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Message-ID: <20121108122345.72aa0f6c@avionic-0020.adnet.avionic-design.de>
Date: Thu, 8 Nov 2012 12:23:45 +0100
From: Alban Bedel <alban.bedel@...onic-design.de>
To: Roland Stigge <stigge@...com.de>
Cc: Thierry Reding <thierry.reding@...onic-design.de>,
LKML <linux-kernel@...r.kernel.org>,
Alexandre Pereira da Silva <aletes.xgr@...il.com>,
Alban Bedel <alban.bedel@...onic-design.de>
Subject: Re: [PATCH] pwm: lpc32xx - Fix the PWM polarity
On Thu, 08 Nov 2012 11:44:48 +0100
Roland Stigge <stigge@...com.de> wrote:
> On 08/11/12 11:33, Alban Bedel wrote:
> > On Thu, 08 Nov 2012 10:51:35 +0100
> > Roland Stigge <stigge@...com.de> wrote:
> >
> >> On 07/11/12 16:25, Alban Bedel wrote:
> >>> Signed-off-by: Alban Bedel <alban.bedel@...onic-design.de>
> >>> ---
> >>> drivers/pwm/pwm-lpc32xx.c | 6 +++++-
> >>> 1 files changed, 5 insertions(+), 1 deletions(-)
> >>>
> >>> diff --git a/drivers/pwm/pwm-lpc32xx.c b/drivers/pwm/pwm-lpc32xx.c
> >>> index adb87f0..0dc278d 100644
> >>> --- a/drivers/pwm/pwm-lpc32xx.c
> >>> +++ b/drivers/pwm/pwm-lpc32xx.c
> >>> @@ -51,7 +51,11 @@ static int lpc32xx_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
> >>>
> >>> c = 256 * duty_ns;
> >>> do_div(c, period_ns);
> >>> - duty_cycles = c;
> >>> + if (c == 0)
> >>> + c = 256;
> >>> + if (c > 255)
> >>> + c = 255;
> >>> + duty_cycles = 256 - c;
> >>
> >> Except for the range check (for the original c > 255), this results in:
> >>
> >> duty_cycles = 256 - c
> >>
> >> except for (c == 0) where
> >>
> >> duty_cycles = 1
> >
> > No it lead to duty_cycles = 0
>
> Let's do it step by step with the above code:
>
> c == 0
>
> >>> + if (c == 0)
> >>> + c = 256;
>
> c == 256
>
> >>> + if (c > 255)
> >>> + c = 255;
>
> c == 255
>
> >>> + duty_cycles = 256 - c;
>
> c == 1
>
> See?
Right, my bad.
> >
> >> which actually is
> >>
> >> duty_cycles = (256 - c) - 255
> >>
> >> (think with the original c)
> >>
> >> i.e. nearly a polarity inversion in the case of (c == 0).
> >>
> >> Why is the case (c == 0) so special here? Maybe you can document this,
> >> if it is really intended?
> >
> > It is intended, the formular for duty value in the register is:
> >
> > duty = (256 - 256*duty_ns/period_ns) % 256
>
> Where does this modulo defined? In the Manual, there is sth. like this
> defined for RELOADV (tables 606+607), but not for DUTY.
>
> Maybe I missed sth. in the manual. Link or hint appreciated!
The manual doesn't mention this explicitly but you can see that without
the modulo when duty_ns==0 DUTY would be 256, but the register is only
8 bits wide (ie. modulo 256). I made a few test and looked at the PWM
output on a scope they confirm this:
DUTY HIGH LEVEL
1 99.9%
25 90.0%
128 50.0%
220 10.0%
255 0.1%
0 0.0%
I'll resubmit the patch with the clamping in the correct order.
Alban
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