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
27
9634E–AT42–06/12
AT42QT2120
GPO: If set to 0, this key is a driven-low output. If set to 1 then the output is driven high. Setting
this bit only has an effect if the EN bit is set to 1.
EN: If set to 0, indicates that this key is to be used as a touch channel. Setting this bit to 1 will
disable the key for touch use and make the channel pin an output.
Note: It is not possible to enable Channel 0 or Channel 1 as an output. Setting the GPO bit on
these channels will only have the effect of disabling the key. When a change is made to
the EN bit a calibration cycle may occur because of the change in the signal values. It is
recommended to manually initiate a calibration cycle after a change is made to the EN
bit regardless of this.
Comms Defaults: All Key Control bytes set to 0x00
Standalone Defaults: Key 0 Control byte = 0x00
Key 1 Control byte = 0x14
Key 2 – 11 Control bytes = 0x04
5.18 Addresses 40 – 51: Pulse/Scale for Keys
PULSE/SCALE: The PULSE/SCALE settings are used to set up a proximity key. In comms
mode the proximity key is set up by configuring a key’s PULSE/SCALE settings via an I
2
C bus.
In standalone mode, default settings make key 0 a proximity key. This cannot be changed.
These bits represent two numbers; the low nibble is SCALE, high nibble is PULSE.
Each acquisition cycle consists signal accumulation and signal averaging. PULSE determines
the number of measurements accumulated, SCALE the averaging factor.
The SCALE factor (averaging factor) for the accumulated signal is an exponent of 2.
PULSE is the number of measurements accumulated and is an exponent of 2.
For example:
Oversampling is used to enhance the resolution of the Analog-to-Digital-Converter (ADC).
Oversampling theory says that for each additional bit of resolution, n, the signal must be
oversampled four times (or 2
2
× n.) If two bits of addition resolution are required then the pulse
setting would be 4 (4
2
= 2
4
). If 3-bits of additional resolution are required the Pulse setting would
be 6 (4
3
= 2
6
). Here the result of each ADC pulse measurement is taken and added to the last.
The oversampling theory also states that this accumulated result must be scaled back by a
factor of 2
n
. The will be the Scale value.
Table5-19 on page28 shows some of the recommended oversampling settings
(1)
.
Table 5-18. Controls for Keys
Address Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
40 PULSE SCALE
::
51 PULSE SCALE
1.Other settings are possible but the Pulse value should never be more than six higher than the Scale
setting as the signal result is stored in a 16-bit variable.