2 - QT140/QT150 SPECIFICS
2.1 SIGNAL PROCESSING
These devices process all signals using 16 bit
math, using a number of algorithms pioneered by
Quantum. These algorithms are specifically
designed to provide for high survivability in the face
of adverse environmental changes.
T hre sh old
Figure 2-1 Drift Compensation
S ig na l
H ys te res is
R efer ence
2.1.1 D
RIFT
C
OMPENSATION
Signal drift can occur because of changes in Cx,
Cs, and Vdd over time. If a low grade Cs capacitor
Ou tpu t
is chosen, the signal can drift greatly with
temperature. If keys are subject to extremes of
temperature or humidity, the signal can also drift. It
conditions. There are three timeout durations available via
is crucial that drift be compensated, else false detections,
strap option: 10s, 60s, and infinite (Table 2-1).
non-detections, and sensitivity shifts will follow.
Max On-Duration works independently per channel; a
Drift compensation (Figure 2-1) is a method that makes the
reference level track the raw signal at a slow rate, only while timeout on one channel has no effect on another channel
except when the AKS feature is impacted on an adjacent
no detection is in effect. The rate of reference adjustment
must be performed slowly else legitimate detections can also key. Note also that the timings in Table 2-1 are dependent on
the oscillator frequency: Doubling the recommended
be ignored. The IC drift compensates each channel
frequency will halve the timeouts.
independently using a slew-rate limited change to the
reference level; the threshold and hysteresis values are
Infinite timeout is useful in applications where a prolonged
slaved to this reference.
detection can occur and where the output must reflect the
Once an object is sensed, the drift compensation mechanism detection no matter how long. In infinite timeout mode, the
designer should take care to be sure that drift in Cs, Cx, and
ceases since the signal is legitimately high, and therefore
Vdd do not cause the device to ‘stick on’ inadvertently even
should not cause the reference level to change.
when the target object is removed from the sense field.
The signal drift compensation is 'asymmetric'; the reference
level drift-compensates in one direction faster than it does in The delay timings for Max On-Duration depend directly on
resonator frequency. Also, if the acquisition burst on one or
the other. Specifically, it compensates faster for decreasing
signals than for increasing signals. Increasing signals should more channels lasts longer than 5.5ms per channel, the
not be compensated for quickly, since an approaching finger specified timings may be longer.
could be compensated for partially or entirely before even
2.1.4 D
ETECTION
I
NTEGRATOR
approaching the sense electrode. However, an obstruction
over the sense pad, for which the sensor has already made
It is desirable to suppress false detections due to electrical
full allowance for, could suddenly be removed leaving the
noise or from quick brushes with an object. To this end,
sensor with an artificially elevated reference level and thus
these devices incorporate a per-key ‘Detection Integrator’
become insensitive to touch. In this latter case, the sensor
counter that increments with each signal detection exceeding
will compensate for the object's removal very quickly, usually the signal threshold (Figure 2-1) until a limit count is
in only a few seconds.
reached, after which an Out pin becomes active. If a ‘no
detect’ is sensed prior to the limit, this counter is reset to
With large values of Cs and small values of Cx, drift
zero. The required limit count is 3.
compensation will appear to operate more slowly than with
the converse.
The Detection Integrator can also be viewed as a
'consensus' vote requiring a detection in three successive
samples to trigger an active output.
2.1.2 T
HRESHOLD
C
ALCULATION
The internal threshold level is fixed at 6 counts for all
2.1.5 F
ORCED
S
ENSOR
R
ECALIBRATION
channels. These IC's employ a fixed hysteresis of 2 counts
below the threshold (33%).
Pin 28 is a Reset pin, active-low, which in cases where
power is clean can be simply tied to Vdd. On power-up, the
device will automatically recalibrate all channels of sensing.
2.1.3 M
AX
O
N
-D
URATION
If a sufficiently large object contacts a key for a prolonged
Pin 28 can also be controlled by logic or a microcontroller to
duration, the signal will trigger a detection output preventing
force the chip to recalibrate, by toggling it low for 5µs then
further normal operation. To cure such ‘stuck key’ conditions, raising it high again.
the sensor includes a timer on each channel to monitor
detection duration. If a detection exceeds the maximum timer
2.1.6 R
ESPONSE
T
IME
setting, the timer causes the sensor to perform a full
Response time is fixed at 99ms at a 10MHz clock. Response
recalibration (if not set for infinite). This is known as the Max
time can be altered by changing the clock frequency;
On-Duration feature.
doubling the frequency to 20MHz will cut the response time
After the Max On-Duration interval, the sensor channel will
to 49ms.
once again function normally, even if partially or fully
obstructed, to the best of its ability given electrode
lQ
6
QT140/150 1.01/1102
QUANTUM [ QUANTUM RESEARCH GROUP ]
