at42qt1481 ATMEL Corporation, at42qt1481 Datasheet - Page 41

• Manufacturer Part Number: at42qt1481
• Description: 48-key Qmatrix Ic
• Manufacturer: ATMEL Corporation

9621B–AT42–06/11
Figure 5-1.
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) causes 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 made asymmetric if desired; 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 Setups parameters. This can be done on a per-key basis.
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 touch
pad. However, an obstruction over the sense pad, for which the sensor has already made full
allowance, could suddenly be removed leaving the sensor with an artificially suppressed
reference level and thus become insensitive to touch. In the latter case, the sensor should
compensate for the object's removal by raising the reference level relatively quickly.
Drift compensation and the detection time-outs work together to provide for robust, adaptive
sensing. The time-outs provide abrupt changes in reference calibration depending on the
duration of the signal event.
NDRIFT and PDRIFT are effective while the part is awake. If sleep is enabled, SDC must also
be configured so drift compensation operates at the desired rate.
NDRIFT Typical values:
NDRIFT Default value:
PDRIFT Typical values:
PDRIFT Default value:
Threshold
Hysteresis
Output
Thresholds and Drift Compensation
Signal
Reference
9 to 11
(2 to 3.3 seconds per count of drift compensation translation via
LUT,
10
(2.5s / count of drift compensation)
9 to 11
(2 to 3.3 seconds per count of drift compensation;
translation via LUT,
10
(2.5s / count of drift compensation)
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