On Mon, Nov 19, 2012 at 10:33:48AM +0100, Lars-Peter Clausen wrote:
> On 11/19/2012 09:21 AM, Thierry Reding wrote:
> > Add support for reading conversion results from the ADC and provide them
> > through a single IIO channel. A proper scaling factor is also exported
> > based on the reference voltage provided by a regulator.
> >
> > Signed-off-by: Thierry Reding <thierry.reding@xxxxxxxxxxxxxxxxx>
> > ---
> [...]
> > +static int adc081c_read_raw(struct iio_dev *iio,
> > + struct iio_chan_spec const *channel, int *value,
> > + int *micro, long mask)
> > +{
> > + struct adc081c *adc = iio_priv(iio);
> > + int err, scale;
> > +
> > + switch (mask) {
> > + case IIO_CHAN_INFO_RAW:
> > + err = i2c_smbus_read_word_swapped(adc->i2c, REG_CONV_RES);
> > + if (err < 0)
> > + return err;
> > +
> > + *value = (err >> 4) & 0xff;
> > + return IIO_VAL_INT;
> > +
> > + case IIO_CHAN_INFO_SCALE:
> > + err = regulator_get_voltage(adc->ref);
> > + if (err < 0)
> > + return err;
> > +
> > + scale = err / 255;
>
> Shouldn't this be 256?
Well, the maximum value that the conversion register can contain is 255,
so if we divide by 256 we'll always be slightly off, right? Or maybe I
misunderstand what the scale factor is supposed to do. From reading the
documentation it seems like user-space is supposed to multiply the raw
value by the scale to obtain the real voltage.
> > +
> > + *value = scale / 1000000;
> > + *micro = scale % 1000000;
>
> scale for voltages is in microvolt, so I think it this is off by a factor of
> 1000.
If scale is supposed to be in microvolts, then this is off by a factor
of 1000000 since regulator_get_voltage() returns the voltage in
microvolts as well.
> For ADCs it often makes sense to use IIO_VAL_FRACTIONAL_LOG2 with the val
> being set to the reference voltage (in mV) and val2 being set to the number
> of bits.
>
> E.g in your case
>
> *val = err / 1000;
> *val2 = 8;
I'm confused, IIO_VAL_FRACTIONAL_LOG2 is supposed to return mV, but
IIO_VAL_INT_PLUS_MICRO will return uV?
And setting val2 to 8 will cause the scaling factor to be 256, and not
255, so the converted value will actually be off (as I mentioned above).
> > +
> > + return IIO_VAL_INT_PLUS_MICRO;
[...]
> > + adc->channel.type = IIO_VOLTAGE;
> > + adc->channel.info_mask = IIO_CHAN_INFO_SCALE_SHARED_BIT |
> > + IIO_CHAN_INFO_RAW_SEPARATE_BIT;
>
> nitpick: Since it is the same for each driver I'd make the channel static const.
Okay, that makes sense.
Thierry
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