qt100 Quantum Research Group, qt100 Datasheet - Page 4

• Manufacturer Part Number: qt100
• Description: Charge-transfer Qtouch? Ic Touch Screen
• Manufacturer: Quantum Research Group

2.1.4 SYNC Mode
It is possible to synchronize the device to an external clock
source by placing an appropriate waveform on the SYNC pin.
SYNC mode can synchronize multiple QT100 devices to each
other to prevent cross-interference, or it can be used to
enhance noise immunity from low frequency sources such as
50Hz or 60Hz mains signals.
lQ
QT100
SNSK
SYNC
QT100
QT100
SNSK
SYNC
SNSK
SYNC
OUT
Figure 2.3 SYNC Mode (triggered by SYNC edges)
Figure 2.2 Fast Mode Bursts (SYNC held high)
Figure 2.1 Low Power Mode (SYNC held low)
QT100
SNSK
QT100
SYNC
Figure 2.4 SYNC Mode (Short Pulses)
SNSK
SYNC
sleep
sleep
~85ms
sleep
~1ms
>10us
sleep
sleep
sleep
>10us
slow mode sleep period
slow mode sleep period
sleep
sleep
fast detect
integrator
>10us
Revert to Slow Mode
Revert to Fast Mode
sleep
4
timer setting the sensor performs a full recalibration. This is
known as the Max On-duration feature and is set to ~80s (at
3V). This will vary slightly with Cs and if SYNC mode is used.
As the internal timebase for Max On-duration is determined by
the burst rate, the use of SYNC can cause dramatic changes
in this parameter depending on the SYNC pulse spacing.
2.4 Detect Integrator
It is desirable to suppress detections generated by electrical
noise or from quick brushes with an object. To accomplish
this, the QT100 incorporates a ‘detect integration’ (DI) counter
that increments with each detection until a limit is reached,
after which the output is activated. If no detection is sensed
prior to the final count, the counter is reset immediately to
zero. In the QT100, the required count is four. In LP mode the
device will switch to Fast mode temporarily in order to resolve
the detection more quickly; after a touch is either confirmed or
denied the device will revert back to normal LP mode
operation automatically.
The DI can also be viewed as a 'consensus' filter, that
requires four successive detections to create an output.
The SYNC pin is sampled at the end of each burst. If the
device is in Fast mode and the SYNC pin is sampled high,
then the device continues to operate in Fast mode
(Figure 2.2). If SYNC is sampled low, then the device
goes to sleep. From then on, it will operate in SYNC mode
(Figure 2.1). Therefore, to guarantee entry into SYNC
mode the low period of the SYNC signal should be longer
than the burst length (Figure 2.3).
However, once SYNC mode has been entered, if the
SYNC signal consists of a series of short pulses (>10µs)
then a burst will only occur on the leading edge of each
pulse (Figure 2.4) instead of on each change of SYNC
signal, as normal (Figure 2.3).
In SYNC mode, the device will sleep after each
measurement burst (just as in LP mode) but will be
awakened by a change in the SYNC signal in either
direction, resulting in a new measurement burst. If SYNC
remains unchanged for a period longer than the LP mode
sleep period (about 85ms), the device will resume
operation in either Fast or LP mode depending on the
level of the SYNC pin (Figure 2.3).
There is no DI in SYNC mode (each touch is a detection)
but the Max On-duration will depend on the time between
SYNC pulses; see Sections 2.3 and 2.4. Recalibration
timeout is a fixed number of measurements so will vary
with the SYNC period.
2.2 Threshold
The internal signal threshold level is fixed at 10 counts of
change with respect to the internal reference level, which
in turn adjusts itself slowly in accordance with the drift
compensation mechanism.
The QT100 employs a hysteresis dropout of two counts
of the delta between the reference and threshold levels.
2.3 Max On-duration
If an object or material obstructs the sense pad the signal
may rise enough to create a detection, preventing further
operation. To prevent this, the sensor includes a timer
which monitors detections. If a detection exceeds the
QT100_3R0.09_0707