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gpio.md

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GPIO control in config.txt

gpio

The gpio directive allows GPIO pins to be set to specific modes and values at boot time in a way that would previously have needed a custom dt-blob.bin file. Each line applies the same settings (or at least makes the same changes) to a set of pins, either a single pin (3), a range of pins (3-4), or a comma-separated list of either (3-4,6,8). The pin set is followed by an = and one or more comma-separated attributes from this list:

  • ip - Input
  • op - Output
  • a0-a5 - Alt0-Alt5
  • dh - Driving high (for outputs)
  • dl - Driving low (for outputs)
  • pu - Pull up
  • pd - Pull down
  • pn/np - No pull

gpio settings are applied in order, so those appearing later override those appearing earlier.

Examples:

# Select Alt2 for GPIO pins 0 to 27 (for DPI24)
gpio=0-27=a2

# Set GPIO12 to be an output set to 1
gpio=12=op,dh

# Change the pull on (input) pins 18 and 20
gpio=18,20=pu

# Make pins 17 to 21 inputs
gpio=17-21=ip

The gpio directive respects the "[...]" section headers in config.txt, so it is possible to use different settings based on the model, serial number, and EDID.

GPIO changes made through this mechanism do not have any direct effect on the kernel — they don't cause GPIO pins to be exported to the sysfs interface, and they can be overridden by pinctrl entries in the Device Tree as well as utilities like raspi-gpio.

Note also that there is a delay of a few seconds between power being applied and the changes taking effect — longer if booting over the network or from a USB mass storage device.

enable_jtag_gpio

Setting enable_jtag_gpio=1 selects Alt4 mode for GPIO pins 22-27, and sets up some internal SoC connections, thus enabling the JTAG interface for the ARM CPU. It works on all models of Raspberry Pi.

Pin # Function
GPIO22 ARM_TRST
GPIO23 ARM_RTCK
GPIO24 ARM_TDO
GPIO25 ARM_TCK
GPIO26 ARM_TDI
GPIO27 ARM_TMS