Datasheet

Backlight Control

The backlight of the 2.8" PiTFT has 4 LEDs in series and it draws ~75mA at all times, controlled by a transistor. The

PiTFT 3.5" display has 6 LEDs in a row, and we use a boost converter to get the 5V from the Pi up to the ~20V needed

to light up all the LEDs.

There might be times you'd like to save some power and turn off the backlight. The screen and touchplate will still

work, you just can't see anything. We designed the board with the STMPE610 touchscreen controller which has 2 extra

GPIO and tied one of them to control the backlight. You can use the command line to control the backlight.

By default, the backlight's on...but you can control it in two ways!

PWM Backlight Control with GPIO 18

If you want precise control, you can use the PWM output on GPIO 18. There's python code for controlling the PWM but

you can also just use the kernel module and shell commands.

You'll need to make sure the STMPE control is not 'active' as the STMPE GPIO overrides the PWM output.

sudo sh -c 'echo "1" > /sys/class/backlight/soc\:backlight/brightness'

(Or if you are running an old kernel before the backlight object, try sudo sh -c "echo 'in' >

/sys/class/gpio/gpio508/direction")

OK now you can set the GPIO #18 pin to PWM mode using WiringPi's gpio command

With these basic shell commands, you can set the GPIO #18 pin to PWM mode with 1000 Hz frequency, set the output

to 100 (out of 1023, so dim!), set the output to 1023 (out of 1023, nearly all the way on) and 0 (off)

If you'd like to not have #18 control the backlight, simply

cut the solder jumper, the tiny trace between the two

large gold pads marked Lite #18

gpio -g mode 18 pwm

gpio pwmc 1000

gpio -g pwm 18 100

gpio -g pwm 18 1023

gpio -g pwm 18 0

https://learn.adafruit.com/adafruit-pitft-28-inch-resistive-touchscreen-display-raspberry-

Page 54 of 70