linux-kernel - Re: [RFC PATCH 1/2] dt-bindings: pwm: imx: Allow switching PWM output between PWM and GPIO

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Message-ID: <20180823123759.68b78e9f@jawa>
Date:   Thu, 23 Aug 2018 12:40:13 +0200
From:   Lukasz Majewski <lukma@...x.de>
To:     Michal Vokáč <michal.vokac@...ft.com>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        devicetree@...r.kernel.org, linux-pwm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Fabio Estevam <fabio.estevam@....com>,
        Lothar Waßmann <LW@...o-electronics.de>
Subject: Re: [RFC PATCH 1/2] dt-bindings: pwm: imx: Allow switching PWM
 output between PWM and GPIO

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@...ft.com>
> ---
>  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@...b4000 {
> @@ -25,3 +31,41 @@ pwm1: pwm@...b4000 {
>  	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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