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?
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.
>
> 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
> + >;
> +};
Best regards,
Lukasz Majewski
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