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.
5 Setups
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.
Positive hysteresis: PHYST is fixed at 12.5% of the positive
threshold value and cannot be altered.
Positive threshold levels are programmed in using the Setup
process on a per-key basis.
User-defined Setups are employed to alter these algorithms
to suit each application. These setups are loaded into the
device in a block load over one of the serial interfaces. The
Setups are stored in an onboard eeprom array. After a
setups block load, the device should be reset to allow the
new Setups block to be shadowed in internal Flash ROM
and to allow all the new parameters to take effect. This reset
can be either a hardware or software reset.
Typical values:
1 to 4
(5 to 8 counts of threshold; 4 is internally added to
PTHR to generate the threshold)
Default value:
(6 counts of threshold)
2
5.3 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.
Refer to Table 5.1, page 24 for a list of all Setups.
Block length issues: The setups block is 247 bytes long to
accommodate 48 keys. This can be a burden on smaller host
controllers with limited memory. In larger quantities the
devices can be procured with the setups block
Drift compensation (Figure 5-1) 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.
preprogrammed from Quantum. If the application only
requires a small number of keys (such as 16) then the
setups table can be compressed in the host by filling large
stretches of the Setups area with nulls.
Many setups employ lookup-table value translation. The
Setups Block Summary on page 26 shows all translation
values.
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.
Default Values shown are factory defaults.
5.1 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.
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 negative threshold level.
The drift compensation mechanism can be made asymmetric
if desired; 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 Setups parameters. This
can be done on a per-key basis.
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.
Specifically, drift compensation should be set to compensate
faster for increasing signals than for decreasing signals.
Decreasing signals should not be compensated quickly,
since an approaching finger could be compensated for
partially or entirely before even touching the touch pad.
However, an obstruction over the sense pad, for which the
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,
page 26.
Figure 5-1 Thresholds and Drift Compensation
Typical values:
3 to 8
(7 to 12 counts of threshold; 4 is internally
added to NTHR to generate the threshold).
Reference
Default value:
(10 counts of threshold)
6
Hysteresis
Threshold
5.2 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
Signal
Output
lQ
20
QT60486-AS R8.01/0105
QUANTUM [ QUANTUM RESEARCH GROUP ]
