qt1100a Quantum Research Group, qt1100a Datasheet - Page 28

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

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 the product of
FDIL and NDIL.
Example: If FDIL = 5 and NDIL = 2, the total DI count
required is 10, even though the device only scanned through
all keys twice.
The DI mechanism 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
mechanism can be used to intentionally slow down touch
response 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. The Fast-DI feature is effectively disabled.
If FDIL m 2, then the fast-DI counter also operates in addition
to the NDIL counter.
If the key has not yet been declared active, the following
takes place:
Once the key is declared active, the following takes place:
4.10 PTHR - Positive Threshold Bits
Byte 30, Bits 7..4
Default value:
Typical value:
The positive threshold is used to provide a mechanism for
recalibration of the reference point when a key signal moves
abruptly to the positive. This condition is abnormal; it usually
occurs after a prolonged touch has caused an automatic
recalibration via the NRD parameter, and a subsequent
removal of touch causes the signal to rise abruptly above the
reference level.
The normal desire is to recover from these events quickly ,
usually in a second or two. PTHR is the upper threshold for
the detection of these anomalies; PHYS is the corresponding
hysteresis value (see below) used to provide a stable
detection criteria. Positive recal delay (PRD) is the timing
function used to time when the key is recalibrated once the
positive excursion has been noted.
For a further description see PHYS below.
LQ
1. If (Signal [NTHR): The signal is strong, and the fast-DI
2. If (Signal > NTHR): Both the fast-DI and normal DI
3. If (Normal DI counter = NDIL): The key is declared
1. If (Signal [ NTHR): The signal is still strong enough to
2. If (Signal > NTHR) and (Signal [ NHYS): The key
3. If (Signal > NHYS): There is insufficient signal to
4. If (Normal DI counter = 0), the key is declared inactive.
counter is incremented towards FDIL. Once FDIL is
reached, the Normal DI counter increments once.
counters are cleared due to lack of sufficient signal.
active.
sustain key detection. The key remains in detection,
and the Normal DI counter increments if it is less than
NDIL.
signal is weak, however the key remains in detection
and the Normal DI counter remains unchanged.
sustain key detection, and the N ormal DI counter is
decremented towards 0.
5
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Positive drift compensation (PDCR) also works to restore
signal levels that are erroneously positive. However this
mechanism is much slower and is used primarily to
compensate for longer term drift factors, whereas PTHR is
used to compensate more quickly for fast rises in signal
value.
The PTHR parameter is global to all keys ; it is a single byte
parameter common to all. It is measured in counts of signal
and can be set from 2..15.
4.11 PHYS - Positive Hysteresis Bits
Byte 30, Bit 3..0
Default value:
Typical value:
Positive hysteresis is used in setting the drop out level at
which a positive detection ceases (see PTHR above).
The value is expressed as a signal count from the positive
threshold value back down towards the reference. Thus
given a scenario:
then the signal has to rise to 73 2 + 5 = 737 to cause a
positive detection. At this point the positive recal delay (PRD)
engages and starts timing the signal excursion. Providing
that the signal level does not fall below 736 (1 count of
hysteresis below 737) the timer will continue until it expires,
at which point the affected key is fully recalibrated (and only
that key).
The PHYS parameter is global to all keys ; it is a single byte
parameter common to all. It is measured in counts of signal
and can be set from 0..15.
4.12 SE, SYNC Control Bits
Byte 32, Bits 1, 0
Default SE value:
Default SYNC value:
The Sync functions allow the device to synchronize to
external clock sources or to other similar devices to prevent
interference effects.
Most interference in capacitance sensors comes from beat
frequency and alias generation due to non-coherency
between the sampling rate of the sensor and an external
noise source. Another similar sensor can be considered a
noise source.
The QT1100A offers two ways to limit this kind of
interference:
1.
2.
The SE (‘Sync Enable’) bit determines whether or not Sync
is used. SYNC can be enabled by setting SE = 1. If SE = 0,
the part runs asynchronously regardless of the state of the
SYNC pin. The SYNC bit determines which mode, 1 or 2, the
device operates in if SE = 1.
Level sensing: Make sure the QT1100A does not
sample at the same time as a similar device, where
physically adjacent keys are concerned.
Edge sensing: Synchronize the QT1100A to an external
repetitive noise source so that there are no beat
frequencies generated and the interference shows up as
a benign DC offset to the acquired signal.
(whichever is greater)
Reference = 732
PTHR = 5 counts
PHYS = 1,
1
10% of PTHR or 1 count
Copyright © 2003-2005 QRG Ltd
0 (off)
0 (level sensing)
QT1100A-ISG R3.02/1105