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

From: Paul Cercueil
Date: Wed Nov 04 2020 - 19:09:34 EST


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 :)

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