1 Overvie
1.1
Parameters
The QT1080 is an easy to use, eight-touch-key sensor IC
based on Quantum’s patented charge-transfer 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.
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 two successive bursts of pulses:
Burst A: Keys 0, 1, 4, 5
Burst B: Keys 2, 3, 6, 7
Bursts always operate in A-B sequence.
Self-calibration: On power-up, all eight keys are
self-calibrated within 350 milliseconds (typical) to provide
reliable operation under almost any conditions.
Autorecalibration: 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).
The device also autorecalibrates a key when its signal
reflects a sufficient decrease in capacitance. In this case the
device recalibrates after ~2 seconds so as to recover normal
operation quickly.
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.
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 2 seconds; in the
opposing direction, it is one count every 500ms.
Detection Integrator (DI) confirmation reduces the effects
of noise on the QT1080. The ‘detect integrator’ 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.
A per-key counter is incremented each time the key has
exceeded its threshold and stayed there for a number of
measurement bursts. When this counter reaches a preset
limit the key is finally declared to be touched.
For example, if the limit value is six, then the device has to
exceed its threshold and stay there for six measurement
bursts in succession without going below the threshold level,
before the key is declared to be touched. If on any
measurement burst the signal is not seen to exceed the
threshold level, the counter is cleared and the process has to
start from the beginning.
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), 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.
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.
Sync Mode: The QT1080 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 two sequential acquire bursts, in sequence A-B
from the Sync signal. Thus, each Sync pulse causes all eight
keys to be acquired.
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 touch
detection, 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.
AKS™ Adjacent Key Suppression is a patent-pending
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.
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 QT1080 has two different AKS groupings of keys,
selectable via option resistors. These groupings are:
# AKS operates in two groups of four keys.
# AKS operates over all eight keys.
These two modes allow the designer to provide AKS while
also providing for shift or function operations.
If AKS is disabled, all keys can operate simultaneously.
Outputs: There are two output modes: one per key, and
binary coded.
One per key output:
In this mode there is one output pin per
key. This mode has two output drive options, push-pull and
open-drain. The outputs can also be made either active-high
or active-low. These options are set via external
configuration resistors.
Binary coded output:
In this mode, three output lines encode
for one possible key in detect. If more than one key is
detecting, only the first one touched will be indicated.
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.
lQ
3 QT1080 R11.06/0806
