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MSP430FR2512: liquid tolerant application

Bart
Expert 6310 points
Part Number: MSP430FR2512
Other Parts Discussed in Thread: TIDM-1021
Team,
We have an application requiring 4 buttons, liquid tolerant. MSP430FR2512 has 4 CAP I/O.\
Due to the liquid tolerance requirement, we must use a mutual mode with which we need RX and TX electrodes working.
How to configure 4 buttons with mutual mode with 4 CAP I/O?
Thanks
  • 0
    TI__Guru 60181 points

    Hi Bart,

    Unfortunately, it may be difficult to achieve reasonable results with only the 4 Captivate pins.  You are probably already familiar with TIDM-1021, Liquid Tolerant Capacitive Touch Keypad Reference Design, (I'll share the link for other readers of this post).  It discusses the differences in self and mutual capacitance for use with liquid tolerant designs.

    Since you can't use mutual mode, about the only thing you can do in self mode to help mitigate the coupling the water creates between the electrode and nearby grounds, is to not have a ground plane on the same layer as the buttons (layer 1).  If it is a 2-layer PCB you will most likely have some type of ground plane on the opposite layer.  If so, use a 20-30% hatched ground instead of a solid-filled plane.  If it is a 4-layer, don't put any grounds on layer 2 and avoid and ground plane on layer 3 if possible.  The further you keep a ground plane away from layer 1, the better.

    Next, if the buttons are on a vertical surface, water won't have a chance to pool on the surface, so this is working in your favor, but if enough water accumulates and runs down the surface of the buttons, there will be false triggers.  If the buttons are on a horizontal surface, this is pretty much not going to work.

    If you don't mind saying, what is the application?

    What are your expectations in terms of levels of moisture?  Mist, splash, spray, continuous...?

    Is this a battery-powered application?  If not, will you need to pass EMI tests?

  • +1 in reply to Dennis Lehman
    TI__Guru 60181 points

    Hi Bart,

    I discussed your application with the author of TIDM-1021 to see if he might have some ideas, but with only 4 channels the application is limited to using self capacitive mode only.  I know using the FR2522 is not possible, but could the customer use (2) FR2512?  This way each FR2512 could control 2 buttons in mutual mode.  Of course, each would have their own TX flooded planes.  Something like this:

    The spare channels can be used as a guard channel, which will notify SW when a person is wiping down the surface for cleaning, or wiping away accumulated moisture.  The idea here is if during a wipe down one or more button may be in detect, but if guard is also in detect, SW simply ignores the button inputs until the user is done wiping down the surface.

    Also, it is suggested to synchronize the capacitive measurements by each of the two MCUs to prevent asynchronous cross coupling during their measurements.

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