linux-kernel - Re: [PATCH] pwm: lpc32xx

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Message-ID: <CAAAP30HVdducnECimoh=Buh3CRTLu0agg4GcKEA7B03EnXuAAw@mail.gmail.com>
Date:	Thu, 8 Nov 2012 11:49:27 -0200
From:	Alexandre Pereira da Silva <aletes.xgr@...il.com>
To:	Roland Stigge <stigge@...com.de>
Cc:	Alban Bedel <alban.bedel@...onic-design.de>,
	Thierry Reding <thierry.reding@...onic-design.de>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] pwm: lpc32xx - Fix the PWM polarity

On Thu, Nov 8, 2012 at 11:12 AM, Roland Stigge <stigge@...com.de> wrote:
> On 08/11/12 12:23, Alban Bedel wrote:
>>>> 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.
>
> Thanks for measuring. With this, your resubmitted patch make much more
> sense now.
>
> Roland

Alban,

I think you should include this measurements on the source code as
comments, for future reference.

Thanks.
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