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

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