1 Overvie
1.1
Differences With QT1080
The QT1081 is a general replacement device for the highly
popular QT1080. It has all of the same features as the older
device but differs in the following ways:
Rs resistors on each channel eliminated
Up to 4x more sensitive for a given value of Cs
Shorter burst lengths, less power for a given value of Cs
‘Burst B’ only mode for lower key counts with less power
The QT1081 should be used over the QT1080 for new
designs due to a simpler circuit, lower power and lower cost.
1.2 Parameters
1.2.1 Introduction
The QT1081 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.
1.2.2 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.
1.2.3 Self-calibration
On power-up, all eight keys are self- calibrated within 300
milliseconds (typical) to provide reliable operation under
almost any conditions.
1.2.4 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.
1.2.5 Drift Compensation
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.
1.2.6 Detection Integrator Confirmation
Detection Integrator (DI) confirmation reduces the effects of
noise on the QT1081. 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.
1.2.7 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.
1.2.8 Sync Mode
The QT1081 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.
1.2.9 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 requested to
return to LP mode.
When four or fewer keys are required, current drain in LP
mode can be further reduced by choosing appropriate
channels on the QT1081.
1.2.10 Adjacent Key Suppression (AKS™)
AKS™ is a Quantum-patented feature that can be enabled via
resistor strap option. 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.
lQ 3 QT1081_1R0.04_0307
