Re: [RFC][PATCH] hwmon: (ina2xx) Improve current and power reading precision

Nicolin Chen
2019-01-17T22:38:33Z

On Fri, Jan 04, 2019 at 05:26:42PM -0800, Nicolin Chen wrote:
> Hi Stefan,
>
> Sorry for a super late reply. I took a long vacation.
>
> On Wed, Nov 21, 2018 at 10:16:09PM +0000, Brüns, Stefan wrote:
> > > > Another concern may be voltage drop over the shunt, but for this case you
> > > > have a nominal voltage of 1.8V, so 30uV are 0.001%.
> > > >
> > > > > When measuring a 1.8v voltage running a small current (e.g. 33 mA),
> > > > > the power value (that's supposed to be 59.4 mW) becomes inaccurate
> > > > > due to the larger scale (25mA for method A; 62.5 mA for method B).
> > >
> > > Just found out that I have typos here: 25mW and 62.5mW.
> > >
> > > > Another look into the datasheet reveals, even at full gain (PGA=1), the
> > > > LSB is 40mV / 2^12 = 40mV / 4096 ~ 10uV. So when the current ADC reads
> > > > out as 3*LSB, this anything between 25mA and 35mA. This is the best case
> > > > figure.
> > > Current read doesn't get affected a lot actually, since hwmon ABI
> > > also reports current value in unit mA. However, the power read is
> > > the matter here. With a 62.5mW power_lsb, power results are kinda
> > > useless on my system.
> >
> > The reported current does not matter here, actually. Internally, the ADC value
> > will have an uncertainty of 10mA (at PGA=1). At 1.8V, your uncertainty is
> > 18mW. And thats *only* the quantization noise. It wont get better than that.
>
> The fact is that I do get better power results after setting the
> calibration value to 0x7ff. That's the necessity of this change.
>
> > Also note, you are apparently using the ina2xx hwmon driver - I strongly
> > advise against it, you should either use the ina2xx driver from the IIO
> > subsystem directly, or use the IIO driver via iio-hwmon.
>
> The IIO version is also using the minimum calibration value. It
> will not solve my problem here.
>
> > There is also always the possibility to read the bus and shunt voltage
> > registers and calculate the power manually.
>
> Won't that be a waste since the hardware could have provided a
> better accuracy? It would need more I2C bus reads and cpu cycles
> for calculation.
>
> I don't get why you're against a setting for calibration value.
> This is how the hardware got designed to cover different cases.
> Since we do have such a case that needs some accuracy, it'd be
> fair to add it into the driver. Plus, the feature won't change
> the minimum calibration value at all -- everyone would be happy.

Stefan,

Would you please kindly give an ack to this intention so that
we can at least move forward for patch review?

Neither changing hardware resistor values nor simply ignoring
the inaccuracy is acceptable for us. Since configuring that
calibration register value can help our use cases, we really
need this setting to be available in the driver.

Guenter,

Do you have any input regarding this change? I would like to
hear an opinion from you.

Thank you