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SubjectRe: [PATCH] iio/adc: ingenic: Fix battery VREF for JZ4770 SoC
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 :)

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.

Cheers,
-Paul


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