AT42QT2640_14 Datasheet, PDF (11/73 Pages) ATMEL Corporation – 64-key 8-slider/wheel QMatrix FMEA

It is essential that the Rx and Ry resistors and Cs capacitors be placed very close to the chip. Placing these parts
more than a few millimeters away opens the circuit up to high frequency interference problems (above 20 MHz) as
the trace lengths between the components and the chip start to act as RF antennas.
The upper limits of Rx and Ry are reached when the signal level and hence key sensitivity are clearly reduced. The
limits of Rx and Ry depend on key geometry and stray capacitance, and thus an oscilloscope is required to
determine optimum values of both.
Dwell time is the duration in which charge coupled from X to Y is captured (Figure 2-5 on page 11). Increasing the
dwell time increases the signal levels lost to higher values of Rx and Ry, as shown in Figure 2-5. Too short a dwell
time causes charge to be 'lost', if there is too much rising edge roll-off. Lengthening the dwell time causes this lost
charge to be recaptured, thereby restoring key sensitivity. In the QT2640 dwell time is adjustable (see Section 5.8 on
page 46).
Dwell time problems can also be solved by either reducing the stray capacitance on the X line(s) (by a layout change
– for example, by reducing X line exposure to nearby ground planes or traces) or the Rx resistor needs to be
reduced in value (or a combination of both approaches).
Figure 2-5. Drive Pulse Roll-off and Dwell Time
X drive
Dwell time
Y gate
Lost charge due to
inadequate settling
before end of dwell time
Note: The Dwell time is a minimum of ~125 ns – see Section 5.8 on page 46
One way to determine X line settling time is to monitor the fields using a patch of metal foil or a small coin over the
key (see Figure 2-6). Only one key along a particular X line needs to be observed, as each of the keys along a
particular X line are identical. The dwell time should exceed the observed 95% settling of the X-pulse by 25% or
more.
AT42QT2640 [PRELIMINARY DATASHEET]
11
9684DX–AT42–12/13