Messages in this thread | | | From | Maciej Purski <> | Subject | Re: [2/3] iio: adc: ina2xx: Adhere to documented ABI, use Ohm instead of uOhm | Date | Wed, 08 Nov 2017 14:22:08 +0100 |
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On 11/06/2017 11:21 AM, Stefan Brüns wrote: > 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 >
Thanks for your explanation. I can do this the way you suggest, so the only change with the original driver would be to make current_lsb (which is a scale value) follow changes of shunt_resistance value from userspace.
But before I'd like to ask Jonathan for opinion on that.
Kind regards,
Maciej
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