Enabling Pullup and Pulldown Resistors on The Raspberry Pi

In January 2015 the first version of the Raspbian operating system with built in support for the device tree was released. The device tree is a data structure for describing hardware. Many aspects of the hardware can be described in this data structure rather than being hard coded into the operating system.

Device tree overlays can be used to apply changes to the kernel's internal device tree representation. For example, a device tree overlay can be used to enable the pullup or pulldown resistor for a GPIO.

Lets say that the pullup and pulldown resistors for GPIO #7, #8, and #9 need to be configured as shown in the following table:

GPIO #	Header Pin #	Pull Type
7	26	pulldown
8	24	pulldown
9	21	pullup

The device tree source file mygpio-overlay.dts for the device tree overlay to acheive this is as follows:

/dts-v1/;
/plugin/;

/ {
    compatible = "brcm,bcm2708";

    fragment@0 {
        target = <&gpio>;
        __overlay__ {
            pinctrl-names = "default";
            pinctrl-0 = <&my_pins>;

            my_pins: my_pins {
                brcm,pins = <7 8 9>;     /* gpio no. */
                brcm,function = <0 0 0>; /* 0:in, 1:out */
                brcm,pull = <1 1 2>;     /* 2:up 1:down 0:none */
            };
        };
    };
};

The device tree compiler compiles the source into a binary form. The compiler itself is installed with the following command:

sudo apt-get install device-tree-compiler

And the overlay is compiled with the following command:

dtc -@ -I dts -O dtb -o mygpio-overlay.dtb mygpio-overlay.dts

The device tree blob mygpio-overlay.dtb produced by the compiler is the binary and should be copied to directory /boot/overlays.

The last piece of the puzzle is adding the following line at the end of /boot/config.txt so that the overlay gets loaded at boot time:

device_tree_overlay=overlays/mygpio-overlay.dtb

For additional information about the device tree see Device Trees, Overlays and Parameters