Re: [PATCH v5] iio: adc: add new lp8788 adc driver

[Jonathan Cameron <jic23@kernel.org>]

On 09/15/2012 01:15 AM, Kim, Milo wrote:
>>> 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 ?
Yes, if there is a temperature sensor attached to an ADC channel it
is not up to the ADC driver to know this, but rather to the consumer driver.
> 
> Best Regards,
> Milo
>