Vacuum sensing 100hz signal fix

[brainstorm]

Remove the 100Hz signal on the SS2/PB3 pin of the ATxmega TinyG MCU.

This PR is dependant on PR #1 .

[ian-arkver]

The last commit (ad38088) does not do what the comment and commit message claim it does.

The DIR register is a bitfield with one bit per pin. If a bit is '1' the pin is an output.

The effect of the statement is to clear bits 0 and 1 while leaving all the other bits unchanged.

[ian-arkver]

I'm not sure why configuring the ADC to differential input using PORTB.0 as the low input would make things more stable. This pin is the BOOT jumper. Is this jumper normally fitted to tie this pin to ground?

[brainstorm]

There's an insane amount of nuance in Atmel ADC configurations and TinyG, hang in there, we are going deep in the rabbit hole and troubleshooting this, check this out if you are interested ;)

For instance, there's another gotcha Anthony found out today:

That 2Mhz signal issue from yesterday? Gone today after removing the capacitor.

Unfortunately our sensor stops responding as soon as it's connected to the TinyG, so we do know it's a software configuration detail, but we are trying a bunch of things and commit messages might not reflect exactly what we are actually doing (i.e we were discussing about making sure that PB3 is NOT an output, I might have gotten distracted when writing the commit message and discussing the topic with the others), sorry ;)

[brainstorm]

After reflashing the "stock" firmware from Mark with ADC and the 100Hz signal and fine tuning the cap and resistor the vacuum sensing works fine, closing.