So I got the PCBs last week. I got 12 total even tough I had only ordered 5. Likely because they ran a white panel just for me. Thanks Seeed!
I scrambled to order the parts and ordered some wrong LEDs. I decided to solder one up without LEDs and just the bare minimum to get cap sensing done and sent over USB. I ended up soldering the ISP-header on the component side as suggested and I am keeping that side open for the time being as I like to poke around the board with a scope.
One of the first things I noticed before soldering was that I failed to take the routing bit size into account when drawing the cut-outs for the LEDs. They will still fit the LEDs fine but they don't look very nice.
So after some wrangling with the code I got the charging and discharging of the sampling capacitor working.
CH1 is the slope pin which is used for discharging the sample capacitor (and then measuring the time it takes to discharge ).
These are the pulses on the drive side.
This is what the bottom of the sampling capacitor looks like. You can see it charge up and then discharge when the slope pin goes high.
Eventually I got the AVR interrupts to work and could actually measure the time to zero crossing. I took some measurements which I can use as a baseline as I add components and change the code.See this IPython notebook for details.
But there you have it. It is sensing touch so I am pretty chuffed! I am yet to discover how robust it is.
EDIT: Code is up on github too: https://github.com/kasbah/nomech_mini-code