When using the EDT1 display adapter, the backlight of the LCD is controlled by an LED driver chip that is connected via I2C to the CPU. In newer software versions like on armStoneA9 we have integrated this as a backlight driver unit, but on armStoneA5 with the Vybrid kernel, this is only visible as the LED controller chip itself and has to be set manually.
Go to the LED controller chip with
There you will find settings for 4 LEDs. The backlight brightness is controlled by led1. Each LED can be set to four values:
0: fully off
1: fully on
2: controlled by individual PWM
3: controlled by group PWM
Our default setting is 1, i.e. fully on. You can show this by calling
So the idea is to switch to PWM mode 2. But to avoid a sudden drop in brightness (the default PWM value is 0), set the brightness before switching the mode. Or simply set the PWM brigthness also to fully on, then switch the mode and then set the final brightness.
From now on you can set any value between 0 and 255 to led1_pwm to control the brightness. Example:
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