qt1100a Quantum Research Group, qt1100a Datasheet - Page 26

• Manufacturer Part Number: qt1100a
• Description: 10 Key Qtouch? Sensor Ic
• Manufacturer: Quantum Research Group

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 due to the design of
the signal conversion process.
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 NDCR and PDCR Setups
parameters. NDCR can be modified on a per-key basis.
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
sensor has already made full allowance , 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.
NDCR and PDCR are configured using parameters in the
Setups block (page 31). The numbers entered for these
parameters are used as an entries to lookup tables found on
page 32 and page 33 respectively. The actual amount of drift
compensation can be read from these tables, which depend
on the burst spacing parameter (BS).
Example: BS = 3.5ms, NDCR = 6; the NDCR rate is 2.15s
per LSB change in the reference level when drift
compensating negatively (same direction as a touch).
Example: BS = 2.5ms, PDCR = 9; the PDCR rate is 1.31s
per LSB change in the reference level when drift
compensating positively (opposing direction 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'.
4.4 NRD - Negative Recal Delay Bits
Bytes 10 - 19, Bits 3..0
Default value:
Typical values:
If a foreign object contacts a key the key's signal may
change enough in the negative direction, the same as a
normal touch, to create an unintended detection. When this
happens it is usually desirable to cause the key to
be recalibrated to restore its function after a time
delay of some seconds.
The Negative Recal Delay timer monitors this
detection duration; if a detection event exceeds the
timer's setting, the key will be 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. It can be disabled if desired by
setting this parameter to zero, so that it will not
recalibrate automatically (infinite timeout).
NRD is configured in the Setups block (page 31).
The numbers entered for this parameter are used
LQ
5
Table 4-5, page 34
Threshold
Hysteresis
Output
26
Figure 4-1 Thresholds and Drift Compensation
as entries to a lookup table (LUT) found on page 34. The
actual delay amount can be determined from this table,
which depends on the burst spacing parameter (BS).
4.5 PRD - Positive Recal Delay Bits
Byte 31, Bits 3..0
Default value:
Typical values:
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
recalibration, and thereafter the signal returned to normal.
As an example of the latter, if a foreign object or a finger
contacts a key for a 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 to the new signal level so as to restore
normal touch operation. The device accomplishes this by
simply setting Reference = Signal.
The time required to detect this condition before recalibrating
is governed by PRD. For this feature to operate, the signal
must rise through the positive threshold level PTHR
continuously for a period PRD.
After the PRD interval has expired and the recalibration has
taken place, the affected key will once again function
normally. This interval affects all keys equally.
PRD is configured in the Setups block (page 31). The
numbers entered for this parameter are used as entries to a
lookup table (LUT) found on page 35. The actual delay can
be determined from this table, which depends on the burst
spacing parameter (BS).
4.6 AKS - Adjacent Key Suppression Bits
Bytes 20 - 29, Bit 7
Default value:
The device incorporates 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
finger might inadvertently activate an adjacent key along with
the desired one.
Signal
Reference
Copyright © 2003-2005 QRG Ltd
7
Table 4-6, page 35
0 (off)
QT1100A-ISG R3.02/1105