AT42QT1040-MMH QS418 Atmel, AT42QT1040-MMH QS418 Datasheet - Page 4

• Manufacturer Part Number: AT42QT1040-MMH QS418
• Manufacturer: Atmel

Specifications of AT42QT1040-MMH QS418

• Lead Free Status / RoHS Status: Compliant

2. Overview of the AT42QT1040
2.1
2.2
2.2.1
2.2.2
2.2.3
2.2.4
4
Introduction
Signal Processing
AT42QT1040
Detect Threshold
Detection Integrator
Burst Length Limitations
Adjacent Key Suppression Technology
The AT42QT1040 (QT1040) is a digital burst mode charge-transfer (QT
driver designed for touch-key applications. The device can sense from one to four keys; one to
three keys can be disabled by not installing their respective sense capacitors. Any of the four
channels can be disabled in this way.
The device includes all signal processing functions necessary to provide stable sensing under a
wide variety of changing conditions, and the outputs are fully debounced. Only a few external
parts are required for operation.
The QT1040 modulates its bursts in a spread-spectrum fashion in order to heavily suppress the
effects of external noise, and to suppress RF emissions.
The internal signal threshold level is fixed at 10 counts of change with respect to the internal
reference level. This in turn adjusts itself slowly in accordance with the drift compensation
mechanism. See
When going out of detect there is a hysteresis element to the detection. The signal threshold
must drop below 8 counts of change with respect to the internal reference level to register as un-
touched.
The device features a detection integration mechanism, which acts to confirm a detection in a
robust fashion. A per-key counter is incremented each time the key has exceeded its threshold,
and a key is only finally declared to be touched when this counter reaches a fixed limit of 5. In
other words, the device has to exceed its threshold, and stay there for 5 acquisitions in
succession without going below the threshold level, before the key is declared to be touched.
Burst length is the number of times the charge transfer process is performed on a given channel;
that is, the number of pulses it takes to measure the key’s capacitance.
The maximum burst length is 2048 pulses. The recommended design is to use a capacitor that
gives a signal of <1000 pulses. Longer bursts take more time and use more power.
Note that the keys are independent of each other. It is therefore possible, for example, to have a
signal of 100 on one key and a signal of 1000 on another.
Refer to Application Note QTAN0002, Secrets of a Successful QTouch™ Design (downloadable
from the Atmel
hence determine the burst length. Refer also to the Touch Sensors Design Guide.
The device includes Atmel’s patented Adjacent Key Suppression (AKS) technology, to allow the
use of tightly spaced keys on a keypad with no loss of selectability by the user.
There is one global AKS group, implemented so that only one key in the group may be reported
as being touched at any one time.
®
website), for more information on using a scope to measure the pulses and
Section 3.1 on page 6
for details on how to adjust each key’s sensitivity.
™
) capacitive sensor
9524B–AT42–04/09