As an example of the latter, if a foreign object or a finger
contacts a key for period longer than the Negative Recal
Delay (NRD), the key is by recalibrated to a new lower
reference level. Then, when the condition causing the
negative swing ceases to exist (e.g. the object is removed)
the signal suddenly swings positive to its normal reference.
It is almost always desirable in these cases to cause the
key to recalibrate quickly so as to restore normal touch
operation. The time required to do this is governed by
PRD. In order for this to work, the signal must rise through
the positive threshold level PTHR continuously for the PRD
period.
After the PRD interval has expired and the
autorecalibration has taken place, the affected key will
once again function normally.
PRD Accuracy: to within ± 50ms
Delay: PRD is fixed at 200ms for all keys,
and cannot be altered.
6.8 Burst Length - BL
The signal gain 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.
QT60xx0 devices use a fixed number of QT cycles which
are executed in burst mode. There can be up to 64 QT
cycles in a burst, in accordance with the list of permitted
values shown in Table 6.2, page 22.
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 the
Negative Threshold level (NTHR). Burst length and NTHR
interact; normally burst lengths should be kept as short as
possible to limit RF emissions, but NTHR should be kept
above 6 to reduce false detections due to external noise.
The detection integrator mechanism also helps to prevent
false detections.
BL Typical values: 1, 2 (32, 48 pulses / burst)
BL Default value: 2 (48 pulses / burst)
BL Possible values: 0, 1, 2, 3 (16, 32, 48, 64
pulses/burst)
6.9 Adjacent Key Suppression - AKS
These devices incorporate adjacent key suppression
(‘AKS’ - patent pending) that can be selected on a per-key
basis. AKS permits the suppression of multiple key
presses based on relative signal strength. This feature
assists in solving the problem of surface moisture which
can bridge a key touch to an adjacent key, causing multiple
key presses. This feature is also useful for panels with
tightly spaced keys, where a fingertip might inadvertently
activate an adjacent key.
AKS works for keys that are AKS-enabled anywhere in the
matrix and is not restricted to physically adjacent keys; the
device has no knowledge of which keys are actually
physically adjacent. When enabled for a key, adjacent key
suppression causes detections on that key to be
suppressed if any other AKS-enabled key in the panel has
a more negative signal deviation from its reference.
This feature does not account for varying key gains (burst
length) but ignores the actual negative detection threshold
setting for the key. If AKS-enabled keys in a panel have
different sizes, it may be necessary to reduce the gains of
larger keys relative to smaller ones to equalize the effects
of AKS. The signal threshold of the larger keys can be
altered to compensate for this without causing problems
with key suppression.
Adjacent key suppression works to augment the natural
moisture suppression of narrow gated transfer switches
creating a more robust sensing method.
AKS Default value: 0 (Off)
6.10 Oscilloscope Sync - SSYNC
Pin 11 (S_SYNC) can output a positive pulse oscilloscope
sync that brackets the burst of a selected key. More than
one burst can output a sync pulse as determined by the
Setups parameter SSYNC for each key.
This feature is invaluable for diagnostics; without it,
observing signals clearly on an oscilloscope for a particular
burst is very difficult.
This function is supported in Quantum’s QmBtn PC
software.
SSYNC Default value: 0 (Off)
6.11 Mains Sync - MSYNC
The Mains Sync feature uses M_SYNC pin 1.
External fields can cause interference leading to false
detections or sensitivity shifts. Most fields come from AC
power sources. RFI noise sources are heavily suppressed
by the low impedance nature of the QT circuitry itself.
Noise such as from 50Hz or 60Hz fields becomes a
problem if it is uncorrelated with acquisition signal
sampling; uncorrelated noise can cause aliasing effects in
the key signals. To suppress this problem the M_SYNC
input allows bursts to synchronize to the noise source.
The noise synchronization operating mode is set by
parameter MSYNC in Setups.
The synchronization occurs only at the burst for the lowest
numbered enabled key in the matrix. The device waits for
the synchronization signal for up to 100ms after the end of
a preceding full matrix scan, then when a negative
synchronization edge is received, the matrix is scanned in
its entirety again.
The sync signal drive should be a buffered logic signal, or
perhaps a diode-clamped signal, but never a raw AC signal
from the mains. The device will synchronize to the falling
edge.
lQ
19 QT60240-ISG R8.06/0906
