© Quantum Research Group Ltd.
the detection integrator (Section 2.6). Larger absolute values when the signal in question has not crossed the negative
of threshold desensitize keys since the signal must travel
farther in order to cross the threshold level. Conversely, lower
thresholds make keys more sensitive.
threshold level (Section 2.1).
The drift compensation mechanism can be made asymmetric
if desired; the drift-compensation can be made to occur in
one direction faster than it does in the other simply by setting
^H and ^I to different settings.
As Cx and Cs drift, the reference point drift-compensates for
these changes at a user-settable rate (Section 2.4); the
threshold level is recomputed whenever the reference point
moves, and thus it also is drift compensated.
Specifically, drift compensation should be set to compensate
faster for increasing signals than for decreasing signals.
Decreasing signals should not be compensated quickly, since
an approaching finger could be compensated for partially or
entirely before even touching the touch pad. However, an
obstruction over the sense pad, for which the sensor has
already made full allowance for, 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
raising the reference level relatively quickly.
The negative threshold is programmed on a per-key basis
using the setup process described in Section 5.
2.2 Positive Threshold
See also command ^B, page 24
The positive threshold is used to provide a mechanism for
recalibration of the reference point when a key's signal moves
abruptly to the positive. These transitions are described more
fully in Section 2.7.
The drift compensation rate can be set for each key
individually, and can also be disabled completely if desired on
a per-key basis.
Positive threshold levels are programmed in using the setup
process described in Section 5 on a per-key basis.
Drift compensation and the detection time-outs (Section 2.5)
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'.
2.3 Hysteresis
See also command ^C and ^D, page 25
Refer to Figure 2-1. QT60xx5 ICs employ programmable
hysteresis levels of 12.5%, 25%, or 50% of the delta between
the reference and threshold levels. There are different
hysteresis settings for positive and negative thresholds which
can be set by the user. The hysteresis is a percentage of the
distance from the threshold level back towards the reference,
and defines the point at which the detection will drop out. A
percentage of 12.5% is less hysteresis than 25%, and the
12.5% hysteresis point is closer to the threshold level than to
the reference level.
Drift compensation can result in reference levels that are at
the boundaries of the 8-bit signal window. When this occurs,
saturation is reached and the drift compensation process
stops. One of two error flags is set when the signal
approaches either end of the signal window; it is up to the
host to read these flags and induce a full recalibration via a
recalibration command at that time (Section 2.10 and
command ‘b’, page 28) for the key in question.
2.5 Detection Recalibration Delay
See also command ^L, page 26
The hysteresis levels are set for all keys only; it is not
possible to set the hysteresis differently from key to key on
either the positive or negative hysteresis levels.
If a foreign object contacts a key the key's signal may change
enough in the negative direction, the same as a normal
touch, to create an unintended detection. When this happens
it is usually desirable to cause the key to be recalibrated in
order to restore its function after a time delay of some
seconds.
2.4 Drift Compensation
See also commands ^H, ^I, page 26
Signals can drift because of changes in Cx and Cs over time
and temperature. It is crucial that such drift be compensated,
else false detections and sensitivity shifts can occur. The
QT60xx5 compensates for drift using setups, ^H and ^I.
The Negative Recal Delay timer monitors this detection
duration; if a detection event exceeds the timer's setting, the
key will be fast-recalibrated within its current 8-bit window.
This form of recalibration is simply one of setting Reference =
Signal, and does not affect Offset or Cz state; as a result this
form of recalibration requires only one burst spacing interval
t
Drift compensation (Figure 2-1) is performed by making the
reference level track the raw signal at a slow rate, but only
while there is no detection in effect. The rate of adjustment
must be performed slowly, otherwise legitimate
detections could be ignored. The devices drift
compensate using a slew-rate limited change to
the reference level; the threshold and hysteresis
values are slaved to this reference.
Figure 2-1 Thresholds and Drift Compensation
When a finger is sensed, the signal falls since the
human body acts to absorb charge from the
Reference
Hysteresis
Threshold
cross-coupling between X and Y lines. An isolated,
untouched foreign object (a coin, or a water film)
will cause the signal to rise very slightly due to an
enhancement of coupling. This is contrary to the
way most capacitive sensors operate.
Signal
Once a finger is sensed, the drift compensation
mechanism ceases since the signal is legitimately
detecting an object. Drift compensation only works
Output
lQ
6
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