JTAG

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notes

Sysfs exports information about devices and drivers from the kernel device model to userspace. Gpio-sysfs is the prefered method for accessing gpio from userspace. It deals with one pin at a time and works in means of reading and writing to files for control.

The control files for each gpio chip is contained within '/sys/class/gpio/'

~ # ls /sys/class/gpio/
export       gpiochip231  gpiochip235  gpiochip240  gpiochip248  unexport

To begin with there is only two files 'export' and 'unexport' and a folder for each gpio, each gpio has an ID number.

In each folder there are files that contains information about the chip:

~ # ls /sys/class/gpio/gpiochip231/
base       label      ngpio      subsystem  uevent
  • 'base' contains the ID of the chip, i.e. for gpiochip231, the ID is 231
~ # cat /sys/class/gpio/gpiochip231/base
231
  • 'label' contains the address (so that you are able to identify which gpio you are dealing with)
~ # cat /sys/class/gpio/gpiochip231/label
/plb@0/gpio@81400000
  • 'ngpio' contains the number of gpio pins available

In order to get access to a GPIO pin it is necessary to export it using the export file:

echo NN > /sys/class/gpio/export

Where NN is the base of the GPIO chip plus the pin number, i.e. for pin0 of gpiochip231 NN=231, for pin1 NN=232, etc.

An example:

echo 231 > /sys/class/gpio/export
echo 232 > /sys/class/gpio/export

This gives access to pin0 and pin1.

When a pin is exported, a folder is created: '/sys/class/gpio/gpioNN'. Within such a folder you have the control files for the pin, 'direction' and 'value'

~ # ls /sys/class/gpio/gpio231/
direction  subsystem  uevent     value

Example, setting direction of pin0 as output:

echo out > /sys/class/gpio/gpio231/direction

Possible commands for direction:

high	 Set GPIO to an output with a starting value of 1
low	 Set GPIO to an output with a starting value of 0
out	 Same as low
in	 Set GPIO to an input

Example, setting value of a pin0 to 1, then reading the value:

echo 1 > /sys/class/gpio/gpio231/value
cat /sys/class/gpio/gpio231/value
1

Current design:

sig    phys    NN
TDI -> pin6 -> 231
TMS -> pin4 -> 232
TDO -> pin3 -> 233 
TCK -> pin1 -> 234

References