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I am attempting to build a Pandora Box. The basic idea of this project is that I am able to use the raspberry pi as a pandora radio device. Inputs are taken in from the user via Normally Open push buttons hooked into the RPi's GPIOs. In this step, we construct a simple circuit for the push button. It seems to be the case that all GPIOs work except SDA and SCL. I have been using a combination of my multimeter and WiringPi to test, and it seems that both these GPIOs hover at 3.3v, even when set to input.

gpio -g mode 2 in
gpio -g mode 3 in

These 2 inputs work as expected when in output mode:

gpio -g mode 2 out
gpio -g write 2 0
gpio -g write 2 1

I suspect that it has something to do with these same inputs being used for I2C by default, but I can't confirm. Has anyone else had this issue? What more can I do to investigate the problem? I am running the latest raspian wheezy

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Unfortunately I ran in to the same problem. I have no idea why. I tried to make them outputs and drive them low they stay at 3.3V. Tried to make them inputs and connected them to ground - still they read "1". I have no idea what is going on with GPIO0 and GPIO1. All other GPIO pins behave as expected. I thought I have burned them before I saw your post. – rumen Apr 12 at 23:36
@rumen Good to hear other people are having the same issues. Krzysztof Adamski's answer below seems to explain the phenomenon. – Jeff Apr 15 at 15:50

1 Answer

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SDA and SCL pins have external 1k8 pull-up resistors connected. This is indeed because they are intended for I²C usage (which requires them). Unfortunately, since they are external, they can't be disabled in software.

Having pull-up resistors means that default state of the pin in HIGH, unless they are pulled to ground. And this is exactly the problem you described.

This may be seen as a disadvantage but for buttons, this can be actually useful. This means that you don't have to use any additional pull-up/pull-down resistors with those pins (but you will have to use inverse logic - 1 when button is not pushed and 0 whet it is).

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That make's sense. I figured after some time that it was an issue with the build in pull up/down resistors, but my knowledge in EE prevented me from 'fixing' it. I ended up mapping my buttons to different ports. Thanks for the explanation. – Jeff Apr 15 at 15:52
@Jeff: Just not that they are not build in the sense of SoC. SoC has building pull-up resistors on all GPIO pins too but they can be disabled pragmatically and they are weaker (10kOhm). But they are build-in in a sense of whole RaspberryPi, of course. – Krzysztof Adamski Apr 15 at 16:15

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