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Thread: On Teensy 3.x, should touchRead use AGND or GND?

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    Senior Member+ MichaelMeissner's Avatar
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    On Teensy 3.x, should touchRead use AGND or GND?

    On the Teensy 3.x, if I was doing touch sensing on the touch pins (0-1, 15-19, 22-23, 25, 32-33) would it be better to use AGND or GND for the ground connection? Obviously, for the LC it is a non-issue, since the LC does not have AGND.

    Also, can we get at least a minimal write up in the library page (http://www.pjrc.com/teensy/td_libs.html) for touchRead? Other than the reference in the pinout card (and of course the source code), there appears to be no PJRC documentation for it.
    Last edited by MichaelMeissner; 12-28-2015 at 02:25 PM.

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    Senior Member Constantin's Avatar
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    A documentation write-up would be a good idea. I have not played with the parameters in real life to see how they react.

    For my humidifier project I am using a cap sense circuit to determine water level heights. I had the readings bounce all over the place until I realized that merely touching the arduino, attaching a laptop to the USB port, etc. caused major swings in the readings. A good ground is essential for stable readings. I abused the heating system / water pipes in the house since I know they are GNDED well. This level of GND'ng is not necessary for handheld applications, however, just water level sensing. Obviously, this could expose the teensy to GND imbalances during lightning strikes... oh well, there goes the Teensy!

    As for handheld applications, I suggest using a black case as they usually contain a lot of carbon black (conductive) to color them. Then expose a GND pad to touch the case. Be aware, however, that the readings may change as a function of whether something is attached to the Teensy or not. That is, either implement a calibration routine where the Teensy establishes baselines as it goes along (which can be difficult) or rig the teensy up in a manner that represents its final incarnation (to keep things constant).

    As for AGND vs. GND, I seem to recall reading in the Freescale manual that the touch library does not use the ADC-related ground pins. I'm happy to be wrong, however!
    Last edited by Constantin; 12-28-2015 at 02:06 PM.

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