jfedor
Posts: 11
Joined: Tue Apr 17, 2012 5:30 pm
Contact: Website

Do I need a pull-up on the RUN pin?

Wed Sep 15, 2021 2:37 pm

Hi,

I designed an RP2040 board that is mostly a copy of the minimal design example from the "Hardware design with RP2040" document. One thing I added was a "reset" button connecting RUN to GND so that I don't have to re-plug the USB cable every time I want to flash new software.

Then I started looking at other boards (Adafruit boards and I think the Pico itself) and I noticed that they all seem to have a pull-up resistor between RUN and 3.3V.

Is that something that's required? On one hand my board seems to be working without it and on the other everyone seems to think it should be there.

trejan
Posts: 3717
Joined: Tue Jul 02, 2019 2:28 pm

Re: Do I need a pull-up on the RUN pin?

Wed Sep 15, 2021 3:55 pm

jfedor wrote:
Wed Sep 15, 2021 2:37 pm
Is that something that's required?
Yes.
https://datasheets.raspberrypi.org/rp2040/rp2040-datasheet.pdf wrote: 2.12.5. External Reset

The chip can also be reset by taking its RUN pin low. Taking RUN low will hold the chip in reset irrespective of the state of the core power supply (DVDD) and the power-on reset / brown-out detection blocks. The chip will come out of reset as soon as RUN is taken high, if all other reset sources have been released. RUN can be used to extend the initial power-on reset, or can be driven from an external source to start and stop the chip as required. If RUN is not used, it should be tied high.

jfedor
Posts: 11
Joined: Tue Apr 17, 2012 5:30 pm
Contact: Website

Re: Do I need a pull-up on the RUN pin?

Wed Sep 15, 2021 4:21 pm

So how come it's not there in the minimal design example?

trejan
Posts: 3717
Joined: Tue Jul 02, 2019 2:28 pm

Re: Do I need a pull-up on the RUN pin?

Wed Sep 15, 2021 4:33 pm

jfedor wrote:
Wed Sep 15, 2021 4:21 pm
So how come it's not there in the minimal design example?
You're still meant to refer to the datasheet. The minimal design example breaks out all the other pins including the RUN pin for you to connect to. Load the PCB in KiCAD.

jfedor
Posts: 11
Joined: Tue Apr 17, 2012 5:30 pm
Contact: Website

Re: Do I need a pull-up on the RUN pin?

Thu Sep 16, 2021 11:06 am

I checked again, there's no external pull-up on the Pico either. Seems to work fine without it.

dbrion06
Posts: 681
Joined: Tue May 28, 2019 11:57 am

Re: Do I need a pull-up on the RUN pin?

Thu Sep 16, 2021 11:48 am

I works fine and will work fine for you -I hope-; now, suppose you sell your card to people living with high EMI, transients....
Would they be happy with a non noise protected card?
Commercial sellers prefer pullupping with a 0.01 anyCountry$ (or less) than having unhappy buyers.

trejan
Posts: 3717
Joined: Tue Jul 02, 2019 2:28 pm

Re: Do I need a pull-up on the RUN pin?

Thu Sep 16, 2021 12:10 pm

jfedor wrote:
Thu Sep 16, 2021 11:06 am
I checked again, there's no external pull-up on the Pico either. Seems to work fine without it.
The Pico datasheet says "RUN is the RP2040 enable pin, and has an internal (on-chip) pull-up resistor to 3.3V of about ~50K Ohms." so presumably it is okay. The RP2040 datasheet still says it should be tied high though.

Any RPT engineer want to give an official statement on this?

hippy
Posts: 10745
Joined: Fri Sep 09, 2011 10:34 pm
Location: UK

Re: Do I need a pull-up on the RUN pin?

Fri Sep 17, 2021 1:54 pm

trejan wrote:
Wed Sep 15, 2021 3:55 pm
jfedor wrote:
Wed Sep 15, 2021 2:37 pm
Is that something that's required?
Yes.
https://datasheets.raspberrypi.org/rp2040/rp2040-datasheet.pdf wrote: 2.12.5. External Reset

The chip can also be reset by taking its RUN pin low. Taking RUN low will hold the chip in reset irrespective of the state of the core power supply (DVDD) and the power-on reset / brown-out detection blocks. The chip will come out of reset as soon as RUN is taken high, if all other reset sources have been released. RUN can be used to extend the initial power-on reset, or can be driven from an external source to start and stop the chip as required. If RUN is not used, it should be tied high.
I believe I have noted previously that "If RUN is not used, it should be tied high" is extremely ambiguous because what is "not used" ?

How is having nothing connected to the RUN pin ("not used") any different to connecting a push button ("used") which isn't being pushed ?
trejan wrote:
Thu Sep 16, 2021 12:10 pm
Any RPT engineer want to give an official statement on this?
There are three possible answers beyond "no comment" -
  • The Pico doesn't have a design flaw, an external pull-up is not a necessity
  • The Pico has a design flaw, an external pull-up is a necessity
  • We will need to investigate
I definitely wouldn't bet money on the second, and unlikely to bet on the third.

I recall the only case where it was suggested an external pull-up could be required would be to overcome any weakness of the internal pull-up in the face of external EMI.

That would fit with what I think the datasheet intended to say; "When there will be no connection to the RUN pin then tie it high to avoid any possibility of interference resetting the chip". Note it says "tied" rather than "pulled".

I would personally consider fitting an external pull-up prudent even if not a necessity. One could provide unpopulated pads or holes to allow an end-user to fit it or not.

trejan
Posts: 3717
Joined: Tue Jul 02, 2019 2:28 pm

Re: Do I need a pull-up on the RUN pin?

Fri Sep 17, 2021 3:58 pm

hippy wrote:
Fri Sep 17, 2021 1:54 pm
I believe I have noted previously that "If RUN is not used, it should be tied high" is extremely ambiguous because what is "not used" ?
It isn't ambiguous as "not used" in chip datasheets means no circuitry is connected to it and it isn't used at all during operation of this device. A normally-open button even if unpressed is still considered circuitry.

lurk101
Posts: 966
Joined: Mon Jan 27, 2020 2:35 pm
Location: Cumming, GA (US)

Re: Do I need a pull-up on the RUN pin?

Fri Sep 17, 2021 4:05 pm

trejan wrote:
Fri Sep 17, 2021 3:58 pm
hippy wrote:
Fri Sep 17, 2021 1:54 pm
I believe I have noted previously that "If RUN is not used, it should be tied high" is extremely ambiguous because what is "not used" ?
It isn't ambiguous as "not used" in chip datasheets means no circuitry is connected to it and it isn't used at all during operation of this device. A normally-open button even if unpressed is still considered circuitry.
In practical terms, tying a push button between RUN and GND without external pull up works just fine. Other commercial RP2040 designs do it that way.

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