Figure 2-4 X-Drive Pulse Roll-off and Dwell Time
Figure 2-6 Recommended Key Structure
‘T’ should ideally be similar to the complete thickness the fields need to
penetrate to the touch surface. Smaller dimensions will also work but will give
less signal strength. If in doubt, make the pattern coarser.
Lost charge due to
inadequate settling
before end of dwell time
X drive
Dwell time
Y gate
Figure 2-5 Probing X-Drive Waveforms with a Coin
2.8 Matrix Series Resistors
The X and Y matrix scan lines should use series resistors
(referred to as Rx and Ry respectively) for improved EMI
performance.
X drive lines require them in most cases to reduce edge rates
and thus reduce RF emissions. Typical values range from 1K to
20K✡.
Y lines need them to reduce EMC susceptibility problems and in
some extreme cases, ESD. Typical Y values range around
1K✡. Y resistors act to reduce 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 for high frequency
interference problems (above 20MHz) as the trace lengths
between the components and the chip start to act as RF
antennae.
2.7 Signal Levels
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 will depending on key geometry and stray
capacitance, and thus an oscilloscope is required to determine
optimum values of both.
Using Quantum’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 from 250 to
750 counts with properly designed key shapes (see appropriate
Quantum app note on matrix key design). 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.
The upper limit of Rx can vary depending on key geometry and
stray capacitance, and some experimentation and an
oscilloscope are required to determine optimum values.
Dwell time (page 22) affects the duration in which charge
coupled from X to Y can be captured. Increasing the dwell will
increase the signal levels lost to higher values of Rx and Ry, as
shown in Figure 2-4. Too short a dwell time will cause charge to
be 'lost', if there is too much rising edge roll-off. Lengthening
the dwell time will cause this lost charge to be recaptured,
thereby restoring key sensitivity. In these devices, dwell time is
adjustable (see Section 5.11) to one of 3 values.
QmBtn software is available free of charge on Quantum’s web
site.
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.
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).
Increasing the burst length (BL) parameter will increase the
signal strengths as will increasing the sampling resistor (Rs)
values.
One way to determine X settling time is to monitor the fields
using a patch of metal foil or a small coin over the key (Figure
lQ
5
QT60486-AS R8.01/0105
QUANTUM [ QUANTUM RESEARCH GROUP ]
