PWM backlight frequency on Apalis

Technical Support
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#1

Hi, we need to set a pwm signal with frequency of 200Hz as a requirement for our LCD driver backlight. Apalis by default is set to 150Hz. We can see the signal in the oscilloscope with default configuration without any issue. We can also change the backlight with no issue, but unfortunately at that frequency the LCD is flickering a bit, so we need to set 200Hz mandatory (tested with function generator).

In order to use 200kHz we have modify the device tree in file “imx6q-apalis-ixora.dts” to:

&backlight {
	brightness-levels = <0 45 63 88 119 158 203 255>;
	default-brightness-level = <4>;
	power-supply = <&reg_3v3_sw>;
	pwms = <&pwm4 5000000  PWM_POLARITY_INVERTED>; 
	status = "okay";
};

After device tree compilation we have tested it but there is no output on the oscilloscope on backlight pin 35 (X19 LDVS connector),

Is there any limitation on the maximum frequency we can use?
How can we get 200Hz PWM signal on that pin? can we modify any pre/scaler to get it work?

Thanks
Kind regards, Saúl Trapiello

#2

Should it be

pwms = <&pwm4 0 5000000  PWM_POLARITY_INVERTED>;

?

#3

Thanks, I deleted the 0 unintentionally. It works now.

#4

Hello saultr,
thank you for your feedback.

Matthias Gohlke