AT42QT2120 Datasheet

11
9634E–AT42–06/12
AT42QT2120
2.9 Calibration
Writing a nonzero value to the calibration byte can force a recalibration at any time. This can be
useful to clear out a stuck key condition after a prolonged period of uninterrupted detection. A
calibration command executes 15 burst cycles at LPM 1 and sets the CALIBRATE bit of the
Detection Status register during the calibration sequence.
Note: A calibration command should be sent whenever Key Control bit 0 (EN) is changed.
This changes the use of the key from a standard touch key to an output pin and
vice-versa.
2.10 Guard Channel
A guard channel to help prevent false detection is available in both modes. This is fixed on key 1
for standalone mode and programmable for comms mode by setting Key Control bit 4 (GUARD)
(see Section 5.17 on page 26).
Guard channel keys should be more sensitive than the other keys and physically bigger.
Because the guard channel key is physically bigger it becomes more susceptible to noise so it
should have a higher Oversampling (see Section 5.18 on page 27) than the other keys. In
standalone mode it is assigned to key 1 and cannot be changed.
In comms mode any key can be selected to be a guard key by setting Key Control bit 4
(GUARD).
The guard channel is connected to a sensor pad which detects the presence of touch. Because
of its larger size and sensitivity it goes into touch before the keys it surrounds (if, for example, a
hand covers all the keys).
Figure 2-1. Guard Channel Example
2.11 Signal Processing
2.11.1 Detect Threshold
The device detects a touch when the signal has crossed a threshold level and remained there
for a specified number of counts (see Section 5.11 on page 24). This can be altered on a
key-by-key basis using the key Detect Threshold I
2
C-compatible commands.
This detect threshold is based on the reference value of the particular key. The delta of the key
is obtained by subtracting the reference value from the signal value (the signal value rises when
touch is present).
Guard channel