E102 Atmel, E102 Datasheet - Page 4

• Manufacturer Part Number: E102
• Description: KIT EVAL/DEV USING QT102-ISG
• Manufacturer: Atmel
• Series: Quantum, QTouch™r

Specifications of E102

• Sensor Type: Touch, Capacitive
• Sensing Range: 1 Button/Key
• Interface: Logic Level
• Voltage - Supply: 3V
• Embedded: No
• Utilized Ic / Part: QT102
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Sensitivity: -

1.5 Forced Sensor Recalibration
The QT102 has no recalibration pin; a forced recalibration is
accomplished when the device is powered up, after the
recalibration timeout or when the auto-off override is released.
However, supply drain is low so it is a simple matter to treat
the entire IC as a controllable load; driving the QT102's V
pin directly from another logic gate or a microcontroller port
will serve as both power and 'forced recal'. The source
resistance of most CMOS gates and microcontrollers are low
enough to provide direct power without problems.
1.6 Drift Compensation
Signal drift can occur because of changes in Cx and Cs over
time. It is crucial that drift be compensated for, otherwise false
detections, nondetections, and sensitivity shifts will follow.
Drift compensation (Figure 1.2) 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 QT102 drift compensates using a
slew-rate limited change to the reference level; the threshold
and hysteresis values are slaved to this reference.
Once an object is sensed, the drift compensation
mechanism ceases since the signal is legitimately
high, and therefore should not cause the
reference level to change.
The QT102's 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.
lQ
Threshold
Output
DD
4
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. Note that the positive and negative drift
compensation rates are different.
1.7 Response Time
The QT102's response time is highly dependent on burst
length, which in turn is dependent on Cs and Cx. With
increasing Cs, response time slows, while increasing levels of
Cx reduce response time.
1.8 Spread Spectrum
The QT102 modulates its internal oscillator by ±7.5 percent
during the measurement burst. This spreads the generated
noise over a wider band reducing emission levels. This also
reduces susceptibility since there is no longer a single
fundamental burst frequency.
Figure 1.2 Drift Compensation
Signal
R eference
QT102_2R3.05_1007
H ysteresis