GPIO

For available GPIO pins on the RasPi see expansion header. For default pull-up/pull-down modes see Table 6-31 of https://www.raspberrypi.org/documentation/hardware/raspberrypi/bcm2835/BCM2835-ARM-Peripherals.pdf.

GPIO programming example

We use BCM pad 18 for testing with a LED + serial resistor. Connect the LED cathode (shorter wire) to GND (J8 pin 14) and LED anode (longer wire) via a serial resistor for current limiting to J8 pin 12 which is routed to BCM pin 18.

expansion header pin	pin funcion	SoC pin	connect
12	GPIO	18	resistor -> LED anode
14	GND		LED cathode

command line access

from the command prompt on your running Pi:

export (==make visible) GPIO pin 18:

root@raspberrypi3:~# echo 18 > /sys/class/gpio/export

make pin 18 an output:

root@raspberrypi3:~# echo out > /sys/class/gpio/gpio18/direction 

set its value to 1 (high), the LED should switch on:

root@raspberrypi3:~# echo 1 > /sys/class/gpio/gpio18/value

set its value to 0 (low), the LED should switch off:

root@raspberrypi3:~# echo 0 > /sys/class/gpio/gpio18/value

If you like bash hacking, let the LED blink from the console. A main loop could look like:

while :
do  
  echo 1 > /sys/class/gpio/gpio18/value
  sleep 1
  echo 0 > /sys/class/gpio/gpio18/value
  sleep 1
done

but it is still missing initialization and termination code:

• export must be done only once in the beginning when /sys/class/gpio/gpio18/ does not yet exists
• it is good style to unexport the pin if no longer needed (which also switches the LED off) In the shell script this can be achieved with a trap line.

A complete example looks like:

# /bin/sh

# signal handler containing termination code
trap "echo 18 > /sys/class/gpio/unexport; exit" SIGHUP SIGINT SIGTERM

# initialization code 
echo 18 > /sys/class/gpio/export
echo out > /sys/class/gpio/gpio18/direction 

# main loop
while :
do  
  echo 1 > /sys/class/gpio/gpio18/value
  sleep 1
  echo 0 > /sys/class/gpio/gpio18/value
  sleep 1
done

timing

Timing can hardly be controlled when using a shell for GPIO toggling. Try the shortest possible pulse:

echo 0 > /sys/class/gpio/gpio18/value
echo 1 > /sys/class/gpio/gpio18/value
echo 0 > /sys/class/gpio/gpio18/value

Using a scope you may see a 3..5 ms wide pulse. But, there is guarantee for timing! So, using the shell interface is mainly useful for testing and slow peripherals like switching a LED on. It cannot be used for PWM or motor control.

access from a C program

This code can be one-to-one translated into C code using POSIX style file access functions. For a complete example see: http://www.netzmafia.de/skripten/hardware/RasPi/RasPi_GPIO_C.html

Note: The C equivalent of a trap command is a signal handler.

timing

The timing may be faster, but remains rather unpredictable because the user mode C app may be interrupted by the Linux scheduler. Moreover, the sleep function and its friends only guarantee minimum timeouts.

kernel module

For best timing control, interrupt handling etc. you should consider writing a Kernel Module.

automatic start

see autostart and daemon