I’ve been working on MicroPython for ESP32 recently and working on implementing some of the crazier hardware on the device.

Capacitive touch sensor controllers are widely available, but having the facility built in making the ESP32 competitive with cheaper SoCs which don’t already include these facilities. While I’ve been looking at the software, it’s been interesting to look at the hardware and see how it works as well.

The sensors work by allowing the pin to freely oscillate, and monitoring the frequency at which it does so. Attach the pin to a conductive pad, and bring something large and conductive, like a finger, close to it, and the pin will oscillate more slowly. The controller can measure this and detect that the pad is being (almost) touched.

Each sample takes about 8ms to complete.

MicroPython

I’m programming the ESP in MicroPython which just makes the experimental setup easier. Not all features are merged in yet, so if you want to use the TouchPad from MicroPython you may need to pull in my esp32-touchpad branch

Test Setup

I started off testing the hardware with flying leads and alligator clips, but since capacitive test sensors are affected by their surroundings it’s hard to get a consistent response in this way. So I made up some a quick test setup out a Sparkfun ESP32 Thing and a bit of single-sided blank PCB cut into four zones with a rotary burr.

test pcb with tape

test pcb with module

The ESP32 board is stood off from the touch board with a layer of foam and double-sided taple, and a couple of layers of insulting tape over the top of the foil simulate putting the pads in an enclosure.

I then logged some measurements with the following MicroPython code:

… which captures 400 readings across the four pins as fast as it can, and then dumps the lot out for pasting into gnuplot. Each sample takes about 8ms, so the capture is over about 3 seconds.

Swiping a finger from GPIO15 to GPIO12 shows the way the multiple oscillators respond:

graph with insulator

The ‘resting’ frequencies of the central two pads are lower than that of the pads at the ends, presumably because they are somewhat coupled to them … you can see that touching GPIO15 has a small effect on GPIO14 as well, and GPIO13 is effected by both GPIO14 and GPIO12. GPIO12 is especially messy, I think because the blue wire connecting it runs behind the pad for GPIO13 for quite a way, inevitably coupling them. Any design using the capacitive sensors is going to have to take board layout seriously!

graph of central sensors

A closer look at just two pads (at twice the sample rate, and a larger number of samples) shows that there’s a smooth crossover between the pads, where your fingertip is close to both of them. The curves are nice and smooth, suggesting that they don’t interfere much. It might be possible to get a longer transfer between the two by interleaving tracks at the edges, perhaps even tapering them together like so:

interleaving pads

The sensors work pretty well right through the PCB, these traces show a touch on the non-copper side. Again, good PCB design practices will be needed to prevent interference!

using sensors through pcb

Seriously Retro

Just out of curiosity, I got out the trusty CRO to have a look at how the signals for the touchpads work. Photographing CROs is not exactly easy, so please forgive these lousy images:

Unfortunately, my CRO is sufficiently old as to have a fairly large effect on the system under test (t.read() was 286, like being touched!), so I’ll have to come back to this with some better test equipment.

This first one shows the pin and pad oscillating happily with no touch. At 10us per division that’s about 14ms per cycle or 71kHz. The vertical axis shows the signal going from about 1.5V to 3.7V above ground. At this point, t.read() returns about 300.

scope trace 1

The second shows the waveform while being touched, at the same setup. The frequency has reduced to about 34us per cycle or 30kHz. At this point, t.read() returns about 230.

scope trace 2

Interestingly, the oscillator output isn’t constant, but occurs in about 8ms bursts with about 30ms ‘off’ between them.

scope bursts