On 08/29/13 13:35, Angelo Compagnucci wrote: > Hi Jonathan, > > Thank you for your reply. > > Ok, I understand your points. > > So in my case scaling is composed by rawvalue*rates_to_lsb/pga, rate > is setted by writing sampling frequency, pga is setted writing > in_voltage_scale. Sorry, just reread this and saw that I'd missed the key point you were raising. Pga is set by writing in_voltage_scale, but not with the pga value but rather with the general scale as a result of both the sampling frequency and the chosen pga scale at the current sampling frequency. Thus if a value is written and accepted to a sysfs attribute, the same value is read back. These interlinked controls can be a pain as you are discovering. The golden rule is that userspace should always reread all control attributes after ever write as anything could in theory of changed. > > 2013/8/29 Jonathan Cameron <jic23@xxxxxxxxxx>: >> Firstly keep this sort Of question on the list as it acts as a reference for others. >> >> Angelo Compagnucci <angelo.compagnucci@xxxxxxxxx> wrote: >>> Hi Jonathan, >>> >>> I carefully read your last email and I have some doubts that I want to >>> share with you. >>> I carefully read the documentation and I read the source code for the >>> most >>> other adc drivers, but really cannot understand how it works the >>> raw/scale >>> duo. >>> >> >From my understanding I have to read the raw vale, read the scale >>> value, >>> make some math and have a final value. >> Yes. That is correct >>> >>> So for the mcp3422 I should use rates_to_lsb as base and pga as a >>> multiplier. >>> I can select rates_to_lsb via sampling rate (because they are coupled) >>> and select pga via in_voltageX_scale read/write operations. >>> >>> So what I have now is wrong, because I'm applying math in scale read, >>> instead that should be done from userspace. >> >> The point is that scale should only change as a result of user intervention and so is considered a slow interface. The raw readings are a the fast path hence we keep that to the minimum. Note your scale maths can be implemented as a const lookup table. >> I would be tempted to do it that way both for speed (not crucial) and simplicity of code. >> Putting a comment in to explain the maths would make it clear where the magic numbers come from. >> >> You are right in that it would be nice to do this in user space but it is very difficult to come up with a clean abi that works in all cases! >>> >>> Am I wrong? >>> >>> Can you point me to a documentation to understand better? >> Err. Nope our documentation is awful unfortunately. >>> >>> Thank you! >> >> -- >> Sent from my Android phone with K-9 Mail. Please excuse my brevity. > > > -- 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
