On 11/15/24 4:34 AM, Guillaume Ranquet wrote: > Some chips of the ad7173 family supports open wire detection. > > Generate a threshold event whenever an external source is disconnected > from the system input on single conversions. > > Signed-off-by: Guillaume Ranquet <granquet@xxxxxxxxxxxx> > --- > Hi. > > This patch adds the openwire detection support for the ad4111 chip. > > The openwire detection is done in software and relies on comparing the > results of two conversions on different channels. > > As presented here, the code sends a THRESH Rising event tied to the > in_voltageX_raw channel on which it happened. > > We think this is not correct but we can't seem to find an implementation > that would be elegant. > > The main idea would be to add a specific channel for openwire detection > but we still would need to have the openwire enablement and threshold > tied to the voltage channel. > > Any thought on this? > Just to spell this out in a bit more detail, my understanding is that the procedure works like this... To detect an open wire on a single-ended input, we measure the voltage on pin VIN0 using channel register 15, the we read the voltage on the same pin again using channel register 0. The datasheet isn't clear on the details, but on one or the other or both of these, the ADC chip is applying some kind of pull up or pull down on the input pin so that each measurement will be nearly the same if there is a wire attached with a 0-10V signal on it. Or if the wire is detached and the pins are left floating, the two measurements will be different (> 300 mV). So this threshold value (the 300 mV) is measured in terms of the difference between two voltage measurements, but of the same input pin. My suggestion is to create extra differential channels like in_voltage0-involtage100_* where 100 is an arbitrary number. These channels would be defined as the difference between the two measurements on the same pin. The event attributes would use these channels since they are triggered by exceeding the threshold value according to this difference measurement. To complicate matters, these chips can also be configured so that two input pins can be configured as a fully differential pair. And we can also do open wire detection on these differential pairs. In this case we would configure input pins VIN0 and VIN1 as a differential pair, then read the difference of those two pins using channel register 1, then read again using channel register 2. The we see if the difference of the two differences exceeds the threshold. In this case, we can't have in_(voltage0-voltage1)-(voltage100-voltage101)_* attributes. So I guess we would have to do something like in_voltage200-voltage300 to handle this case? (voltage200 would represent the first measurement of voltage0-voltage1 and voltage300 would represent the 2nd measurement of the same).
