Default GPIO output

[PhillyNJ]

Hi,

If nothing is hooked up to a GPIO pin, does the OS read it as HIGH or LOW. I am trying to run a python script to shut down the Pi safely when a GPIO is set to high. With no GPIO pins connected, pin 2 is reading HIGH. Is this the normal behavior?

Code: Select all

import RPi.GPIO as GPIO
import time
import os
GPIO.setmode(GPIO.BCM)
GPIO.setup(2, GPIO.IN)
GPIO.setup(23, GPIO.OUT)
print("Shutdown script started. Wait for shutdown request")
while True:
  GPIO.output(23, GPIO.HIGH) // sent HIGH to arduino - lets arduino know PI is running
  if(GPIO.input(2)):
     print("Request from button to shutdown")
     os.system("sudo shutdown -h now")
     break
time.sleep(1)

[RaTTuS]

it will float if it has not been set so could be read as high or low IIRC

[PhillyNJ]

it will float if it has not been set so could be read as high or low IIRC

How do I set the Pin LOW, when it is setup for IN?

[jojopi]

The SoC has the facility to enable a weak pull-up or pull-down on any GPIO configured as an input. This is equivalent to connecting the pin to 3.3V or 0V with a resistor of around 50kΩ, and will hold the pin at the required level until pulled more strongly by an external circuit. To enable this with RPi.GPIO:

Code: Select all

GPIO.setup(11, GPIO.IN, pull_up_down=GPIO.PUD_DOWN)

However, GPIO2/3 (or 0/1 on Revision1 boards) have physical 1.8kΩ pull-up resistors soldered on the board, for use with I²C. A weak pull-down will not overcome this, so these two GPIOs always read high when not connected.

[gordon@drogon.net]

The SoC has the facility to enable a weak pull-up or pull-down on any GPIO configured as an input. This is equivalent to connecting the pin to 3.3V or 0V with a resistor of around 50kΩ, and will hold the pin at the required level until pulled more strongly by an external circuit. To enable this with RPi.GPIO:

Code: Select all

GPIO.setup(11, GPIO.IN, pull_up_down=GPIO.PUD_DOWN)

However, GPIO2/3 (or 0/1 on Revision1 boards) have physical 1.8kΩ pull-up resistors soldered on the board, for use with I²C. A weak pull-down will not overcome this, so these two GPIOs always read high when not connected.

Not (always) true. As of the foundations 3.6 kernel release BCM_GPIO pins 0 and 1 are set to outpus and driven low. So if you have a Revision 1 board then that trick isn't advisable anymore )-:

-Gordon

[jojopi]

Not (always) true. As of the foundations 3.6 kernel release BCM_GPIO pins 0 and 1 are set to outpus and driven low. So if you have a Revision 1 board then that trick isn't advisable anymore )-:

I did not intend to suggest that GPIO0/1/2/3 would be high even when driven low, just that they cannot be weakly pulled down. I am not sure what trick you mean, or whether it was ever advisable.

Also, I have a Rev1 board running 3.6.11+ #371 kernel, and GPIO0/1 are not driven low at any time during boot.

[gordon@drogon.net]

Also, I have a Rev1 board running 3.6.11+ #371 kernel, and GPIO0/1 are not driven low at any time during boot.

Hm. Now you've got me there.

When I first got the 3.6 release something set GPIO pins 0 & 1 to outputs and set them low. I checked this on a couple of boards - I noticed it immediately as I had a ladder board pluged into one and normally on a reboot the small LEDs come on (dimly) - they're on BCM_GPIO 0 & 1... they were off. I even posted about it: http://www.raspberrypi.org/phpBB3/viewt ... 24&t=35275

Oh well. Possibly another update since then has reverted that change.

As you were.

-Gordon

[PhillyNJ]

Thanks guys for the help.