However, an obstruction over the 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.
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.
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'.
If Signal >NThr then the fast-DI counter is cleared due to
lack of touch.
Disabling a key: If NDIL =0, the key becomes disabled.
Keys disabled in this way are pared from the burst sequence
in order to improve sampling rates and thus response time.
See Section 2.2, page 3.
NDRIFT Typical values:
(2 to 3.3 seconds per count of drift compensation)
NDRIFT Default value: 10
(2.5s / count of drift compensation)
9 to 11
NDIL Typical values:
NDIL Default value:
FDIL Typical values:
FDIL Default value:
2, 3
2
4 to 6
5
PDRIFT Fixed value:
0.4 secs
Note: This value cannot be altered and does not appear
in the Setups block.
5.4 Detect Integrators - NDIL, FDIL
NDIL is used to enable or disable keys and to provide signal
filtering. To enable a key, its NDIL parameter should be
non-zero (ie NDIL=0 disables a key). See Section 2.2.
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.
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 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.
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
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..254. NRD above 0 is expressed in 0.5s increments.
Thus if NRD =120, the timeout value will actually be 60
seconds. 255 is not a legal number to use.
NRD Typical values:
NRD Default value:
NRD Range:
20 to 60 (10 to 30 seconds)
20 (10 seconds)
0..254 (∞, 0.5 .. 127s)
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’.
5.6 Positive Recalibration Delay - PRD
A recalibration occurs 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).
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.
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.
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.
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.
If FDIL = 1, the device functions conventionally; each
channel acquires only once in rotation, and the normal detect
lQ
19
QT60248-AS R4.02/0405
QUANTUM [ QUANTUM RESEARCH GROUP ]
