MSP430FR2512: Touch on metal and proximity detection?

Hi Anthonly, thank you for reaching out with your questions regarding metal touch and the MSP430FR2512.

It is possible to do proximity detection with metal touch. By theory, the metal surface creates one plate of a capacitor while the self capacitive sensing electrode forms the other. The metal surface must be grounded in order for this to work.

To achieve proximity with this same metal surface, it must be allowed to float (not tied to ground) and is used as an electrode to measure changes in capacitance as your hand or finger comes near the surface. Connecting this to a self capacitive channel and you have created a proximity sensor with a large surface area. The trick is how to switch back and forth between the two scenarios. It turns out that when any CapTIvate channel is not being actively scanned, you can program the default "off" state to be either floating or grounded. BTW, the default state is grounded.

Here is how this would work.
Let's say you have an electrode (copper pattern on a pcb representing the button ) and a metal plate just above the surface of the electrode.
In the CapTIvate design center you will create two sensors, both self capacitive. Make the first one a button, the other a proximity sensor. The pcb copper electrode "button" sensor is connected to one CapTIvate channel representing the button, the metal plate is connected to another one representing the proximity sensor.

Now while measuring the "button" electrode the second CapTIvate channel is grounded by default, tying the metal plate to ground and creating a capacitor between the "button" electrode and ground. Next, while measuring the proximity sensor, the first channel switches to ground and the metal plate is now the proximity electrode relative to the neighboring grounds in the system and environment.

Remember, the differences between the two electrodes is the "button" measures the changes in capacitance between the copper electrode on the pcb and the grounded metal plate. Pushing on the metal changes the distance between "ground" and the electrode, thus changing the capacitance. The proximity measures the change in capacitance between the floating metal plate and the surrounding environment and is why when you approach the metal surface it can detect your presence.

Now for the real world. The concern with this implementation is the CapTIvate channel connected to the metal surface is now a direct conduit for ESD strikes directly into the MSP, which is really bad and should always be avoided. Yes, you can provided ESD protective devices on this channel on the pcb to alleviate this potential problem, but in my opinion this is really asking for trouble.

Lastly, the MSP430FR25xx_26xx family with CapTIvate are very flexible, allowing you to have any combination of sensors on any block.
For the FR2512 specifically, this is no problem with only the two blocks, each with two channels since you only need two channels total so you can choose one pin from each block or both pins from same block. The Captivate design center can auto assign the best possible connections for you or you can choose them manually.

Hope this helps.