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

[Artur Rojek]

On 2020-11-05 01:09, Paul Cercueil wrote:
> Hi Artur,
>
> Le mer. 4 nov. 2020 à 23:29, Artur Rojek <contact@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx>
> > > Signed-off-by: Paul Cercueil <paul@xxxxxxxxxxxxxxx>
> > > ---
> > >  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@xxxxxxxxxxxxxx>
>
> 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