AT42QT2120 Datasheet
Table Of Contents
- Features
- 1. Pinouts and Schematics
- 2. Overview
- 3. Wiring and Parts
- 4. I2C-compatible Communications (Comms Mode Only)
- 5. Setups
- 5.1 Introduction
- 5.2 Address 0: Chip ID
- 5.3 Address 1: Firmware Version
- 5.4 Address 2: Detection Status
- 5.5 Addresses 3 – 4: Key Status
- 5.6 Address 5: Slider Position
- 5.7 Address 6: Calibrate
- 5.8 Address 7: Reset
- 5.9 Address 8: Low Power (LP) Mode
- 5.10 Address 9 – 10: Toward Touch and Away from Touch Drift (TTD, ATD)
- 5.11 Address 11: Detection Integrator (DI)
- 5.12 Address 12: Touch Recal Delay (TRD)
- 5.13 Address 13: Drift Hold Time (DHT)
- 5.14 Address 14: Slider Options
- 5.15 Address 15: Charge Time
- 5.16 Address 16 – 27: Detect Threshold (DTHR)
- 5.17 Addresses 28 – 39: Key Control
- 5.18 Addresses 40 – 51: Pulse/Scale for Keys
- 5.19 Address 52 – 75: Key Signal
- 5.20 Address 76 – 99: Reference Data
- 6. Specifications
- Appendix A. I2C-compatible Operation
- Associated Documents
- Revision History
23
9634E–AT42–06/12
AT42QT2120
Figure 5-1. Thresholds and Away From Touch Drift
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 increases due to capacitance being added to Cx. An
isolated, untouched foreign object (a coin, or a water film) will cause the signal to drop very
slightly due to an enhancement of coupling.
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 TTD and ATD Setup
parameters. This is a global configuration.
Specifically, drift compensation should be set to compensate faster for decreasing signals than
for increasing signals. Increasing signals should not be compensated quickly, since an
approaching finger could be compensated for partially or entirely before even touching the
touchpad (Toward Touch Drift (TTD)).
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 this latter case, the sensor should
compensate for the object's removal by lowering the reference level relatively quickly (Away
from Touch Drift (ATD)).
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'.
If ATD or TTD is set to 0 then the drift compensation in the respective direction is disabled.
Note: it is recommended that the drift compensation rate be more than four times the LP mode
period. This is to prevent undersampling, which decreases the algorithm's efficiency.
Default TTD: 20 (3.2 s / reference level)
Default ATD: 5 (0.8 s / reference level)
Threshold
Signal
Hysteresis
Reference
Output