Spread Spectrum operation: The bursts operate over a
spread of frequencies, so that external fields will have
minimal effect on key operation and emissions are very
weak. Spread spectrum operation works with the DI
mechanism to dramatically reduce the probability of false
detection due to noise.
1 Overview
The QT1101 is an easy to use, ten touch-key sensor IC
based on Quantum’s patented charge-transfer (‘QT’)
principles for robust operation and ease of design. This
device has many advanced features which provide for
reliable, trouble-free operation over the life of the product.
Sync Mode: The QT1101 features a Sync mode to allow the
device to slave to an external signal source, such as a mains
signal (50/60Hz), to limit interference effects. This is
performed using the SYNC/LP pin. Sync mode operates by
triggering three sequential acquire bursts, in sequence
A-B-C from the Sync signal. Thus, each Sync pulse causes
all ten keys to be acquired.
Burst operation: The device operates in ‘burst mode’. Each
key is acquired using a burst of charge-transfer sensing
pulses whose count varies depending on the value of the
reference capacitor Cs and the load capacitance Cx. In LP
mode, the device sleeps in an ultra-low current state
between bursts to conserve power. The keys signals are
acquired using three successive bursts of pulses:
Low Power (LP) Mode: The device features an LP mode for
microamp levels of current drain with a slower response
time, to allow use in battery operated devices. On detection
of touch, the device automatically reverts to its normal mode
and asserts the DETECT pin active to wake up a host
controller. The device remains in normal, full acquire speed
mode until another pulse is seen on its SYNC/LP pin, upon
which it goes back to LP mode.
Burst A: Keys 0, 1, 4, 5
Burst B: Keys 2, 3, 6, 7
Burst C: Keys 8, 9
Bursts always operate in A-B-C sequence.
Self-calibration: On power-up, all ten keys are
self-calibrated within 450ms typical to provide reliable
operation under almost any conditions.
AKS™ Adjacent Key Suppression is a patented feature
that can be enabled via jumper resistors. AKS works to
prevent multiple keys from responding to a single touch, a
common complaint about capacitive touch panels. This can
happen with closely spaced keys, or with control surfaces
that have water films on them.
Auto-recalibration: The device can time out and recalibrate
each key independently after a fixed interval of continuous
touch detection, so that the keys can never become ‘stuck
on’ due to foreign objects or other sudden influences. After
recalibration the key will continue to function normally. The
delay is selectable to be either 10s, 60s, or infinite
(disabled).
AKS operates by comparing signal strengths from keys
within a group of keys to suppress touch detections from
those that have a weaker signal change than the dominant
one.
The device also auto-recalibrates a key when its signal
reflects a sufficient decrease in capacitance. I n this case the
device recalibrates after ~2 seconds so as to recover normal
operation quickly.
The QT1101 has two different AKS groupings of keys,
selectable via option resistors. These groupings are:
Drift compensation operates to correct the reference level
of each key slowly but automatically over time, to suppress
false detections caused by changes in temperature,
humidity, dirt and other environmental effects.
AKS operates in three groups of keys.
AKS operates over all ten keys.
These two modes allow the designer to provide AKS while
also providing for shift or function operations.
The drift compensation is asymmetric. In the increasing
capacitive load direction the device drifts more slowly than in
the decreasing direction. In the increasing direction, the rate
of compensation is one count of signal per two seconds. In
the opposing direction, it is one count every 500ms.
If AKS is disabled, all keys can operate simultaneously.
Outputs: The QT1101 has a serial output using one or two
wires, RS-232 data format, and automatic baud rate
detection. A simple protocol is employed.
Detection Integrator (DI) confirmation reduces the effects
of noise on the QT1101 outputs. The DI mechanism requires
consecutive detections over a number of measurement
bursts for a touch to be confirmed and indicated on the
outputs. In a like manner, the end of a touch (loss of signal)
has to be confirmed over a number of measurement bursts.
This process acts as a type of ‘debounce’ against noise.
The QT1101 operates in slave mode, i.e. it only sends data
to the host after receiving a request from the host.
An additional /CHANGE (state changed) signal allows the
use of the serial interface to be optimised, rather than being
polled continuously.
Simplified Mode: To reduce the need for option resistors,
the simplified operating mode places the part into fixed
settings with only the AKS feature being selectable. LP
mode is also possible in this configuration. Simplified mode
is suitable for most applications.
In normal operation, both the start and end of a touch must
be confirmed for six measurement bursts. In a special ‘Fast
Detect‘ mode (available via jumper resistors) (Tables 1.2 and
1.6), confirmation of the start of a touch requires only two
sequential detections, but confirmation of the end of a touch
is still six bursts.
Fast detect is only available when AKS is disabled.
Lq
2
QT1101 R4.06/0806
QUANTUM [ QUANTUM RESEARCH GROUP ]
