© Quantum Research Group Ltd.
3.5.2 NOISE
C
OUPLING
INTO Y LINES
Figure 3-3 Relationship of X and Y signals
External noise, sometimes caused by ground bounce due to
injected line noise, can couple into the Y lines and cause
signal interference in extreme cases. Such noise can be
readily suppressed by adding a 100pF capacitor from each Y
line to a ground plane near the QT60xx5.
'n' pulses / burst
R2R Value
Xa
Yb
3.6 Burst Length & Sensitivity
See also Command ^F, page 25
Amp out
The signal gain in volts / pF of Cx for each key is controlled
by circuit parameters as well as the burst length.
The burst length is simply the number of times the
charge-transfer (‘QT’) process is performed on a given key.
Each QT process is simply the pulsing of an X line once, with
a corresponding Y line enabled to capture the resulting
charge passed through the key’s capacitance Cx.
Xa
Yb
Yb'
Yb
QT60xx5 devices use a finite number of QT cycles which are
executed in a short burst. There can be from 1 to 64 cycles in
a burst, in accordance with the list of permitted values shown
for command ^F, page 25. If burst length is set to zero, the
burst is disabled but its time slot in the scanning sequence of
all keys is preserved so as to maintain uniform timing.
3.4.2 NOISE
C
OUPLING
I
NTO
X
LINES
External noise, sometimes caused by ground bounce due to
injected line noise, can couple into the X lines and cause
signal interference in extreme cases. Such noise can be
readily suppressed by the use of series resistors as
described above. Adding a small capacitor to the matrix line
on the QT60xx5 side of the R, for example 100pF to ground
near the QT60xx5, will greatly help to reduce such effects.
Increasing burst length directly affects key sensitivity. This
occurs because the accumulation of charge in the charge
integrator is directly linked to the burst length. The burst
length of each key can be set individually, allowing for direct
digital control over the signal gains of each key individually.
Apparent touch sensitivity is also controlled by Negative
Threshold (Section 2.1). Burst length and negative threshold
interact; normally burst lengths should be kept as short as
possible to limit RF emissions, but the threshold setting
should be kept above a setting of 6 to limit false detections.
The detection integrator can also prevent false detections at
the expense of slower reaction time (Section 2.6).
3.5 'Y' Gate Drives
There are 8 'Y' gate drives (YC0..YC7) which are active-high;
only one of these lines is used during a burst for a particular
key. These lines are used to control the PLD to ground all
unselected ‘Y’ lines, making them low impedance. The
selected ‘Y’ line in the matrix remains unclamped by the PLD
during the rising edge of the ‘X’ drive line, during the time that
the coupled charge from a single key is fed to the charge
integrator via the 8:1 analog mux.
3.7 Intra-Burst Spacing
See also Command ^M, page 27
The time between X drive pulses during a burst is the
intra-burst pulse spacing. This timing has no noticeable effect
on performance of the circuit, but can have an impact on the
nature of RF spectral emissions from the matrix panel. The
setting of this function can be from 2µs through 10µs, loosely
corresponding to fundamental emission frequencies from
500kHz and 100kHz respectively.
There are also 3 Y-encoded lines YS0..YS2 which select the
correct switch to actuate in the analog mux for the desired ‘Y’
line. Line ‘YG’ from the controller acts to trigger the PLD’s
pulse generation circuit, whose pulse width following the rise
of an ‘X’ line is dependent on an RC time constant. This
pulse, ‘YE’, drives the enable pin of the QS3251 mux low
(switch on) just before a positive-going ‘X’ drive pulse, and
high again (switch off) just after the ‘X’ drive pulse. The time
from the rising edge of an ‘X’ signal to the rising edge of ‘YE’
is referred to as the dwell time, and this parameter has a
direct effect on the ability of the circuit to suppress moisture
films (see Sections 3.9 and 3.13).
Longer spacings require more time to execute and can limit
the operational settings of burst length and/or burst spacing
(Section 5.7).
The intra-burst QT spacing has no effect on sensitivity or
water film suppression and is not particularly important to the
sensing function other than described above.
After the ‘YE’ pulse has ceased, the controller and circuit act
to ground all ‘Y’ lines via the PLD just before the ‘X’ drive
signal goes low; this restores the charge across the matrix
keys to a null state, making them ready for another sample.
3.8 Burst Spacing
See also Command ^G, page 25
The interval of time from the start of one burst to the start of
the next is known as the burst spacing. This is an alterable
parameter which affects all keys.
3.5.1 RFI FROM Y LINES
Y lines are 'virtual grounds' and do not radiate a significant
amount of RFI; in fact, they act as ‘sinks’ for RFI emitted by
the X lines since they are virtual grounds. Series-R in the Y
lines is not required for RFI suppression, and in fact series-R
can introduce cross-talk among keys.
Shorter spacings result in faster response time, but due to
increasing timing restrictions at shorter spacings burst
lengths are restricted, limiting the amount of gain that can be
lQ
12
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