Y gate signals can be manipulated externally so that the gate

dwell time is very short to suppress the effects of surface

conductivity due to water films. See Section 2.3.

1.6 SIGNAL PROCESSING

The QT60320D calibrates and processes all signals using a

number

of

algorithms

pioneered

by

Quantum.

These

algorithms are specifically designed to provide for high

survivability in the face of adverse environmental challenges.

The QT60320D is fully self-calibrating. On powerup the IC

scans the matrix key by key and sets appropriate calibration

points for each in accordance with setup information in its

internal eeprom, or on the fly from a host MPU. Since the

circuit can tolerate a very wide dynamic range, it is capable of

adapting to a wide mix of key sizes and shapes having wildly

varying Cx coupling capacitances. No special operator or

factory calibration or circuit tweak is required to bring keys

into operation, except for a gain and threshold batch setup

which can be performed in seconds from a file saved on a

PC. Once set, there should never be a need to readjust these

parameters.

Signal drift can occur because of changes in Cx and Cs over

time. It is crucial that drift be compensated for, otherwise

false detections, non-detections, and sensitivity shifts will

follow.

Drift compensation (Figure 1-7) 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 QT60320D drift compensates using a

slew-rate limited change to the reference level; the threshold

and hysteresis values are slaved to this reference.

When a finger is sensed, the signal falls since the human

body acts to absorb charge from the cross-coupling between

X and Y lines. An isolated, untouched foreign object (a coin,

or a water film) will cause the signal to rise

slightly due to the enhanced coupling thus

created. These effects are contrary to the way

most capacitive sensors operate.

Once a finger is sensed, the drift compensation

mechanism

ceases

since

the

signal

is

legitimately low, and therefore should not cause

the reference level to change.

The

QT60320's

drift

compensation

is

'asymmetric': the drift-compensation occurs in

one direction faster than it does in the other.

Specifically, it compensates faster for increasing

signals than for decreasing signals. Decreasing

signals should not be compensated for quickly,

since an approaching finger could be compensated for

partially or entirely before even touching the sense 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 will compensate for the object's

removal by raising the reference level quickly.

The threshold value is established as an offset to the

reference level. As Cx and Cs drift, the reference drift

compensates with the changes and the threshold level is

automatically recomputed in real time so that it is never in

error. Since key touches result in negative signal swings, the

threshold is set below the signal reference level.

The QT60320D employs a hysteresis of 25% of the delta

between the reference and threshold levels. The signal must

rise by 25% of the distance from threshold to reference

before the detection event drops out and the key registers as

untouched.

If a foreign object contacts a key the signal may change

enough to create a 'false' detection lasting for the duration of

the contact. To overcome this, the IC includes a timer which

monitors detection duration. If a detection exceeds the timer

setting, the timer causes the sensor to perform a full

recalibration. This is known as the Max On-Duration feature.

After the Max On-Duration interval has expired and the

recalibration has taken place, the affected key will once again

function normally even if still contacted by the foreign object,

to the best of its ability. The Max On-Duration is fixed at 10

seconds of continuous detection.

To suppress false detections caused by spurious events like

electrical noise, the QT60320D incorporates a detection

integration counter that increments with each detection

sample until a limit is reached, at which point a detection is

confirmed. If no detection is sensed on any of the samples

prior to the final count, the counter is reset immediately to

zero, forcing the process to restart. The required count is 4.

4

QT60320D R1.11/12.07.03

Figure 1-7 Drift Compensation

Threshold

Signal

Hysteresis

Reference

Output

Figure 1-6 Sample Electrode Geometries

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