AT42QT2120 Datasheet

13
9634E–AT42–06/12
AT42QT2120
2.11.6 Drift Hold Time
Drift Hold Time (DHT) is used to restrict drift on all keys while one or more keys are activated.
DHT restricts the drifting on all keys until approximately four seconds after all touches have been
removed.
This feature is particularly useful in cases of high-density keypads where touching a key or
hovering a finger over the keypad would cause untouched keys to drift, and therefore create a
sensitivity shift, and ultimately inhibit touch detection. In Comms mode this value is settable, see
Section 5.13 on page 24.
The QT2120 will remain in fast mode (LP = 1) for the duration of the DHT counter. The total DHT
time is 160 ms × DHT value. The default setting for DHT is 25, so 160 ms × 25 = ~4 seconds.
The QT2120 will not drift or re-enter slow LP mode during this time.
2.11.7 Hysteresis
Hysteresis is fixed at 12.5% of the Detect Threshold. When a key enters a detect state once the
DI count has been reached, the Detect threshold (DTHR) value is changed by a small amount
(12.5% of DTHR) in the direction away from touch. This is done to effect hysteresis and so
makes it less likely a key will dither in and out of detect. DTHR is restored once the key drops out
of detect.
Note: The minimum value for hysteresis is 2 counts.
3. Wiring and Parts
3.1 Rs Resistors
Series resistors Rs (Rs0 – Rs11 for comms mode and Rs0 – Rs6 for standalone mode) are
in line with the electrode connections and should be used to limit electrostatic discharge (ESD)
currents and to suppress radio frequency interference (RFI). Series resistors are recommended
for noise reduction. They should be approximately 4.7 k to 20 k each. For maximum noise
rejection the value may be up to 100 k. Care should be taken in this case that the sensor keys
are fully charged. The Charge Time may need to be increased (see Section 5.15 on page 26).
Each count increase will extend the charge pulse by approximately 1 µs.
3.2 LED Traces and Other Switching Signals
Digital switching signals near the sense lines induce transients into the acquired signals,
deteriorating the signal-to-noise (SNR) performance of the device. Such signals should be
routed away from the sensing traces and electrodes, or the design should be such that these
lines are not switched during the course of signal acquisition (bursts).
LED terminals which are multiplexed or switched into a floating state, and which are within, or
physically very near, a key (even if on another nearby PCB) should be bypassed to either Vss or
Vdd with at least a 10 nF capacitor. This is to suppress capacitive coupling effects which can
induce false signal shifts. The bypass capacitor does not need to be next to the LED, in fact it
can be quite distant. The bypass capacitor is noncritical and can be of any type.
LED terminals which are constantly connected to Vss or Vdd do not need further bypassing.