GPIO pull-up / down issue

Dario_Martino · May 12, 2019, 11:44am

Hi everyone, I have a problem setting up Pull-up / down resistors for GPIO buttons.

So, my buttons without any resistor work fine, but as there’s no resistors, i occasionally have unwanted button presses.

So I decided to put a pull resistor to any button. The problem is that when I actually push the buttons, sometimes they work, sometimes they don’t.

I’ve made a PCB like this:

POS ------ SW -------- GPIO ------- 10k res -------- NEG
POS ------ SW -------- GPIO ------- 10k res -------- NEG
POS ------ SW -------- GPIO ------- 10k res -------- NEG

Positive and negative are shared, and I tried both pull-up and pull-down just inverting positive and negative.

Any idea what i may have done wrong?

Udo_Hartmann · May 12, 2019, 1:30pm

afaik Raspberry comes with both pullup and pulldown resistors built in, you only have to switch which of both should be active (in software)

Dario_Martino · May 12, 2019, 2:35pm

Udo, thanks for the answer.
I’ve searched everywhere, but I didn’t find how to enable the internal resistors in openhabian. Any hints?

Udo_Hartmann · May 12, 2019, 4:43pm

github.com

raspberrypilearning/physical-computing-guide/blob/master/pull_up_down.md

# Pull Up and Pull Down Resistors

When a GPIO pin is in input mode and not connected to 3.3 volts or ground, the pin is said to be **floating**, meaning that it has no fixed voltage level. That's no good for what you want, as the pin will randomly float between `HIGH` and `LOW`. You need to categorically know the state of the pin. So you need to fix the voltage level to `HIGH` or `LOW`, and then make it change **only** when (in the case of this guide) a button is pressed or a pair of wires are manually connected.

This can be done in two ways:

- A pull up circuit

  Wire the GPIO pin to 3.3 volts through a large (10kΩ) resistor so that it always reads `HIGH`. Then we can short the pin to ground by closing the circuit so that the pin will go `LOW`.

  ![](images/pull_up.png)

- A pull down circuit

  Wire the GPIO pin to ground through a large (10kΩ) resistor so that it always reads `LOW`. Then you can short the pin to 3.3 volts by closing the circuit so that it goes `HIGH`. When the circuit is closed, there is a lower resistance path to 3.3 volts, and therefore the pin will read `HIGH`.

  ![](images/pull_down.png)

  Note: The 1kΩ R2 resistor is there in both circuits to give the GPIO pin a fail-safe protection, in case we mistakenly set the pin to be in OUTPUT mode.

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Afaik there is no option at all to do this in openHABian.

Dario_Martino · May 14, 2019, 10:06am

So, just for you to know, there IS a way to enable a raspberry’s internal pull up/down resistors in an openhabian system (actually any debian based system).

I may write a tutorial about it since there’s lack of information on the subject.

I’m a bit disappionted that your last reply just linked generic infos on pull up/down resistors that, if you read my first post, I already knew how they work. Luckily i’m stubborn and kept searching/studying.

Udo_Hartmann · May 14, 2019, 10:54am

Well, in fact the link is about how to enable internal pullup/pulldown resistors per pin, although you will need a python script. This sort of scripts is not part of openhabian (as far as I know).

I may have misunderstood your first posting

Dario_Martino · May 14, 2019, 11:01am

My apologies, as I’ve read the truncated file without opening the actual link.

Anyways I solved with wiringPI and a bash script.

rossko57 · May 14, 2019, 12:42pm

Out of curiosity - are the pullups configured active in some way by default, then? Why didn’t your external pullups work as expected?

Udo_Hartmann · May 14, 2019, 4:25pm

afaik by default there are some GPIs konfigured as pullup, some others as pulldown, so it depends on which GPI is used…