A General Purpose Input/Output (GPIO) is a flexible software-controlled digital signal. They are provided from many kinds of chip, and are familiar to Linux developers working with embedded and custom hardware. Each GPIO represents a bit connected to a particular pin, or “ball” on Ball Grid Array (BGA) packages. Board schematics show which external hardware connects to which GPIOs. Drivers can be written generically, so that board setup code passes such pin configuration data to drivers.
Often different aspects of the GPIO need to be controlled, such as:
In the ADSP-SC58x processors there are 7 gpio blocks, called PORTA..PORTG. Each PORTx interface has 16 GPIO pins. PORT(A-G) pins are brought out on the 110 pins of SC589-EZKIT(PA0-PG13) and 80 pins of SC584-EZKIT(PA0-PD16). While in the ADSP-SC573 processors there are only 6 gpio blocks, called PORTA..PORTF.
Standard Linux kernel have inside a special interface allow to access to GPIO pins. You can use kernel menuconfig (linux-menuconfig) to verify that these interfaces are active in your kernel and, if necessary, enable them.
The kernel tree path is the following:
Symbol: GPIO_SYSFS [=y] Prompt: /sys/class/gpio/... (sysfs interface) Defined at drivers/gpio/Kconfig:51 Depends on: GPIOLIB && SYSFS && EXPERIMENTAL Location: -> Kernel configuration -> Device Drivers -> GPIO Support (GPIOLIB [=y])
There are three kinds of entry in /sys/class/gpio:
This is in addition to standard files including the “device” symlink.
The control interfaces are write-only:
/sys/class/gpio/ "export" ... Userspace may ask the kernel to export control of a GPIO to userspace by writing its number to this file. Example: "echo 19 > export" will create a "gpio19" node for GPIO #19, if that's not requested by kernel code. "unexport" ... Reverses the effect of exporting to userspace. Example: "echo 19 > unexport" will remove a "gpio19" node exported using the "export" file. GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42) and have the following read/write attributes: /sys/class/gpio/gpioN/ "direction" ... reads as either "in" or "out". This value may normally be written. Writing as "out" defaults to initializing the value as low. To ensure glitch free operation, values "low" and "high" may be written to configure the GPIO as an output with that initial value. Note that this attribute *will not exist* if the kernel doesn't support changing the direction of a GPIO, or it was exported by kernel code that didn't explicitly allow userspace to reconfigure this GPIO's direction. "value" ... reads as either 0 (low) or 1 (high). If the GPIO is configured as an output, this value may be written; any nonzero value is treated as high. If the pin can be configured as interrupt-generating interrupt and if it has been configured to generate interrupts (see the description of "edge"), you can poll(2) on that file and poll(2) will return whenever the interrupt was triggered. If you use poll(2), set the events POLLPRI and POLLERR. If you use select(2), set the file descriptor in exceptfds. After poll(2) returns, either lseek(2) to the beginning of the sysfs file and read the new value or close the file and re-open it to read the value. "edge" ... reads as either "none", "rising", "falling", or "both". Write these strings to select the signal edge(s) that will make poll(2) on the "value" file return. This file exists only if the pin can be configured as an interrupt generating input pin. "active_low" ... reads as either 0 (false) or 1 (true). Write any nonzero value to invert the value attribute both for reading and writing. Existing and subsequent poll(2) support configuration via the edge attribute for "rising" and "falling" edges will follow this setting. GPIO controllers have paths like /sys/class/gpio/gpiochip42/ (for the controller implementing GPIOs starting at #42) and have the following read-only attributes: /sys/class/gpio/gpiochipN/ "base" ... same as N, the first GPIO managed by this chip "label" ... provided for diagnostics (not always unique) "ngpio" ... how many GPIOs this manges (N to N + ngpio - 1)
# cat /sys/kernel/debug/gpio GPIOs 0-15, platform/31004000.gport, adi-gpio: GPIOs 16-31, platform/31004080.gport, adi-gpio: gpio-30 (mdio-reset ) out hi GPIOs 32-47, platform/31004100.gport, adi-gpio: GPIOs 48-63, platform/31004180.gport, adi-gpio: GPIOs 64-79, platform/31004200.gport, adi-gpio: GPIOs 80-95, platform/31004280.gport, adi-gpio: GPIOs 96-101, platform/31004300.gport, adi-gpio: GPIOs 480-495, i2c/0-0022, mcp23017, can sleep: GPIOs 496-511, i2c/0-0021, mcp23017, can sleep: