On Thursday, November 2, 2017 10:04:01 AM CET Maciej Purski wrote: > On 10/14/2017 08:27 PM, Stefan Bruens wrote: > > On Montag, 9. Oktober 2017 11:29:43 CEST Maciej Purski wrote: > >> On 10/01/2017 09:48 PM, Stefan Brüns wrote: > >>> According to the ABI documentation, the shunt resistor value should be > >>> specificied in Ohm. As this is also used/documented for the MAX9611, > >>> use the same for the INA2xx driver. > >>> > >>> This poses an ABI break for anyone actually altering the shunt value > >>> through the sysfs interface, it does not alter the default value nor > >>> a value set from the devicetree. > >>> > >>> Minor change: Fix comment, 1mA is 10^-3A. > >> > >> I have just a minor issue. There could be an inconsistency with units as > >> in > >> my patch I make current_lsb adjustable and I need it to be in uA (it used > >> to be hardcoded as 1 mA so to achieve better precision we need smaller > >> units). So in order to keep calibration register properly scaled, I > >> convert > >> uOhms to mOhms on each set_calibration(). So if both my changes and your > >> changes were applied, on each shunt_resistore_store we would be > >> performing > >> multiplication by 10^6 and then in set_calibration() division by 10^3 > >> which > >> seems odd to me. > >> > >> I guess we could keep it as shunt_resistor_ohms instead of > >> shunt_resistor_uohm. We could avoid performing division on each > >> shunt_resistor_show() and perform multiplication by 10^3 only once in > >> set_calibration() on each > >> shunt_resistore_store(). We could then change the default value and > >> perform > >> division only on probing, when reading the shunt_resistance from device > >> tree. > >> > >> There are many other options. It's not a major issue so maybe we could > >> leave it as it is or you could suggest some changes in my patch. > > > > Sorry it took me so long to answer ... > > > > The current fixed current_lsb of 1mA is indeed a bad choice for everything > > but a shunt resistor value of 10mOhm, as it truncates the current value. > > So what is a *good* choice? > > > > One important point is the current register is merely more than a > > convenience register. At least for the INA219/220, it provides nothing > > not achievable in software, and for the INA226 family it only has added > > value if the current is varying faster than the readout frequency and the > > averaging is used. > > > > The precision of the current register is limited by the precision of the > > shunt voltage register, and may be reduced by the applied > > scaling/calibration factor. > > > > The precision of the shunt voltage register is fixed at 10uV (INA219) > > resp. > > 2.5uV (INA226). Changing conversion time (both) and PGA (219) affects the > > noise and offset, but the lsb value is still fixed. > > > > If one wants to carry over the shunt voltage register precision into the > > current register, its important no (or hardly any) truncation happens. The > > terms therefor are given in the manual, formulas 8.5.1 (4) resp 7.5.1 (3): > > > > INA219: current = shunt_voltage * cal_register / 4096 > > INA226: current = shunt_voltage * cal_register / 2048 > > > > So any cal value smaller than 4096 (2048) will introduce truncation > > errors, > > larger values may introduce overflows, if the full input range is used. > > Now, would it not be wise to always use 4096 (2048) for the calibration > > value? > > > > The raw values from the IIO subsystem are meaningless without their > > accompanying scale factor. Instead of changing the calibration value, why > > not just change the reported scale factor? > > > > More opinions are very welcome. > > > > Kind regards, > > > > Stefan > > Thanks for the reply. > > I agree that cal_register set to 4096 (2048) allows us to eliminate > truncaction error. However according to your suggestion, if we made cal_reg > a fixed value, then current_lsb and r_shunt should be also a fixed value, > as they are related according to formula 8.5 (1) > > cal_register = 0.00512 / (current_lsb * r_shunt) A fixed cal_register only means the current_lsb is implied by the selected shunt resistor value. If you insert 2048 into the equation above, you get: current_lsb = 2.5 * 1e-6 * r_shunt, and using Ohms law to replace r_shunt, thats exactly the resolution of the shunt_voltage register as specified in the datasheet. The higher the shunt resistor value, the smaller the current_lsb. > Therefore, changing the scale value wouldn't affect the calib_reg value, so > it wouldn't give the user any information on the actual current_lsb of the > device. The real value is calculated like this by the user: > > processed_value = raw_value * scale > > I think that even after changing the scale value processed_value is expected > to be approximately the same. A fixed cal_register means you change the current_lsb by changing the shunt resistor. This exposes the full ADC resolution. The current_lsb *is* the scale value. Kind regards, Stefan -- Stefan Brüns / Bergstraße 21 / 52062 Aachen home: +49 241 53809034 mobile: +49 151 50412019
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