On Mon, 18 Nov 2024 21:48, David Lechner <dlechner@xxxxxxxxxxxx> wrote: >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). Hi Jonathan, I was wondering if you had any opinions on this RFC? Thx, Guillaume.
