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

[Michal VokÃÄ]

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 PITAÂto 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 VokÃÄ <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
> > +	>;
> > +};