Low Power Reset Circuit

NASI · ‎2022-01-14

Hello,

Could anybody please look at the circuit, and if it is correct.

I am not sure if C5 is necessary to be there.

Thank you.

Peter BENSCH · ‎2022-01-14

The reset devices were not developed as an addition to the STM32, but for other devices that still need an external reset and that too with the old method of a pull-up, as was often used in the last century.

In this respect, the internal pull-up is nice, but redundant in the context of the STM32 - as, by the way, is the external reset device. All STM32s have a comparable reset circuit and external pull-ups are redundant on the NRST.

At NRST you only need the 100nF capacitor, if you also want to trigger a manual reset, you can also connect a button - preferably with a 100 ohms series resistor to protect the button contact.

Regards

/Peter

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LCE · ‎2022-01-14

STM181x website:

Open-drain, active-low (STM1811,1816)

I don't know about the STM32's internal pull-up resistor on NRST, but I would definitely place one, something like 10k between NRST and +3V3.

And then 100nF would be really over the top, I usually place some 100pF just for deglitching and for... it's always good to have the option to place a cap somewhere. ;)

Peter BENSCH · ‎2022-01-14

If you really want to use an external reset device in addition to the internal reset circuit, you don't need C5 (if the line to NRST is not too long).

Is there a reason you need to use an external reset?

And no, contrary to what @Community member says, you definitely don't need a pull-up at NRST.

Regards

/Peter

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LCE · ‎2022-01-14

That's why I wrote "I don't know about the STM32's internal pull-up resistor on NRST"...

Have all STM32 types the (almost) same NRST input circuit with a weak pull-up?

The OP didn't give the exact STM32 type, and I wouldn't trust something I haven't checked.

NASI · ‎2022-01-14

Thank you for your respond @Community member and @Peter BENSCH

I don't need a pull-up at NRST, I am agree. As STM1816 has a pull-up inside.

I don't have any reason to use an external reset. The NRST is connected to be used in debug reset, and therefore I thought maybe it is good to have an extern reset.

Thank you.

LCE · ‎2022-01-14

Okay, interesting that also the STM1816 has an internal pull-up.

I once did the mistake and thought that an "open-drain" labelled device would come with an internal pull-up, and it did not.

Peter BENSCH · ‎2022-01-14

The reset devices were not developed as an addition to the STM32, but for other devices that still need an external reset and that too with the old method of a pull-up, as was often used in the last century.

In this respect, the internal pull-up is nice, but redundant in the context of the STM32 - as, by the way, is the external reset device. All STM32s have a comparable reset circuit and external pull-ups are redundant on the NRST.

At NRST you only need the 100nF capacitor, if you also want to trigger a manual reset, you can also connect a button - preferably with a 100 ohms series resistor to protect the button contact.

Regards

/Peter

In order to give better visibility on the answered topics, please click on Accept as Solution on the reply which solved your issue or answered your question.

NASI · ‎2022-01-14

Thank you @Peter BENSCH