Application note - GPIO on fitlet2 with FC-SCG running Linux

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  • This application note applies to fitlet2 with FC-SCG FACET-Card running Linux.
  • It provides an example of how to use the supported I2C GPIO extender feature of FC-SCG under Linux.

I2C GPIO extender binding and usage

1. Make sure that the kernel you use supports the necessary driver.
Check the following entries in the kernel config: 

CONFIG_GPIO_PCA953X=[y]
CONFIG_I2C=y
CONFIG_I2C_CHARDEV=y

2. Make sure that the appropriate I2C bus is enabled in BIOS settings:
Chipset -> South Cluster Configuration -> LPSS Configuration LPSS I2C #[1-8] Support is set to ACPI mode or PCI mode Set LPSS I2C #[1-8] Speed is set to Standard Mode of Fast Mode
NOTE: It’s strongly recommended that all buses are enabled in the range of 1-7

3.Find the GPIO extender on an I2C bus: 

readonly dev_func='17.2' 
busnum=$(ls -l /sys/bus/i2c/devices/ | sed -ne 
"/${dev_func}\/i2c_designware/ s/^.*i2c-\([[:digit:]]\{1,2\}\)$/\1/p") 
busaddr=$(i2cdetect -y -r $busnum | sed -ne 
's/^.*\(\(27\|4e\)\).*$/0x\1/Ip') 
busaddr=$((busaddr)) 
printf "Device found @ 0x%x on bus %i\n" $busaddr $busnum

Note: PCA9335 chip should be found at address 0x27, whereas XRA1201 chip at 0x4e

4. Bind the above found device to driver (superuser privileges are necessary) 

echo pca9535 $busaddr >/sys/bus/i2c/devices/i2c-${busnum}/new_device

Then check the kernel debug messages for a line like the following: 

i2c i2c-7: new_device: Instantiated device pca9535 at 0x27

5. To find the appropriate GPIO, issue the command: 

ls /sys/bus/i2c/drivers/pca953x/

Then look the output for a line like: 

7-0027

And then issue the command: 

ls /sys/class/gpio/gpiochip*/device -l

Afterwards, look at the output for a line like the following: 

/sys/class/gpio/gpiochip251/device -> ../../../7-0027

The bus number and address has to be the same in both cases (7 and 27 in the example). Hence, the gpiochip251 is to be sought; the individual GPIO line number are from 251 to 251+15

6. The GPIO found may be operated over sysfs interface in the following way: issue the commands: 

export n=251 # desired GPIO number
echo $n >/sys/class/gpio/export
echo out >/sys/class/gpio/gpio$n/direction # desired direction in/out
cat /sys/class/gpio/gpio$n/value # read the input value
echo 1 > /sys/class/gpio/gpio$n/value # write a value for output
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