linux-kernel - RE: [PATCH v5] iio: adc: add new lp8788 adc driver

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Date:	Sat, 15 Sep 2012 00:15:01 +0000
From:	"Kim, Milo" <Milo.Kim@...com>
To:	Lars-Peter Clausen <lars@...afoo.de>
CC:	Jonathan Cameron <jic23@...nel.org>,
	Jonathan Cameron <jic23@....ac.uk>,
	"linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v5] iio: adc: add new lp8788 adc driver

> > The LP8788 has 13 ADC input selection.
> >
> > ADC selection:
> > Battery voltage, general ADC1 and so on.
> >
> > ADC result:
> > Result = MAX_VALUE * (raw + 0.5) / 4095  except ADC is the charger
> voltage
> > If the ADC input is the charger voltage,
> > Result = MAX_VALUE * (raw + 0.5) / (4095 * 0.48)
> >
> > The raw value is from the registers.
> > It has the range between 0 to 4095. (12bits)
> >
> > MAX_VALUE is constant for each selection.
> > For the battery voltage, there are three ADC inputs. 5.0/5.5/6.0V
> > Battery voltage for Max 5.0V = 5.0
> > Battery voltage for Max 5.5V = 5.5
> > Battery voltage for Max 6.0V = 6.0
> > Charger = 6.0
> > ADC1 = 2.5
> >
> > I'm afraid I still misunderstand how IIO ADC works.
> > Could you me some guide how to setup the scale in the driver?
> 
> The scale is a fixpoint value, which should be multiplied with the raw
> value
> to get the result in the proper unit. The unit depends on the channel
> type,
> e.g. for voltage it is mV and for temperature it is C. The number of
> decimal
> places for the fixed point value depends on whether you return
> INT_PLUS_MICRO it's 6, if you return INT_PLUS_NANO it is 9. The digits
> before the decimal point are stored in "val" the digits after the
> decimal
> point are stored in "val2".
> 
> E.g. if you have
> *val = 1;
> *val2 = 1256;
> return INT_PLUS_MICRO;
> 
> your scale factor is 1.001256, if you'd return INT_PLUS_NANO you scale
> factor would be 1.0000001256 instead.
> 
> In your case you could for example calculate the voltage scales as:
> 
> tmp = MAX_VALUE * 1000000 / 4095;
> *val = tmp / 1000000;
> *val2 = tmp % 1000000;
> 
> This assumes that MAX_VALUE is in millivolt.
> 
> E.g. if MAX_VALUE is 5.0V you should get a scale of 1.220703 (val = 1,
> val2
> = 220703). Since your MAX_VALUE is fixed you can probably just pre-
> calculate
> the result of MAX_VALUE * 1000000 / 4095 for each channel, similar like
> you
> already did with your lp8788_scale table.

Superb! Thanks a lot for your clear explanation.

I have one more question about the temperature ADC.

The LP8788 has no dedicated temperature ADC, but it provides four
general inputs - ADC1 to 4.
The IIO consumer can get the battery temperature among general ADC inputs.
I think the ADC driver should not assume that ADC1..4 is for the temperature,
because the selection is configurable in the platform side.

Then how can I set the unit of ADC MAX_VALUE?
I would set the unit of ADC MAX_VALUE to mili (same as voltage) and
the result is converted by the IIO consumer manually.
Does it make sense ?

Best Regards,
Milo

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