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
12
9634E–AT42–06/12
AT42QT2120
In standalone mode the detect threshold is set to a fixed value of 10 counts of change with
respect to the internal reference level for the guard channel and 10 counts for the other six keys
(including proximity channel). The reference level has the ability to adjust itself slowly in
accordance with the drift compensation mechanism.
The drift mechanism will drift toward touch at a rate of 160 ms x 20 = 3.2 seconds (Towards
Touch Drift (TTD) register) and away from touch at a rate of 160 ms x 5 = 0.8 seconds (Away
from Touch Drift (ATD) register). These values are fixed in standalone mode but can be
configured in comms mode see Section 5.10 on page 22.
2.11.2 Detect Integrator
The device features a fast detection integrator counter (DI filter), which acts to filter out noise at
the small expense of a slower response time. The DI filter requires a programmable number of
consecutive samples confirmed in detection before the key is declared to be touched. The
minimum number for the DI filter is 1. A setting of 0 for the DI also defaults to 1. The DI has a
maximum usable value of 32. Values above this will prevent a key from entering touch.
The DI is also implemented when a touch is removed.
2.11.3 Cx Limitations
The recommended range for key capacitance Cx is 1 pF – 30 pF. Larger values of Cx will give
reduced sensitivity.
2.11.4 Touch Recalibration Delay
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 Touch Recalibration Delay (TRD) and is set to approximately 30 s in standalone
mode.
In comms mode this feature can be changed by setting a value in the range 1 – 255 (160 ms –
40,800 ms) in steps of 160 ms. A setting of 0 disables the TRD.
TRD is a global setting and applies to all keys.
2.11.5 Away from Touch Recalibration
If a keys signal jumps in the negative direction (with respect to its reference) by more than the
Away from Touch Recalibration setting (25% of detect threshold), then a recalibration of that key
takes place.
Note: The minimum Away from Touch Recalibration is hard limited to 4 counts.