On 03/07/2013 18:39, Hector Palacios wrote: > Dear Lars, > > On 07/03/2013 12:46 PM, Lars-Peter Clausen wrote: >> The scale attribute reports the scale of the raw value. To gain the >> value of >> the result in mV you have to multiply the raw attribute with the scale >> attribute. The scale attribute does not return the result of a >> conversion with >> a scale different applied. > > Thank you for the guidance, I misunderstood the whole thing. > >> I suggest you implement this in two steps. First implement the scale >> property >> for all channels. Use a LUT for each channel's fullscale value (in mV) >> and >> implement reading the scale like this. >> >> case IIO_CHAN_INFO_SCALE: >> *val = lut[chan->channel]; >> *val2 = chan->scan_type.realbits; >> return IIO_VAL_FRACTIONAL_LOG2; >> >> The next step is to make the scale modifiable for channels which >> support this. > > I'm sorry, it's the first time I work with an iio driver and I'm afraid > I don't understand this. > Let's say the fullscale value for channel 0 is 1.85V by default (when > divide_by_two is disabled) and 3.7V (when enabled). > > The channel's realbits is currently set to 18, because (although the ADC > resolution is 12 bits) the device can read several samples and return an > accumulated value in 18 bits. > > Now, considering the default case where 1.85V is the max ref voltage and > considering that the driver only reads one sample during the raw_read > operation (12 bits), what value should a read of 'scale' return? > > scale = Vref_mv / ((1 << 12) - 1) = 1850 /4095 = 0.45177045 ? > That's right. Although, I would think that all the values are on 18bits, so you probably want to use chan->scan_type.realbits there. 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; > That is, the value to multiply the raw value to get the voltage in mV? > So that if reading the 'raw' returns say 425 and reading scale returns > 0.45177 the measured voltage is 425 * 0.45177 = 192mV? > Right. > > Then let's say I have implemented a function to modify the scale (which > should eventually allow me to enable/disable the by two divisor). What > is the user supposed to write to the 'scale'? Is he supposed to supply a > decimal number like 0.45177 * 2 = 0.903? This would be pretty odd. > Should I create a table instead so that writing a 0 means divisor > disabled and writing 1 means divisor enabled, for example? > That is why you should provide an in_voltageX_scale_available file, providing a list of valid values. > Does the iio provide any descriptor that automatically shows the value > in mV (already converted)? > You can use IIO_CHAN_INFO_PROCESSED to expose that information. -- Alexandre Belloni, Free Electrons Embedded Linux, Kernel and Android engineering http://free-electrons.com -- To unsubscribe from this list: send the line "unsubscribe linux-iio" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
