The signal drift compensation is 'asymmetric'; the reference
level drift-compensates in one direction faster than it does in
the other. Specifically, it compensates faster for decreasing
signals than for increasing signals. Increasing signals should
not be compensated for quickly, since an approaching finger
could be compensated for partially or entirely before even
approaching the sense electrode. However, an obstruction
over the sense pad, for which the sensor has already made
full allowance, could suddenly be removed leaving the
sensor with an artificially elevated reference level and thus
become insensitive to touch. In this latter case, the sensor
will compensate for the object's removal very quickly, usually
in only a few seconds.
With large values of Cs and small values of Cx, drift
compensation will appear to operate more slowly than with
the converse.
Drift Compensation in Slow Mode: Drift compensation
rates in Slow mode are preserved if there is no Sync signal,
and the rates are derived from the ~90ms Sleep interval.
However if there is a Sync signal, then drift compensation
rates are derived from an assumption that the Sync
periodicity is ~18ms (which is corresponds to 55.5Hz). Thus,
drift compensation timings in Sync mode are correct for an
~18ms Sync period but different (slower or faster) for other
Sync periods. For example a Sync period of 36ms would
halve the expected drift compensation rates.
2.1.3 Threshold Level
The internal threshold level is fixed at 12 counts for all four
channels. The hysteresis is fixed at 2 counts (17%).
2.1.4 Max On-Duration
If a sufficiently large object contacts a key for a prolonged
duration, the signal will trigger a detection output preventing
further normal operation. To cure such ‘stuck key’ conditions ,
the sensor includes a timer on each channel to monitor
detection duration. If a detection exceeds the maximum timer
setting, the timer causes the sensor to perform a full
recalibration (if not set for infinite). This is known as the Max
On-Duration feature.
After the Max On-Duration interval, the sensor channel will
once again function normally, even if partially or fully
obstructed, to the best of its ability, given electrode
conditions. There are three timeout durations available via
strap option: 10s, 60s, and infinite (Table 2.2).
Max On-Duration works independently per channel; a
timeout on one channel has no effect on another channel.
Note also that the timings in Table 2.2 are dependent on the
oscillator frequency in Fast mode. Doubling the
recommended frequency will halve the timeouts. This is not
true in Slow mode.
Infinite timeout is useful in applications where a prolonged
detection can occur and where the output must reflect the
detection no matter how long. In infinite timeout mode, the
designer should take care to ensure that drift in Cs, Cx, and
Vdd do not cause the device to ‘stick on’ inadvertently even
when the target object is removed from the sense field.
Timeouts are approximate and can vary substantially over
Vdd and temperature, and should not be relied upon for
critical functions. Timeouts are also dependent on operating
frequency in Fast mode.
Max On-Duration in Slow Mode: When Sync mode is used
in Slow mode, the Max On-Duration timings are derived from
the Sync period. The device assumes the Sync periodicity is
18ms (midway between 50Hz and 60Hz sync timings). Thus,
Max On-Duration timings in Sync mode are correct for an
18ms Sync period but different (shorter or longer) for other
Sync periods. For example a Sync period of 36ms would
double all expected Max On-Duration timings.
2.1.5 Detection Integrator
It is desirable to suppress false detections due to electrical
noise or from quick brushes with an object. To this end,
these devices incorporate a per-key ‘Detection Integrator’
counter that increments with each signal detection exceeding
the signal threshold (Figure 2.1) until a limit count is reached,
after which an Out pin becomes active. If a ‘no detect’ is
sensed even once prior to the limit, this counter is reset to
zero and no detect output is generated. The required limit
count is 6.
The Detection Integrator can also be viewed as a
'consensus' vote requiring a detection in successive samples
to trigger an active output.
In Slow mode, the detect integrator forces the device to
operate faster to confirm a detection. The six successive
acquisitions required to affirm a detection are done without
benefit of a low power sleep mode between bursts.
2.1.6 Forced Sensor Recalibration
Pin 13 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.
Pin 13 can also be controlled by logic or a microcontroller to
force the chip to recalibrate, by toggling it low for 10µs or
more, then raising it high again.
2.1.7 Fast Positive Recalibration
If the sensed capacitance becomes lower by 5 counts than
the reference level for 2 seconds, the sensor will consider
this to be an error condition and will force a recalibration on
the affected channel.
2.2 Options
These devices are designed for maximum flexibility and can
accommodate most popular sensing requirements via option
pins.
The option pins are read on power-up and about once every
10 seconds while the device is not detecting touch on any
channel. Options are set using high value resistors
connected to certain SNS pins, to either Vdd or Vss. These
options are read 25 times over 250µs to ensure that they are
not influenced by noise pulses. All 25 samples must agree.
However, large values of Cx on the SNS wires can load
down the pins to the point where the 1M pull-up resistors
cannot pull high fast enough, and the pins are read
erroneously as a result. Cx should be below 50pF to prevent
errors; this value can be read with a conventional
capacitance meter with the QT240 removed.
The option setting resistors are mandatory and cannot be
deleted. They must be strapped to either Vdd or Vss.
lQ
5 QT240R R1.11/1006
