Messages in this thread | | | Subject | Re: [RFC PATCH 1/2] dt-bindings: pwm: imx: Allow switching PWM output between PWM and GPIO | From | Michal Vokáč <> | Date | Thu, 23 Aug 2018 13:21:04 +0200 |
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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@ysoft.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@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 >> + >; >> +};
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