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

[Jonathan Cameron <jic23@kernel.org>]

On Thu, 05 Nov 2020 01:27:51 +0100
Artur Rojek <contact@artur-rojek.eu> wrote:

> 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 
> >>> \x7fADC.")
> >>> 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 
> >>> \x7fb/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.

Applied to the fixes-togreg branch of iio.git

Thanks,

Jonathan

> 
> Cheers,
> Artur
> > 
> > Cheers,
> > -Paul