Datasheet

13
9596A–AT42–10/10
AT42QT1070
2.12.4 Max On Duration
If an object or material obstructs the sense pad the signal may rise enough to create a detection,
preventing further operation. To prevent this, the sensor includes a timer which monitors
detections. If a detection exceeds the timer setting the sensor performs a key recalibration. This
is known as the Max On duration feature and is set to approximately 30s in standalone mode.
In comms mode this feature can be changed by setting a value in the range 1 – 255
(160 ms – 40800 ms) in steps of 160 ms. A setting of 0 disables the Max On Duration
recalibration feature.
Note: If bit 4 of address 53 is clear then a recalibration of all keys occurs on Max On Duration
instead of the individual key recalibration.
2.12.5 Positive Recalibration
If a keys signal jumps in the negative direction (with respect to its reference) by more than the
Positive Recalibration setting (4 counts), then a recalibration of that key takes place.
2.12.6 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.
2.12.7 Hysteresis
Hysteresis is fixed at 12.5 percent of the Detect Threshold. When a key enters a detect state
once the DI count has been reached, the NTHR value is changed by a small amount
(12.5 percent of NTHR) in the direction away from touch. This is done to affect hysteresis and so
makes it less likely a key will dither in and out of detect. NTHR is restored once the key drops out
of detect.
3. Wiring and Parts
3.1 Rs Resistors
Series resistors Rs (Rs0 – Rs6 for comms mode and Rs0 – Rs4 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.
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).