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 10 µs 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.

5

QT240R R1.11/1006