On 23.8.2018 12:40, Lukasz Majewski wrote:
Hi Lukasz, thanks for the reply!
Hi Michal,
Output of the PWM block of i.MX SoCs is always zero volts when the
block is disabled. This can caue issues when inverted PWM polarity is
needed. With inverted polarity a duty cycle = 0% corresponds to solid
high level on the output. If the PWM is dissabled its output
instantly goes to solid zero which corresponds to duty cycle = 100%.
To have a trully inverted PWM output configure the PWM pad as a GPIO
with pull-up. Then switch the pad to PWM output whenever non-zero
duty cycle is needed.
Just to ask - Is your display equipped with power supply enable/disable
pin?
No it is not. The backlight on my display is just a bunch of serial-
parallel connected LEDs with separate GND and VCC pins on a separate flex
cable. And the display itself also does not have a reset or enable signal.
It is a PITA to use it I must say..
As fair as I remember the trick to avoid flickering the display
was to disable the display (enable-gpio property) and set the PWM PIN
as GPIO to high in u-boot.
Yes, I know about that. I can not use this as the PWM output is the only
signal I have to control the backlight. I mentioned that somewhere in the
previous discussion with Lothar.
I also think this could be useful not only for backlight. Any circuit that
requires truly inverted PWM signal can use it. I see it as an enhancement
to what you with Lothar have already done ;)
Signed-off-by: Michal Vokáč <michal.vokac@xxxxxxxxx>
---
Documentation/devicetree/bindings/pwm/imx-pwm.txt | 44
+++++++++++++++++++++++ 1 file changed, 44 insertions(+)
diff --git a/Documentation/devicetree/bindings/pwm/imx-pwm.txt
b/Documentation/devicetree/bindings/pwm/imx-pwm.txt index
c61bdf8..3b1bc4c 100644 ---
a/Documentation/devicetree/bindings/pwm/imx-pwm.txt +++
b/Documentation/devicetree/bindings/pwm/imx-pwm.txt @@ -14,6 +14,12
@@ See the clock consumer binding,
Documentation/devicetree/bindings/clock/clock-bindings.txt
- interrupts: The interrupt for the pwm controller
+Optional properties:
+- pinctrl: For i.MX27 and newer SoCs. Add extra pinctrl to configure
the PWM
+ pin to gpio function. It allows control over the pin output level
when the
+ PWM block is disabled. This is meant to be used if inverted
polarity of the
+ PWM signal is required. See "Inverted PWM output" section bellow.
+
Example:
pwm1: pwm@53fb4000 {
@@ -25,3 +31,41 @@ pwm1: pwm@53fb4000 {
clock-names = "ipg", "per";
interrupts = <61>;
};
+
+Inverted PWM output
+-------------------
+
+The i.MX SoC has such limitation that whenever a pad is configured
as a PWM +output, the output level is always zero volts when the PWM
block is disabled. +The zero output level is actively driven by the
output stage of the PWM block +and can not be overridden by pull-up.
It also does not matter what PWM polarity +a PWM client (e.g.
backlight) requested. +
+To gain control of the PWM output level in disabled state two
pinctrl states +can be used. The "default" state and the "pwm" state.
In the default state the +PWM output is configured as a GPIO with
pull-up. In the "pwm" state the output +is configured as a PWM
output. This setup assures that the PWM output is at +the required
level that corresponds to duty cycle = 0 when PWM is disabled. +E.g.
at boot. +
+Example:
+
+&pwm1 {
+ pinctrl-names = "default", "pwm";
+ pinctrl-0 = <&pinctrl_backlight_gpio>;
+ pinctrl-1 = <&pinctrl_backlight_pwm>;
+}
+
+pinctrl_backlight_gpio: pwm1grp-gpio {
+ fsl,pins = <
+ /* GPIO with 22kOhm pull-up */
+ MX6QDL_PAD_GPIO_9__GPIO1_IO09 0xF008
+ >;
+};
+
+pinctrl_backlight_pwm: pwm1grp-pwm {
+ fsl,pins = <
+ /* PWM output */
+ MX6QDL_PAD_GPIO_9__PWM1_OUT 0x8
+ >;
+};
