6 Setups
6.1 Introduction
The devices calibrate and process all signals using a
number of algorithms specifically designed to provide for
high survivability in the face of adverse environmental
challenges. They provide a large number of processing
options which can be user-selected to implement very
flexible, robust keypanel solutions.
User-defined Setups are employed to alter these
algorithms to suit each application. These setups are
loaded into the device over the I
2
C serial interfaces. The
Setups are stored in an onboard EEPROM array.
Many setups employ lookup-table value translation.
Table 6.2, the Setups Lookup Table on page 22 shows all
translation values. The default values are the factory
defaults.
Refer to Table 6.1 for all Setups.
6.2 Negative Threshold - NTHR
The negative threshold value is established relative to a
key’s signal reference value. The threshold is used to
determine key touch when crossed by a negative-going
signal swing after having been filtered by the detection
integrator. Larger absolute values of threshold desensitize
keys since the signal must travel farther in order to cross
the threshold level. Conversely, lower thresholds make
keys more sensitive.
As Cx and Cs drift, the reference point drift-compensates
for these changes at a user-settable rate; the threshold
level is recomputed whenever the reference point moves,
and thus it also is drift compensated.
The amount of NTHR required depends on the amount of
signal swing that occurs when a key is touched. Thicker
panels or smaller key geometries reduce ‘key gain’, i .e.
signal swing from touch, thus requiring smaller NTHR
values to detect touch.
The negative threshold is programmed on a per-key basis
using the Setup process. See
Table 6.2, page 22.
Negative hysteresis: NHYST is fixed at 12.5 percent of
the negative threshold value and cannot be altered.
Typical values: 3 to 8
(7 to 12 counts of threshold; 4 is internally added to
NTHR to generate the threshold).
Default value: 6
(10 counts of threshold)
6.3 Positive Threshold - PTHR
The positive threshold is used to provide a mechanism for
recalibration of the reference point when a key's signal
moves abruptly to the positive. This condition is not
normal, and usually occurs only after a recalibration when
an object is touching the key and is subsequently removed.
The desire is normally to recover from these events
quickly.
Positive hysteresis: PHYST is fixed at 12.5 percent of the
positive threshold value and cannot be altered.
Positive threshold levels are all fixed at six counts of signal
and cannot be modified.
6.4 Drift Compensation - NDRIFT, PDRIFT
Signals can drift because of changes in Cx and Cs over
time and temperature. It is crucial that such drift be
compensated, else false detections and sensitivity shifts
can occur.
Drift compensation (Figure 6.1) is performed by making the
reference level track the raw signal at a slow rate, but only
while there is no detection in effect. The rate of adjustment
must be performed slowly, otherwise legitimate detections
could be ignored. The devices drift compensate using a
slew-rate limited change to the reference level; the
threshold and hysteresis values are slaved to this
reference.
When a finger is sensed, the signal falls since the human
body acts to absorb charge from the cross-coupling
between X and Y lines. An isolated, untouched foreign
object (a coin, or a water film) will cause the signal to rise
very slightly due to an enhancement of coupling. This is
contrary to the way most capacitive sensors operate.
Once a finger is sensed, the drift compensation
mechanism ceases since the signal is legitimately
detecting an object. Drift compensation only works when
the signal in question has not crossed the neg ative
threshold level.
The drift compensation mechanism can be asymmetric; the
drift-compensation can be made to occur in one direction
faster than it does in the other simply by changing the
NDRIFT and PDRIFT Setup parameters. This can be done
on a per-key basis.
lQ
17 QT60240-ISG R8.06/0906
Figure 6.1 Thresholds and Drift Compensation
Threshold
Signal
Hysteresis
Reference
Output
