Re: [PATCH RFC] iio: mxs-lradc: add scaling to enable divide_by_two operation

[Alexandre Belloni <alexandre.belloni@free-electrons.com>]

Hi,

On 04/07/2013 10:58, Hector Palacios wrote:
> 
> The field of the register that stores the sample is 18 bits, but each
> sample is only 12 bits. The read_raw of a given channel will only read
> one sample and thus returns a value between 0 and 4095.
> If I use realbits here it will divide the Vref_mv by (1 << 18) and will
> give me a wrong scale value.
> Either we change the 'realbits' to 12 which is the real resolution of
> the ADC, or we leave it as 18 and use a 12 in this operation.
>

As said I didn't have a deep look at it so you are probably right.

>> Using IIO_VAL_FRACTIONAL_LOG2 allows you to avoid doing that computation
>> in your driver (sometimes, it is actually difficult to get it right
>> because you have to use 64 bits numbers).
>>
>> Also, it simplifies things a bit, you can then use something like:
>>
>> case IIO_CHAN_INFO_SCALE:
>> *val = lut[chan->channel];
>> *val2 = chan->scan_type.realbits - is_divide_by_two;
>> return IIO_VAL_FRACTIONAL_LOG2;
> 
> Yes, but not all channels have a by two divisor. Some have a by 4, so
> I'll need to work out a formula per channel divisor.
> When using IIO_VAL_FRACTIONAL_LOG2, val must be given in mV?
>

val is in mV indeed.

My first go at it was something along the lines:

*val = 1850;
*val2 = 12 - divider[chan->channel] - is_divide_by_two;

But as Lars suggested you could also have something like:

*val = Vmax[chan->channel];
*val2 = 12 - is_divide_by_two;



-- 
Alexandre Belloni, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com