Is it OK to tie NRST lines together for 2 or moreSTM32F042 processors ?

[Gudgel.boB]

Is it OK to tie NRST lines together for 2 or more processors ?

In this case, they are STM32F042C6T6

I would think so but just thought I'd ask.

boB

[Tesla DeLorean]

I'd generally avoid doing that, the pin's bi-directional and if one triggers the POR/BOR or Watchdogs, it's going to take the other(s) with it.

[Gudgel.boB]

Okeee Dokee then.

Not a problem if it did but....

Thanks

[Tesla DeLorean]

Just one of those potentially unanticipated consequences.

Driving the pin high with a push-pull driver, that can cause real issues.

[Gudgel.boB]

I thought these were open-collector only pull-downs inside ?

Maybe not... Haven't looked lately.

[TDK]

They are open-drain. As long as you're okay with a reset on one resetting all the others, it's fine to tie them together.

Forcing NRST high externally will have bad consequences no matter how many lines are tied together.

[Gudgel.boB]

Yes, I would never drive the RST input HIGH as the open drain/collector could be damaged.

I just looked at the datasheet and did not see an open drain connected to this pin from the WDT or anything else in that particular piece of documentation.

I was getting into a bit of a tight spot on the PCB layout and was looking for another option and one less transistor for the external reset to these processors

[Tesla DeLorean]

>>Yes, I would never drive the RST input HIGH as the open drain/collector could be damaged.

Not saying you would, but I've seen it done a lot of times so it's worth a word of caution, ie GPIO-PP on a host MCU, ARM chip doesn't reset, and continues executing..

The STM32 has an internal pull-up. Seen a lot of boot issues where BOOTx is left floating, as the core is (semi)viable at quite low voltages.

[Gudgel.boB]

I don't think that the person that originally did this board schematic added an external pull-up resistor.

While I am working on this, maybe I should add one if there is room.

BTW, this is supposed to reset the two parts from the orange Pi zero that is connected to it if for some reason the STM32's crash.... Which it looks like it did yesterday.

Looks like I might just be able to reboot the two STM32s by toggling the 3.3V regulator ENable line from the All Winner's I/O on the OPI which would be a much easier solution for me having to add more parts and software that would have to be done on the OPI anyway. Pin PL5/S_JTAG_CK/S_PL_EINT5

Too many options.... So little time 😀