QTouch Schematic and Layout Checklist [APPLICATION NOTE]
Atmel-42094C-QTouch-Schematic-and-Layout-Checklist-ApplicationNote_072014
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For lower cost products
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Any load on GPIO pins does not source or sink more than ~1mA
If there is any load on a discrete out pin of the microcontroller, ensure that the load does not sink or source too
much current directly from chip. This can cause ground bounce which would in turn affect QTouch
measurements. Typically the current should be limited to ~1mA, but this can vary depending on the part and
sensitivity of channels. If required use transistors etc. to lower load current.
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No additional circuitry on sense lines
Placing ESD protection circuitry, MUX, etc are not recommended since they add parasitic capacitance and may
introduce noise. Placing option resistors on the sense lines to connect the pins to different circuitry is acceptable.
But the sensitivity of the channel needs to be re-tuned if options change.
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Nearby LEDs have a bypass capacitor
The changing capacitance from switching LEDs can cause detection instability and stuck-on state in nearby
sensors. This is particularly true if LEDs are pulled down or up to switch on, but are allowed to float when off.
If such LEDs are less than 4mm away from capacitive sensors, they must be bypassed with a capacitor that has
a typical value of 1nF.
Bypass capacitor does not need to be physically close to the LED.
Figure 2-8.
Bypass Capacitors on Nearby LEDs