On 08/02/11 09:11, Michael Hennerich wrote: > On 08/01/2011 07:21 PM, Jonathan Cameron wrote: >> Hi Michael / All, >> >> We have a quite a few capacitance drivers. They are all simple and so >> would be easy to clean up, except for the fact that we don't have an abi >> for them. >> >> So lets make one up. How about the following? Main choice is the units... >> Doing it with Farads leaves us with a lot of needed decimal places, but then >> we need a lot anyway for these so what the heck. We are going to need those >> types with holes in them.. > > Hi Jonathan, > > These devices typically feature an single digit pF input range(2..8 pF). > Going with a scale in Farads is probably not going to work. > What do you mean by types with holes in them? IIO_VAL_INT_PLUS_FEMTO where FEMTO bit has maximum of 999999999 Thus there are quite a few leading zeros. > We need something scanf() and friends can eat... Yup. > In addition we have enough scale bits before the decimal point as well. Please give an example why. Do we have a calibscale type attribute here? > I would make the scale targeting pF of uF. I thought about that but then we are back to having a fairly illogical set of units for the different types of devices. > > >> What: /sys/bus/iio/devices/iio:deviceX/in_capacitanceY_raw >> What: /sys/bus/iio/devices/iio:deviceX/out_capacitanceY_raw >> What: /sys/bus/iio/devices/iio:deviceX/in_capacitanceY-capacitanceZ_raw >> KernelVersion: 3.1 >> Contact: linux-iio@xxxxxxxxxxxxxxx >> Description: >> Raw (unscaled no bias removal etc) capacitance value from/to >> channel Y. After application of _offset and _scale, units will >> be Farads. >> >> Additional entries for: >> >> What: /sys/bus/iio/devices/iio:deviceX/in_capacitanceY_offset >> What: /sys/bus/iio/devices/iio:deviceX/in_capacitanceY_scale >> >> >> With the ad7745 the capdac does seem to be available off chip. > Why do you think the CAPDAC is off-chip on the AD7745? > The CAPDAC can be seen as a negative capacitance connected internally > to the Cin pin. Oops. That was meant to be doesn't not does... Sorry for the confusion. > >> If I understand >> their use correctly it could just be treated as a _calibbias parameters? > Yes - it can be seen as a bias - however why do you have out_capacitanceY_raw then? :) Because I hadn't looked at the datasheet properly when I wrote that bit and forgot to go back and edit it. This really wasn't my most coherent email ever. Google isn't feeding me any digital to capacitance devices so looks like the output side of things is irrelevant. >> (the complexity being that there are two..) > Yes - one for each Cin(+|-) pin. > More tricky on the AD7746, since there are only two CAPDACs for two Cin(+|-) pin pairs. As you state below that you tend to save and restore the zero offset, can we not do the same with the capdac values? If so they can be treated from a userspace point of view as completely independent. > >> Naturally there is also a >> calibration register so this gets somewhat tricky... > The calibration register typically holds the zero-scale calibration coefficient. > It's automatically updated following the capacitance offset calibration. > Tricky here is that there is only one on the AD7746, so the values must be saved > and restored when switching between the input pairs. > >> Is there an optimum >> combination for a given desire measurement range? >> >> > Yes there is - large offsets > 1 pF should be eliminated by the CAPDACs. Fair enough. So what is the conclusion for what interface elements we actually need to control this? > > from the datasheet: > > CAPACITIVE SYSTEM OFFSET CALIBRATION The capacitive offset is > dominated by the parasitic offset in the application, such as the > initial capacitance of the sensor, any parasitic capacitance of > tracks on the board, and the capacitance of any other connections > between the sensor and the CDC. Therefore, the AD7745/AD7746 are not > factory calibrated for capacitive offset. It is the user’s > responsibility to calibrate the system capacitance offset in the > application. Any offset in the capacitance input larger than ±1 pF > should first be removed using the on-chip CAPDACs. The small offset > within ±1 pF can then be removed by using the capacitance offset > calibration register. One method of adjusting the offset is to > connect a zero-scale capacitance to the input and execute the > capacitance offset calibration mode. The calibration sets the > midpoint of the ±4.096 pF range (that is, Output Code 0x800000) to > that zero-scale input. Another method would be to calculate and write > the offset cali-bration register value, the LSB is value 31.25 aF > (4.096 pF/217). The offset calibration register is reloaded by the > default value at power-on or after reset. Therefore, if the offset > calibration is not repeated after each system power-up, the > calibration coefficient value should be stored by the host controller > and reloaded as part of the AD7745/AD7746 setup. On the AD7746, the > register is shared by the two capacitive channels. If the capacitive > channels need to be offset calibrated individually, the host > controller software should read the AD7746 capacitive offset > calibration register values after performing the offset calibration > on individual channels and then reload the values back to the AD7746 > before executing a conversion on a different channel. >> All comments welcom. >> Jonathan >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-iio" in >> the body of a message to majordomo@xxxxxxxxxxxxxxx >> More majordomo info at http://vger.kernel.org/majordomo-info.html >> > > -- To unsubscribe from this list: send the line "unsubscribe linux-iio" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
