Managing GPIO

In Wiren Board part GPIO output to terminal blocks, connectors on the expansion modules, the other part is used for business purposes. GPIO are also used to control the switching transistors for low-voltage load.

The correspondence between numbers GPIO Linux and their location and function can be found in the table WB SH 3.5: List of GPIO or WB rev. 2.8: List of GPIO.

In Wiren Board rev. 2.8 8 GPIO on a special connector connected via MCP23S08 and temporarily do not work with standard Linux API. How to work with them there on the pageMCP23S08.

All GPIO (as well as other ports Wiren Board) working with voltage 3.3V. In no case do not connect directly to the GPIO signals with voltage over 3.3V! In case you want to connect devices operating at a higher voltage, you must use matching circuits or connected (for 5V) through a resistor of 20 kOhm or more

On some GPIO (particularly at the 3 GPIO, launched into terminals in Wiren Board rev. 2.8) can be programmatically set to lift 47K + 3.3V. See. Pin_pull-up.

sysfs interface under Linux

GPIO under Linux are supported through sysfs-interface.

To work through a certain GPIO sysfs it must be exported:

Here and further N - number of gpio

echo N > /sys/class/gpio/export

Exported gpio appear in the catalog /sys/class/gpio :

root@wirenboard:~# ls -1 /sys/class/gpio/
export
gpio32
gpiochip0
gpiochip120
gpiochip32
gpiochip64
unexport

В директории /sys/class/gpioN теперь находятся файлы для работы с GPIO (где N - номер GPIO, как и было сказано ранее): The directory /sys/class/gpioN are now files to work with GPIO (where N - No. of GPIO, as was said earlier):

root@wirenboard:~# ls -1 /sys/class/gpio/gpioN/
active_low
device
direction
edge
power
subsystem
uevent
value

Setting the direction of GPIO (input / output) is made by writing to the file direction </ b>

echo in > /sys/class/gpio/gpioN/direction #set the GPIO to input
echo out > /sys/class/gpio/gpioN/direction # set the GPIO to output 

Reading and Setting the GPIO is made using the file value.

Reading:

echo in > /sys/class/gpio/gpioN/direction # Set GPIO a number N to input 
cat /sys/class/gpio/gpioN/value # returns 1 or 0

Record:

echo out > /sys/class/gpio/gpioN/direction # set GPIO No. N to output 
echo 0 > /sys/class/gpio/gpioN/value # set logic 0 (low voltage) on the GPIO No. N
echo 1 > /sys/class/gpio/gpioN/value # set logical 1 (high voltage) to GPIO No. N

Sysfs interface and interrupt

Via sysfs interface, you can request a change of state interrupt processor.

Setting the interrupt is done by writing the values in the file "edge". Values can be:

• "none" - disable interrupt
• "rising" - enable interrupt on falling edge
• "falling" - enable interrupt on the rising edge
• "both" - enable interrupt on both edges.

Example:

root@wirenboard:~# echo 3 >  /sys/class/gpio/export # export GPIO No. 3 (TB10 in WB3.3)
root@wirenboard:~# cat /sys/class/gpio/gpio3/edge   # check the status of the interrupt
none
root@wirenboard:~# echo falling > /sys/class/gpio/gpio3/edge # set interrupt falling edge
root@wirenboard:~# cat /proc/interrupts  | grep gpiolib # interrupt appears in the list. 26 - internal interrupt, 0 - the number of events
 26:          0  gpio-mxs   3  gpiolib
root@wirenboard:~# cat /proc/interrupts  | grep gpiolib # After several events, 76 - the number of events
 26:         76  gpio-mxs   3  gpiolib

Interrupts can be caught from userspace using the system call poll () and select () on file value. Example of look here [1]

see also elinux.org

Direct access by processor memory

This method is strongly NOT recommended for use without sufficient reason. To work in C / C ++ is to use the files to work through sysfs, as described in the previous section.

Driving GPIO can use the direct access to the processor registers, bypassing the Linux interface/dev/mem. At the same time, compared to the work via sysfs minimization of costs. This method can be used, if you need a very fast access to GPIO, such bitbang protocols or PWM. It should be borne in mind that the process scheduler can still contribute to the work program of significant delays. It is recommended to make time-critical tasks in the kernel.

See [2] , [3]

GPIO and Device Tree

Pointing to the GPIO Device Tree you need to configure the GPIO to work in a mode of software SPI, I2C, GPIO to be used as the source of the interrupt, etc. For example on a pin 10 @ UEXT1 (CS) and pins 5 @ UEXT2 (SCL), 6 @ UEXT2 (SDA), 10 @ UEXT2 (CS) derived processor GPIO lines. They can be configured to use such as a chip-select for as SPI or I2C.

GPIO processor and peripherals are divided into banks (gpio chip). GPIO processor split into 3 banks for 32 GPIO: gpio0, gpio1, gpio2. Addressing GPIO in Device Tree comes to the number of the bank and the number of GPIO * in * the bank.

Example 1

We define signal 6 @ UEXT2 (SDA) as the source of the interrupt driver mrf24j40. According to the table List of GPIO, signal corresponds to 53 GPIO of processor. 53 belongs to the second bank gpio (32 to 63). GPIO number inside the bank 53-32 = 21:

				6lowpan@0 {
					compatible = "microchip,mrf24j40";
					spi-max-frequency = <100000>;
					reg = <6>;
					interrupt-parent = <&gpio1>;
					interrupts = <21 0>;
				};