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Date:   Thu, 05 Nov 2020 01:27:51 +0100
From:   Artur Rojek <contact@artur-rojek.eu>
To:     Paul Cercueil <paul@crapouillou.net>
Cc:     Jonathan Cameron <jic23@kernel.org>,
        Lars-Peter Clausen <lars@metafoo.de>,
        Peter Meerwald-Stadler <pmeerw@pmeerw.net>, od@zcrc.me,
        linux-iio@vger.kernel.org, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org
Subject: Re: [PATCH] iio/adc: ingenic: Fix battery VREF for JZ4770 SoC
In-Reply-To: <F3RAJQ.DZODDTV09KM21@crapouillou.net>
References: <20201104192843.67187-1-paul@crapouillou.net>
 <cb8b8ff426500db61c61b51413f3746c@artur-rojek.eu>
 <F3RAJQ.DZODDTV09KM21@crapouillou.net>
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On 2020-11-05 01:09, Paul Cercueil wrote:
> Hi Artur,
> 
> Le mer. 4 nov. 2020 à 23:29, Artur Rojek <contact@artur-rojek.eu> a 
> écrit :
>> Hi Paul,
>> 
>> On 2020-11-04 20:28, Paul Cercueil wrote:
>>> The reference voltage for the battery is clearly marked as 1.2V in 
>>> the
>>> programming manual. With this fixed, the battery channel now returns
>>> correct values.
>>> 
>>> Fixes: a515d6488505 ("IIO: Ingenic JZ47xx: Add support for JZ4770 SoC 
>>> ADC.")
>>> Cc: <stable@vger.kernel.org>
>>> Signed-off-by: Paul Cercueil <paul@crapouillou.net>
>>> ---
>>>  drivers/iio/adc/ingenic-adc.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>> 
>>> diff --git a/drivers/iio/adc/ingenic-adc.c 
>>> b/drivers/iio/adc/ingenic-adc.c
>>> index ecaff6a9b716..19b95905a45c 100644
>>> --- a/drivers/iio/adc/ingenic-adc.c
>>> +++ b/drivers/iio/adc/ingenic-adc.c
>>> @@ -71,7 +71,7 @@
>>>  #define JZ4725B_ADC_BATTERY_HIGH_VREF_BITS	10
>>>  #define JZ4740_ADC_BATTERY_HIGH_VREF		(7500 * 0.986)
>>>  #define JZ4740_ADC_BATTERY_HIGH_VREF_BITS	12
>>> -#define JZ4770_ADC_BATTERY_VREF			6600
>>> +#define JZ4770_ADC_BATTERY_VREF			1200
>>>  #define JZ4770_ADC_BATTERY_VREF_BITS		12
>>> 
>>>  #define JZ_ADC_IRQ_AUX			BIT(0)
>> 
>> I thought we set it to 6600 because GCW Zero was not showing correct 
>> battery values at 1200.
>> But if you verified that 1200 works with JZ4770, then:
>> Acked-by: Artur Rojek <contact@artur-rojek.eu>
> 
> Yes, IIRC we were trying to figure out the range and settled with
> [-3.3V,+3.3V] since it would give "plausible" values but which were
> never quite right. The doc does say that the voltage is (hw_val /
> 4096) * 1.2V, but also says that the ADC operated with 3.3V power
> supply, I guess we got confused. We never considered the battery could
> not be connected directly to the ADC's VBAT pin, so a 1.2V reference
> didn't make sense at that time. I guess we need to learn about
> electronics :)
Yes we do :)
> 
> It turns out the battery is connected to the VBAT pin with a 1 MOhm
> resistor, and the VBAT pin is also pulled low with a 332 kOhm
> resistor. So a fully charged battery with 4.2V reads as (4.2V *
> 332000) / (1332000) = 1.05V, which totally fits in a [0V,+1.2V] range.
> 
> With that same 4.2V battery I get a hardware value of about 3584, and
> (3584 / 4096) * 1.2V == 1.05V, which matches the value computed above.
> So the battery reading looks accurate this time.
Excellent! Thanks for the detailed explanation.

Cheers,
Artur
> 
> Cheers,
> -Paul