TS-TPC-7990 DIO

The i.MX6 and FPGA GPIO are accessed using the kernel's sysfs GPIO interface. See the kernel's documentation here for more detail. This interface provides a set of files and directories for interacting with GPIO which can be used from any language that can write files.

To interact with a pin, first export it to userspace:

echo "48" > /sys/class/gpio/export

If you receive a permission denied on a pin that means it is claimed by another kernel driver. In this case, you can "cat /sys/kernel/debug/gpio" to see what driver has claimed it. If it succeeds you will have a /sys/class/gpio/gpio48/ directory. The relevant files in this directory are:

 direction - "in", "out" (out = low), "low", "high"
 value - write "1" or "0", or read "1" or "0" if direction is in
 edge - write with "rising", "falling", or "none"

With direction you can set it as an output and set the direction at the same time with high/low. If you just specify out this is the same as low.

# Set as a low output
echo "out" > /sys/class/gpio/gpio48/direction
# Set GPIO 48 high
echo "1" > /sys/class/gpio/gpio48/value
# Set GPIO 48 low
echo "0" > /sys/class/gpio/gpio48/value

# Read the value of GPIO 48
echo "in" > /sys/class/gpio/gpio48/direction
cat /sys/class/gpio/gpio48/value

# You can set it as a high output by setting the direction:
echo "high" > /sys/class/gpio/gpio48/direction

As an output, the in can be written to 0 for low (GND), or 1 for high (3.3V). The GPIO pins directly off of the i.MX6 processor support an absolute maximum of -0.5 to 3.6V. It is also possible to use any processor GPIO as an interrupt by writing the edge value, and then using select() or poll() on the value file for changes.