Figure 2.4 SYNC Mode (Short Pulses)

SNSK

QT100

SYNC

>10us

>10us

>10us

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.

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

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.

4

QT100_3R0.09_0707

Figure 2.1 Low Power Mode (SYNC held low)

SYNC

SNSK

QT100

sleep

sleep

sleep

fast detect

integrator

OUT

~85ms

Figure 2.2 Fast Mode Bursts (SYNC held high)

SNSK

QT100

SYNC

~1ms

Figure 2.3 SYNC Mode (triggered by SYNC edges)

SYNC

SYNC

SNSK

QT100

SNSK

QT100

slow mode sleep period

sleep

sleep

sleep

sleep

sleep

sleep

Revert to Fast Mode

Revert to Slow Mode

slow mode sleep period