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

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Date:   Thu, 23 Aug 2018 13:19:35 +0200
From:   Lothar Waßmann <LW@...O-electronics.de>
To:     Lukasz Majewski <lukma@...x.de>
Cc:     Michal Vokáč <michal.vokac@...ft.com>,
        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>
Subject: Re: [RFC PATCH 1/2] dt-bindings: pwm: imx: Allow switching PWM
 output between PWM and GPIO

Lukasz Majewski <lukma@...x.de> wrote:

> 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.
> 
Unfortunately there are both types of displays. Some with an ENABLE
pin, some without.


Lothar Waßmann
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