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arun23
arun23
Associate
January 4, 2016
Question

Usage of MultiLayer Varistor for ESD protection on the NRST signal of STM32 M0 microcontroller

  • January 4, 2016
  • 4 replies
  • 951 views
Posted on January 04, 2016 at 16:11

Hi,

We are using STM32L051C6 microcontroller in our design. We also use the STWD100 Watchdog chip too provide a reset when the code hangs up. This is added as a redundance to the internal watchdog chip of the microcontroller. We also have put a multilayer varistor (DC voltage of 5.6V) on the NRST signal for ESD protection purpose. Additional RC delay network is added prior to the Varistor on the NRST signal.

Is this a recommended practice to use a varistor on the NRST signal of the microcontroller for ESD protection purpose. The circuit is powered up using a 3V battery.

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    4 replies

    Tesla DeLorean
    Tesla DeLorean
    Guru
    January 4, 2016
    Posted on January 04, 2016 at 19:14

    Can people touch it?

    Using R-C networks on NRST pins is notorious for creating all types of issues. If you need the signal to be low for a prescribed amount of time, use a POR chip/circuit.

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    arun23
    arun23Author
    Associate
    January 5, 2016
    Posted on January 05, 2016 at 11:16

    Hi

    RC network was a recommendation from STWD100 chip. The query is whether it is advisable to use a Varistor across the NRST and GND for ESD protection purposes.

    Thanks, 

    Tesla DeLorean
    Tesla DeLorean
    Guru
    January 5, 2016
    Posted on January 05, 2016 at 14:27

    To which I asked ''Can people touch it''

    Trying to glean there whether you have some button people are going to be pressing, or if they are hot plugging it into something. Where is the ESD coming from?

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    gregstm
    gregstm
    Senior II
    January 6, 2016
    Posted on January 06, 2016 at 01:47

    Very hard to decide on your question with the little information given, so I will talk in general.

    I think keeping it simple is the best policy - just the simple capacitor recommended by the data sheet is all that is required. All the extra stuff will cause more trouble than it is worth.

    If it is for a product, I would keep the reset well clear of people's fingers - the user shouldn't need to touch it. If they need to reset the device tell them to remove the battery for a while. Keep the SURFACE MOUNT capacitor very close to the micro with lots of ground infill.

    The only time I would use a varistor or similar is for a reset button on a distant panel, and you were trying to stop the ESD entering the board, at the edge of the board.

    If you intend having the varistor near the micro then I doubt that would be very effective. An ESD near the micro will induce unwanted voltages everywhere and bypass the varistor. ESD behaves in mysterious ways.

    Rather than go to huge efforts to stop a random reset - assume you will get them and make sure the software handles them gracefully.

    I used to test my hardware with a piezo sparker from a dismantled gas lighter - crude and destructive but it helps you appreciate how ESD behaves.

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