Changes between Version 8 and Version 9 of ventana/LVDS

Timestamp:

07/17/2019 10:46:35 PM (5 years ago)

Author:

Bobby Jones

Comment:

Add backlight troubleshooting section

ventana/LVDS

@@ -152 +152 @@
  * touchscreen's are typically I2C or USB based devices with drivers in the {{{drivers/input/touchscreen}}} directory
 
-
 [=#troubleshooting]
+=== Troubleshooting Backlight Portion ===
+In some cases the device tree node used by the linux {{{pwm-backlight}}} driver may need to be adjusted if 
+"flickering" exists ([https://i.stack.imgur.com/LF3Y4.jpg example]). This phenomenon is caused by a pwm frequency setting that is too low for the connected display's backlight which creates a visual artifact that may be detectable by the human eye. 
+
+As described in the pwm device tree bindings [https://raw.githubusercontent.com/torvalds/linux/master/Documentation/devicetree/bindings/pwm/pwm.txt documentation], the backlight node of the device tree has a property labelled {{{pwms}}}. This property has includes the {{{pwm-specifier}}} which typically encodes the chip-relative PWM number and the PWM
+period in nanoseconds.
+{{{
+backlight {
+        compatible = "pwm-backlight";
+        pwms = <&pwm1 0 5000000>;               /* 1 / (5000000 / 1000000000) = 200 Hz */
+        brightness-levels = <
+                0  1  2  3  4  5  6  7  8  9
+                10 11 12 13 14 15 16 17 18 19
+                20 21 22 23 24 25 26 27 28 29
+                30 31 32 33 34 35 36 37 38 39
+                40 41 42 43 44 45 46 47 48 49
+                50 51 52 53 54 55 56 57 58 59
+                60 61 62 63 64 65 66 67 68 69
+                70 71 72 73 74 75 76 77 78 79
+                80 81 82 83 84 85 86 87 88 89
+                90 91 92 93 94 95 96 97 98 99
+                100
+                >;
+        default-brightness-level = <100>;
+};
+}}}
+
+If you want to make an adjustment to this frequency, you can use the {{{fixfdt}}} script in the bootloader. For example, to change the backlight frequency to {{{10kHz}}} you would run the following command from the bootloader:
+{{{#!bash
+# The third value of the pwms property is in nanoseconds so to convert to Hz:
+# 1 / (100000 / 1000000000) = 10 kHz
+setenv fixfdt "fdt addr ${fdt_addr}; fdt set /backlight pwms <0x3d 0 100000>"
+}}}
+
 === Troubleshooting Touchscreen Portion ===
 There is a command line tool called {{{evtest}}} that can be used to check for touch events.