linux-kernel - Re: [PATCH 2/2] hwmon: (ina2xx) Make calibration register value fixed

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Message-ID: <1765425.OcFVFKOaMc@pebbles>
Date:   Sat, 25 Nov 2017 21:49:14 +0100
From:   Stefan Brüns <stefan.bruens@...h-aachen.de>
To:     Guenter Roeck <linux@...ck-us.net>
CC:     Maciej Purski <m.purski@...sung.com>,
        Jonathan Cameron <jic23@...nel.org>,
        <linux-iio@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-hwmon@...r.kernel.org>,
        Javier Martinez Canillas <javier@....samsung.com>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Hartmut Knaack <knaack.h@....de>,
        "Jean Delvare" <jdelvare@...e.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Subject: Re: [PATCH 2/2] hwmon: (ina2xx) Make calibration register value fixed

On Saturday, November 25, 2017 7:50:16 PM CET Guenter Roeck wrote:
> On 11/22/2017 07:32 AM, Maciej Purski wrote:
> > Calibration register is used for calculating current register in
> > hardware according to datasheet:
> > current = shunt_volt * calib_register / 2048 (ina 226)
> > current = shunt_volt * calib_register / 4096 (ina 219)
> > 
> > Fix calib_register value to 2048 for ina226 and 4096 for ina 219 in
> > order to avoid truncation error and provide best precision allowed
> > by shunt_voltage measurement. Make current scale value follow changes
> > of shunt_resistor from sysfs as calib_register value is now fixed.
> > 
> > Power_lsb value should also follow shunt_resistor changes as stated in
> > datasheet:
> > power_lsb = 25 * current_lsb (ina 226)
> > power_lsb = 20 * current_lsb (ina 219)
> > 
> > Signed-off-by: Maciej Purski <m.purski@...sung.com>
> 
> Setting the calibration register to a specific value may optimize precision,
> but limits the supported value range, which is the whole point of providing
> a calibration register. What am I missing here ?

For the current register, any different calibration register value is 
completely useless - smaller values just truncate the register value, larger 
values adds noise in the lsbs. Both registers (current/shunt voltage) are 
16bit, and nothing is going to change that.

There is a *very* small allowed power operating area where scaling down would 
be of any use:
1. Bus voltage exceeds 25 (20) Volts
2. Shunt voltage is about 82 (320) mV

In this case the power register overflows. More specifically, if:
(shunt_voltage [mv] / 81.92) * (bus_voltage [V] / 25) > 1

Maximum allowed bus voltage is 36 V resp 26 V. (ina226/219).

Kind regards,

Stefan

-- 
Stefan Brüns  /  Bergstraße 21  /  52062 Aachen
home: +49 241 53809034     mobile: +49 151 50412019
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