On Sun, Jun 26, 2022 at 2:20 PM Jonathan Cameron <jic23@xxxxxxxxxx> wrote: > > From: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx> > > Address an ABI gap for device where the offset of both lines in a > differential pair may be controlled so as to allow a wider range of > inputs, but without having any direct effect of the differential > measurement. > > _offset cannot be used as to remain in line with existing usage, > userspace would be expected to apply it as (_raw + _offset) * _scale > whereas _zeropoint is not. i.e. If we were computing the differential > in software it would be. > ((postive_raw + _zeropoint) - (negative_raw + zeropoint) + _offset) * _scale > = ((postive_raw - negative_raw) + _offset) * _scale > = (differential_raw + _offset) * _scale > > Similarly calibbias is expected to tweak the measurement seen, not > the adjust the two lines of the differential pair. > > Needed for in_capacitanceX-capacitanceY_zeropoint for the > AD7746 CDC driver. ... > +What: /sys/.../iio:deviceX/in_capacitanceY-capacitanceZ_zeropoint > +KernelVersion: 5.19 5.20? > +Contact: linux-iio@xxxxxxxxxxxxxxx > +Description: > + For differential channels, this an offset that is applied > + equally to both inputs. As the reading is of the difference > + between the two inputs, this should not be applied to the _raw > + reading by userspace (unlike _offset) and unlike calibbias > + it does not affect the differential value measured because > + the effect of _zeropoint cancels out across the two inputs > + that make up the differential pair. It's purpose is to bring makes > + the individual signals, before the differential is measured, > + within the measurement range of the device. The naming is > + chosen because if the separate inputs that make the > + differential pair are drawn on a graph in their > + _raw units, this is the value that the zero point on the > + measurement axis represents. It is expressed with the > + same scaling as _raw. -- With Best Regards, Andy Shevchenko
