lkml.org 
[lkml]   [2018]   [Aug]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [RFC PATCH 1/2] dt-bindings: pwm: imx: Allow switching PWM output between PWM and GPIO
From
Date
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
>> + >;
>> +};

\
 
 \ /
  Last update: 2018-08-23 13:22    [W:0.126 / U:0.728 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site