nrf: implement touchio.TouchIn

[dhalbert]

The COMP (comparator) peripheral description says it does "Single-pin capacitive sensor support".

[nickzoic]

OK, having a look into this now ... progress will be at https://github.com/nickzoic/micropython/tree/nickzoic/circuitpython-nrf-touchin-1048

Nordic API ref: https://github.com/NordicPlayground/nrf52-capsense-example

[nickzoic]

Hmmm, request from @ptorrone was:

please use the "Single-pin capacitive sensor support" - no external resistor is required then!

... but in https://devzone.nordicsemi.com/tutorials/b/design-examples/posts/capacitive-touch-on-the-nrf52-series there's the following update:

Important updates:

Due to erratum 84 (COMP: ISOURCE not functional), capacitive sensing using the relaxation oscillator method is not usable in a real product with the nRF52832, as it will only work reliably in room temperature. The ISOURCE feature is not present in the nRF52840 nor nRF52810.
After this tutorial was written, SDK 12 was released with support for capacitive touch using the Capacitive Sensor driver and Capacitive Sensor library.

That erratum being:

The programmable current source (ISOURCE) has too high varation. Variance over temp is >20 times specified nominal value
... but ISOURCE not being present at all in the '840 worries me more.

This is also mentioned at: https://www.nordicsemi.com/DocLib/Content/SDK_Doc/nRF5_SDK/v15-2-0/lib_csense_lowlevel

Note
Until Anomaly 84 (ISOURCE not functional) is fixed, the COMP solution cannot provide production-level quality. Use the SAADC solution instead.

[ladyada]

seems unclear, whats the SAADC method?

[nickzoic]

That's the version with the resistors, using another pin to charge the pads and then the saadc to measure the rate at which it discharges

The comp method may be okay if we're clever about self-calibration, I'll try it out ...

[dhalbert]

The current CircuitPython "calibration" on SAMD21 is pretty simple: when the TouchIn object is created, we assume that it's not being touched. Touch threshold is that value + 100. But TouchIn.raw_value is available for people who want or need it, and the threshold can also be adjusted.

If the nRF errata are saying that the readings are really erratic, yeah, that could be a significant issue.

[nickzoic]

On Wed, Dec 5, 2018, at 00:32, Dan Halbert wrote: The current CircuitPython "calibration" on SAMD21 is pretty simple: when the TouchIn object is created, we assume that it's not being touched. Touch threshold is that value + 100. But TouchIn.raw_value is available for people who want or need it, and the threshold can also be adjusted.> If the nRF errata are saying that the readings are really erratic, yeah, that could be a significant issue.

Yeah, it sounds like the current source drifts *a lot* with die temperature, so so will the frequency of the relaxation oscillator.The SAADC approach is a kludge around this problem by using an I/O pin + resistor as the current source. *BUT* Temperature should change relatively slowly, so assuming all the current sources drift the same amount, we can maybe get around this problem by providing *relative* touch sensing. *HOWEVER* The other concern is that the 52840 doesn't have the current source available at all, so this technique won't work. If you're planning on migrating the nRF feathers to the '40 then we'd need to use the I/O pin + resistor approach.

[ladyada]

yah we require it to work on all nrf boards. if we have to do it, we can make it a common module so any chips can use it? ive done the technique w the original cirplay board. you just need a 1mohm res and fast reading w/no interrupts, so it should be in c not python

[nickzoic]

On Wed, Dec 5, 2018, at 11:04, ladyada wrote: yah we require it to work on all nrf boards. if we have to do it, we can make it a common module so any chips can use it? ive done the technique w the original cirplay board. you just need a 1mohm res and fast reading w/no interrupts, so it should be in c not python

OK no worries, I'll scrap the ISOURCE approach and implement just the ADC approach.Yes, I think that this technique should work on pretty much any chips if their ADC is fast and precise enough.

[nickzoic]

OK so I've got as far as confirming that yes, charging and discharging A0 through a 1Mohm resistor gives enough of a signal that we could call it a touch sensor. Python code is actually (just) quick enough to catch this!

At least, I think that's what I'm seeing ... it's pretty naïve so it could also be measuring electrical noise coupled into the pin. Sadly my 'scope leads are too high capacitance to show a signal variation on the screen. I've got to try with a bigger resistance, too: it'll discharge slower but that'll give me more time to measure a difference. The C implementation should make this clearer though because I'll be able to get a bit more sophisticated with measuring the discharge rate.

A bigger problem though: the touchio.TouchIn API is all about wrapping a single Pin object. This method needs another pin as well, to be the "drive". This can be a spare digital pin, which can drive many touch pads each through a resistor. So for this version we might need to do something like:

t0 = touchio.TouchIn(pad=board.A0, drive=board.D15)
t1 = touchio.TouchIn(pad=board.A1, drive=board.D15)

... which would share D15 between the two pads on A0 and A1. A TouchIn created without a drive pin would have to work some other way (perhaps ISOURCE on the '832 if it works at all) or raise an exception on platforms with no such mechanism (like the '840)

(it occurs to me that you could use a pair of analog pins and a single resistor to provide two touch pads, by switching each pin between being an analog input and a digital output in turn. That would mean driving the capacitance of the touch pad directly though, which is a bit of a waste of electrons and increases the risk of shorting the driven pin accidentally.)

[ladyada]

yeah i like that API. i think you probably have it working but you need to do it in C for sure, you can see the arduino version of this code here:
https://github.com/PaulStoffregen/CapacitiveSensor

[nickzoic]

Yep, no worries. I should be able to get this done in the next couple of days ...