at42qt2160 ATMEL Corporation, at42qt2160 Datasheet - Page 28

• Manufacturer Part Number: at42qt2160
• Description: Qslide?, 16-key Qmatrix? Sensor Ic
• Manufacturer: ATMEL Corporation

7.13
28
Address 15...16: Neg/Pos Drift Compensation
AT42QT2160
Table 7-13.
Signals can drift because of changes in Cx and Cs over time and temperature. It is crucial that
such drift be compensated, else false detections and sensitivity shifts can occur.
Drift compensation (see
signal at a slow rate, but only while there is no detection in effect. The rate of adjustment must
be performed slowly, otherwise legitimate detections could be ignored. The parameters can be
configured in increments of 0.16s.
Figure 7-1.
The device drift compensates using a slew-rate limited change to the reference level; the
threshold and hysteresis values are slaved to this reference.
When a finger is sensed, the signal falls since the human body acts to absorb charge from the
cross-coupling between X and Y lines. An isolated, untouched foreign object (a coin, or a water
film) will cause the signal to rise very slightly due to an enhancement of coupling. This is contrary
to the way most capacitive sensors operate.
Once a finger is sensed, the drift compensation mechanism ceases since the signal is
legitimately detecting an object. Drift compensation only works when the signal in question has
not crossed the negative threshold level.
The drift compensation mechanism can be asymmetric; the drift-compensation can be made to
occur in one direction faster than it does in the other simply by changing the NDRIFT and
PDRIFT Setup parameters. This is a global configuration.
Specifically, drift compensation should be set to compensate faster for increasing signals than
for decreasing signals. Decreasing signals should not be compensated quickly, since an
approaching finger could be compensated for partially or entirely before even touching the
touchpad (NDRIFT).
Threshold
Hysteresis
Address
Output
15
16
Neg/Pos Drift Compensation
Thresholds and Drift Compensation
b7
0
0
Signal
Figure
Reference
b6
7-1) is performed by making the reference level track the raw
b5
b4
NDRIFT
PDRIFT
b3
b2
b1
9502A–AT42–07/08
b0