{"id":259,"date":"2015-01-11T23:13:09","date_gmt":"2015-01-11T23:13:09","guid":{"rendered":"http:\/\/int03.co.uk\/blog\/?p=259"},"modified":"2015-12-29T15:58:33","modified_gmt":"2015-12-29T15:58:33","slug":"raspberry-pi-gpio-states-at-boot-time","status":"publish","type":"post","link":"https:\/\/int03.co.uk\/blog\/2015\/01\/11\/raspberry-pi-gpio-states-at-boot-time\/","title":{"rendered":"Raspberry Pi GPIO states at boot time"},"content":{"rendered":"

When external hardware is connected to the Raspberry Pi, it can be important to know the initial state of the GPIO pins at boot time. For example, if we have a motor connected to a GPIO pin, it could be a problem if the motor starts running as the system boots up. Ideally, we would want all the pins defined as inputs at start up, but that doesn’t seem to be the case with some of the Pi GPIO pins.<\/p>\n

Fortunately, there’s now a solution in the form of the device tree pin configuration<\/a>. This involves creating a pin configuration source file, and compiling that to a binary “blob” which is loaded by the OS at start up. This source file can configure pins as inputs, outputs or in special function modes (UART serial port for example). It can also configure whether a pin has pull-up or pull-down enabled.<\/p>\n

This proved very useful for my Espiresso project<\/a>, when I needed a pin to enable the pump at start up. Having run out of general GPIO pins, I was forced to use GPIO15 (RXD) which is configured as a pull-up UART pin by default. This would have resulted in the pump running as the system booted. However, by editing the device tree configuration, it was possible to set this pin as an input with pull-down mode, which prevents the external hardware being switched on during boot.\u00a0The GPIO pin can then be reconfigured as an output by the application, after the Pi has booted.<\/p>\n

As an update to this, there have been a couple of important developments with device-tree since I wrote this, which I’ll summarise briefly here:<\/p>\n