General purpose input output

GPIOs

This document introduces handling of GPIO signals that are conected to Zynq-7000 PS EMIO block and is accesible as general purpose input / output pins on Extension conector E1 with Linux gpio subsystem userspace interfaces.

There are two interfaces legacy sysfs interface and new character device based one.

PINS

Pins connected to the PL block require FPGA code to function. If the pin signals are wired directly (in the FPGA sources) from PS based EMIO signals to the FPGA pads, then they can be managed using Linux drivers intended for the PS block. This is currently done with two fpga projects: classic and mercury.

Appropriate fpga bitstream can be applied using bash command.

redpitaya> overlay.sh v0.94

redpitaya> cat /opt/redpitaya/fpga/fpga_0.94.bit > /dev/xdevcfg

Although the Zynq SoC provides 118 GPIO lines, only 16 are accessible to the user. They can be found on the extension connector E1, as pins DIO0_P to DIO7_P and DIO0_N to DIO7_N. These pins can be identified by different numbering/naming schemes:

line number, used by the gpiod tools, libgpiod, and the underlying character device interface
sysfs number, used by the legacy sysfs interface: this is the line number offset by 906
EMIO signal name, documented in the device tree
GPIO number, also documented in the device tree
pin name, used in the SCPI interface, in the APIs (prefixed with RP_), and throughout the Red Pitaya documentation.

Note

Red Pitaya board models that use Zynq 7020 provide access to more GPIO pins:

SDRlab 122-16, STEMlab 125-14 4-Input, STEMlab 125-14-Z7020-LN - 22 pins

SIGNALlab 250-12 - 19 pins

This is the mapping between all these numbering/naming schemes:

Line number	sysfs number	FPGA signal name	EMIO signal	GPIO number	Pin name
62 - 69	968 - 975	exp_p_io[7:0]	EMIO8 - EMIO15	GPIO 0 - GPIO 7	DIO0_P - DIO7_P
70 - 77	976 - 983	exp_n_io[7:0]	EMIO16 - EMIO23	GPIO 8 - GPIO 15	DIO0_N - DIO7_N

The command gpioinfo, from the package gpiod, can be used to list the available lines, with their line number, EMIO signal name and GPIO number:

redpitaya> gpioinfo | grep GPIO
        line  62: "EMIO8  (GPIO 0)" unused input active-high
        line  63: "EMIO9  (GPIO 1)" unused input active-high
        ...

Linux access to GPIO

SYSFS access

This document is used as reference: Linux+GPIO+Driver

Bash example for writing to and reading from pin DIO0_P (sysfs number 968):

#export pin 968
$ echo "968" > /sys/class/gpio/export
#set direction to output
$ echo "out" > /sys/class/gpio/gpio968/direction
#set pin to LOW
$ echo "0" > /sys/class/gpio/gpio968/value
#set pin to HIGH
$ echo "1" > /sys/class/gpio/gpio968/value
#set pin direction to input
$ echo "in" > /sys/class/gpio/gpio968/direction
#output pin value
$ cat /sys/class/gpio/gpio968/value
#when done with pin you should unexport it with
$ echo 968 > /sys/class/gpio/unexport

Please check the SYSFS GPIO C example code for more information.

Character device access

Character device userspace access to gpio kernel subsystem is confirmed working on kernels newer and including 4.8.

References: GPIO for Engineers and Maker

The Linux kernel contains GPIO utilities in its tools directory.

We isolated the sources and created a library from gpio-utils.c and executables from other source files.

Source code

You can access the source code and precompiled binaries on our GitHub in the gpio-utils repository.