Guru 74080 points
    Does "open" mean the RST pin is then floating?
  • 0 in reply to Pallav Gandhi
    Guru 74080 points
    A high impedance input should never be left floating.
  • 0 in reply to Pallav Gandhi
    Guru 46710 points

    Pallav Gandhi said:
    after a long run resister gets open i.e. 47k becomes open

    Simple suggestion here would be - don' t use disappearing resistors. That's it.

  • 0 in reply to Ilmars
    Guru 58965 points
    What is a "disappearing resistor"? Is it written in c and declared "volatile"?

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