Datasheet

5
AT42QT1040 [DATASHEET]
9524D–AT42–05/2013
2. Overview of the AT42QT1040
2.1 Introduction
The AT42QT1040 (QT1040) is a digital burst mode charge-transfer (QT
™
) capacitive sensor 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 de-bounced. 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.
2.2 Signal Processing
2.2.1 Detect Threshold
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 Section 3.1 on page 7 for details
on how to adjust the sensitivity of each key.
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.
2.2.2 Detection Integrator
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.
2.2.3 Burst Length Limitations
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 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
website), for more information on using a scope to measure the pulses and hence determine the burst length. Refer
also to the Touch Sensors Design Guide.
2.2.4 Adjacent Key Suppression Technology
The device includes the Atmel-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.
The use of AKS is selected by connecting a 1 M resistor between Vdd and the SNSK0 pin (see Section 4.1 on
page 9 for more information). When AKS is disabled, any combinations of keys can enter detect.