Advantech RSB-4210 Evaluation Kit User Manual download pdf (Page 63)

55 RSB-4210 User Manual

Chapter 3 Software Functionality

can be written generically, so that board setup code passes such pin configuration

data to drivers.

3.10.2 Paths in Sysfs

There are three kinds of entry in /sys/class/gpio:

 Control interfaces used to get userspace control over GPIOs;

 GPIOs themselves; and

 GPIO controllers ("gpio_chip" instances).

That's in addition to standard files including the "device" symlink. The control inter-

faces are write-only:

/sys/class/gpio/

"export" ... Userspace may ask the kernel to export con-

trol of

a GPIO to userspace by writing its number to this

file.

#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 user-

space.

"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. Writ-

ing 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 GPIOis configured as an out-

put, 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 POLL-

ERR. 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.

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