Datasheet

12
9596A–AT42–10/10
AT42QT1070
A channel set as the guard channel (there can only be one) is prioritised when the filtering of
keys going into detect is taking place. So if a normal key is filtering into touch (touch present but
DI has not been reached) and the key set as the guard key begins filtering in, then the normal
key’s filter is reset and the guard key filters in first.
The guard channel is connected to a sensor pad which detects the presence of touch and
overrides any output from the other keys.
Figure 2-1. Guard Channel Example
2.12 Signal Processing
2.12.1 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 5.10 on page 21). This can be altered on a
key-by-key basis using the key threshold I
2
C-compatible commands.
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 20 counts for the other four
keys. 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 18 = 2.88 seconds and away
from touch at a rate of 160 ms x 6 = 0.96 seconds. The 160 ms is based on 20 x 8 ms cycles. If
the cycle time exceeds 8 ms then the overall times will be extended to match.
2.12.2 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 2. Settings of 0 and 1 for the DI also default to 2. The DI is
also implemented when a touch is removed.
2.12.3 Cx Limitations
The recommended range for key capacitance Cx is 1 pF – 30 pF. Larger values of Cx will give
reduced sensitivity.
Guard channel