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MSP430FR2633: CapTIvate MCU Devkit Questions -- Automatic Resets/Recalibrations and Large-Scale Design Noise Considerations

Chana Pickett
Prodigy 40 points
Part Number: MSP430FR2633

Hi,

We're working with a very large (30" x 42") mutual capacitive grid that is made up of rows and columns of copper channels attached to acrylic sheets. 

We had a few questions about some strange things we're noticing when we use it with CapTivate:

Question 1
We've noticed that sometimes when a user touches a node on the grid, CapTivate seems to re-calibrate to this new value so that when the hand is removed the node that was being touched reads as 'off' and the surrounding nodes (which were previously off) register as touches. (In other words, the outputs being read from the board invert from their previous states, kind of like a scale resetting when the 'tare' button is pressed while a weight is on top of it. When the weight is removed from the scale, it reads a negative value equal to the object's weight.)


This seems to be unrelated to the amount of time a user leaves their hand on the board, so we're thinking the reset isn't related to sensor timeout parameters. 


Are there any other automatic resets built into CapTIvate that might be causing this issue?

If so, is there a location in the auto generated code where we find them? (We'd like to set a counter to see how frequently this automatic reset happens; or to disable it altogether, if possible.) 

For some reason this doesn't seem to show up when we have a 2x2 grid, but it becomes very apparent when the grid is larger than that. 

Question 2
We're also noticing an issue where sometimes an entire row will light up and remain in a 'touch' state, even when no one is physically touching the board. 

With a large-scale board compared to PCB, are there any hardware design considerations that may contribute to these errors?  


We're concerned that working with more than 2 channels on a large scale (>4" channel spacing) may cause problems for the MCU. 

We're trying to figure out how much of our problem is hardware design error and how much might be related to tuning values within CapTIvate's software. 



Thank you very much for your time!

  • 0
    TI__Guru 56685 points
    Hello Chana,

    At first glance here, your first issue still seems like sensor timeouts are too short. What parameters do you have for Sensor Timeout Threshold and your measurement frequency?

    For your second issue, I think this is a matter of noise susceptibility and possible crosstalk. With your sensors being as big as they are, I imagine your traces/wires to the MCU are getting long. You can adjust some of your touch threshold tunings to help with this. You can also try to enable Modulation under the Conversion Control tab to help with some of the noise. Beyond that, some HW adjustments to your sensors may be needed. Some sort of shielding may be required for your wires from the MSP430 to your sensors (depending on how you are doing this now). Also see the Design Guide for more information on how to handle noise from a layout perspective. I know your panel is not a giant pcb, but some fo the same principles can be applied to the panel to help out.
  • 0 in reply to Jace H
    TI__Guru 56685 points
    Hello Chana,

    Has the above information been helpful?
  • 0
    Prodigy 40 points

    Hi Jace,

    Thank you for the responses, and sorry for the belated reply. We just got around to testing a few of the variables you mentioned.

    We were able to check that the sensor timeout threshold was set to its highest value and the measurement frequency was set to 4 MHz. Could this still create an issue with timeout parameters? (Or, if not, are there any other possible resets in the code that are unrelated to the amount of time a user touches a sensor?)


    As far as crosstalk is concerned, we did notice a lot of noise coming from the wires, so we're looking into shielded coax cables. Could the unshielded wires cause some of the crosstalk issues we're seeing? 

    There also seemed to be errors when the board was physically deformed (for example, by pressing too hard on the surface), so we are looking into adhesive sheeting (the 3M 467 MP recommended in the design guide: http://www.digikey.com/catalog/en/partgroup/467mp-and-468mp-series/6393) to see if that will fix the problem. 

    We're also potentially looking into better contacts between the channels and wires since right now we're just rolling them up into conductive copper tape that hangs off the edges of the board. 


    Another issue we are concerned about is the amount of current needed to drive such long copper channels. Do you know what the output current/voltage ranges from the board are when charging/discharging mutual capacitive grids? (We saw a tuning value of 20 uA, but weren't sure where in the design guide we could find more information about those discharging/charging current values.)


    Thanks,
    Chana

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