sense pad, for which the sensor has already made full
allowance for, could suddenly be removed leaving the
sensor with an artificially suppressed reference level and
thus become insensitive to touch. In this latter case, the
sensor should compensate for the object's removal by
raising the reference level relatively quickly.
Drift compensation and the detection time-outs work
together to provide for robust, adaptive sensing. The
time-outs provide abrupt changes in reference calibration
depending on the duration of the signal 'event'.
NDRIFT Typical values: 9 to 11
(2 to 3.3 seconds per count of drift compensation)
NDRIFT Default value: 10
(2.5s / count of drift compensation)
PDRIFT Typical values: 3 to 5
(0.4 to 0.8 seconds per count of drift compensation;
translation via LUT, page 19)
PDRIFT Default value: 4
(0.6s / count of drift compensation)
5.4 Detect Integrators - NDIL, FDIL
To suppress false detections caused by spurious events
like electrical noise, the device incorporates a 'detection
integrator' or DI counter mechanism that acts to confirm a
detection by consensus (all detections in sequence must
agree). The DI mechanism counts sequential detections of
a key that appears to be touched, after each burst for the
key. For a key to be declared touched, the DI mechanism
must count to completion without even one detection
failure.
The DI mechanism uses two counters. The first is the ‘fast
DI’ counter FDIL. When a key’s signal is first noted to be
below the negative threshold, the key enters ‘fast burst’
mode. In this mode the burst is rapidly repeated for up to
the specified limit count of the fast DI counter. Each key
has its own counter and its own specified fast-DI limit
(FDIL), which can range from 1 to 15. When fast-burst is
entered the QT device locks onto the key and repeats the
acquire burst until the fast-DI counter reaches FDIL, or, the
detection fails beforehand. After this the device resumes
normal keyscanning and goes on to the next key.
The ‘Normal DI’ counter counts the number of times the
fast-DI counter reached its FDIL value. The Normal DI
counter can only increment once per complete scan of all
keys. Only when the Normal DI counter reaches NDIL does
the key become formally ‘active’.
The net effect of this is that the sensor can rapidly lock
onto and confirm a detection with many confirmations,
while still scanning other keys. The ratio of ‘fast’ to ‘normal’
counts is completely user-settable via the Setups process.
The total number of required confirmations is equal to FDIL
times NDIL.
If FDIL = 5 and NDIL = 2, the total detection confirmations
required is 10, even though the device only scanned
through all keys only twice.
The DI is extremely effective at reducing false detections at
the expense of slower reaction times. In some applications
a slow reaction time is desirable; the DI can be used to
intentionally slow down touch response in order to require
the user to touch longer to operate the key.
If FDIL = 1, the device functions conventionally; each
channel acquires only once in rotation, and the normal
detect integrator counter (NDIL) operates to confirm a
detection. Fast-DI is in essence not operational.
If FDIL m 2, then the fast-DI counter also operates in
addition to the NDIL counter.
If Signal [ NThr: The fast-DI counter is incremented
towards FDIL due to touch.
If Signal >NThr then the fast-DI counter is cleared due to
lack of touch.
NDIL Typical values: 2, 319
NDIL Default value: 2
FDIL Typical values: 4 to 6
FDIL Default value: 5
5.5 Negative Recal Delay - NRD
If an object unintentionally contacts a key resulting in a
detection for a prolonged interval it is usually desirable to
recalibrate the key in order to restore its function, perhaps
after a time delay of some seconds.
The Negative Recal Delay timer monitors such detections;
if a detection event exceeds the timer's setting, the key will
be automatically recalibrated. After a recalibration has
taken place, the affected key will once again function
normally even if it is still being contacted by the foreign
object. This feature is set on a per-key basis using the
NRD setup parameter.
NRD can be disabled by setting it to zero (infinite timeout)
in which case the key will never auto-recalibrate during a
continuous detection (but the host could still command it).
NRD is set using one byte per key, which can range in
value from 0..255. NRD is expressed in 0.5s increments.
Thus if NRD =120, the timeout value will actually be 60
seconds.
NRD Typical values: 20 to 60 (10 to 30 seconds)
NRD Default value: 20 (10 seconds)
5.6 Positive Recalibration Delay - PRD
A recalibration can occur automatically if the signal swings
more positive than the positive threshold level. This
condition can occur if there is positive drift but insufficient
positive drift compensation, or, if the reference moved
negative due to a NRD auto-recalibration, and thereafter
the signal rapidly returned to normal (positive excursion).
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 can suddenly swing back positive to near 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.
lQ
14 QT60486-AS 0.07/1103
Advanced information; subject to change
