at42qt100a ATMEL Corporation, at42qt100a Datasheet - Page 9

• Manufacturer Part Number: at42qt100a
• Description: Qtouch? Charge Transfer Ic
• Manufacturer: ATMEL Corporation

3.7
3.8
3.9
3.9.1
3.9.2
9531A–AT42–02/09
Response Time
Spread Spectrum
Output Features
Output
HeartBeat
™
Output
The QT100A’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. 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.
The QT100A's response time is highly dependent on run mode and 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. The response time will also be a lot slower in LP or Sync mode due to a
longer time between burst measurements.
The QT100A 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.
The output of the QT100A is active-high upon detection. The output will remain active-high for
the duration of the detection, or until the Max On-duration expires, whichever occurs first. If a
Max On-duration timeout occurs first, the sensor performs a full recalibration and the output
becomes inactive (low) until the next detection.
The QT100A output has a HeartBeat “health” indicator superimposed on it in both LP and Sync
modes. This operates by taking the output pin into a three-state mode for 15 µs, once before
every QT burst. This output state can be used to determine that the sensor is operating properly,
or it can be ignored, using one of several simple methods.
The HeartBeat indicator can be sampled by using a pull-up resistor on the OUT pin
on page
other circuit. The pulses will only be visible when the chip is not detecting a touch.
10), and feeding the resulting positive-going pulse into a counter, flip flop, one-shot, or
AT42QT100A
(Figure 3-6
9