at42qt1481 ATMEL Corporation, at42qt1481 Datasheet - Page 11

• Manufacturer Part Number: at42qt1481
• Description: 48-key Qmatrix Ic
• Manufacturer: ATMEL Corporation

2.8
2.9
9621B–AT42–06/11
Signal Levels
Matrix Series Resistors
Using Atmel’s QmBtn software it is easy to observe the absolute level of signal received by the
sensor on each key. The signal values should normally be in the range of 250 to 750 counts with
properly designed key shapes (see the Touch Sensors Design Guide, available on Atmel’s
website www.atmel.com). However, long adjacent runs of X and Y lines can also artificially boost
the signal values, and induce signal saturation: this is to be avoided. The X-to-Y coupling should
come mostly from intra-key electrode coupling, not from stray X-to-Y trace coupling.
QmBtn software is available free of charge on Atmel’s website.
The signal swing from the smallest finger touch should preferably exceed 10 counts, with 15
being a reasonable target. The signal threshold setting (NTHR) should be set to a value
guaranteed to be less than the signal swing caused by the smallest touch.
Increasing the burst length (BL) parameter increases the signal strengths as will increasing the
sampling resistor (Rs) values.
The X and Y matrix scan lines should use series resistors (Rx0 – Rx7 and Ry0 – Ry5
respectively) for improved EMC performance
X drive lines require Rx in most cases to reduce edge rates and thus reduce RF emissions.
Values range from 1 k to 100 k, typically 1 k.
Y lines need Ry to reduce EMC susceptibility problems and in some extreme cases, ESD.
Typical Y values are about 1 k. Y resistors act to reduce noise susceptibility problems by
forming a natural low-pass filter with the Cs capacitors.
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
page
Ry, as shown in
rising edge roll-off. Lengthening the dwell time causes this lost charge to be recaptured, thereby
restoring key sensitivity. In the QT1481 dwell time is adjustable (see
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).
12). Increasing the dwell time increases the signal levels lost to higher values of Rx and
Figure
2-5. Too short a dwell time causes charge to be 'lost', if there is too much
(Figure 1-1 on page
5).
Section 5.13 on page
AT42QT1481
(Figure 2-5 on
46).
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