sx8646 Semtech Corporation, sx8646 Datasheet - Page 14

• Manufacturer Part Number: sx8646
• Description: Ultra Low Power, Capacitive Button Touch Controller 8 Sensors With Enhanced Led Drivers
• Manufacturer: Semtech Corporation

SX8646
Ultra Low Power, Capacitive Button Touch Controller
(8 sensors) with Enhanced LED Drivers
ADVANCED COMMUNICATIONS & SENSING
DATASHEET
3.5 Sensors on the PCB
The capacitive sensors are relatively simple copper areas on the PCB connected to the eight SX8646 capacitive
sensor input pins (CAP0…CAP7).The sensors are covered by isolating overlay material (typically 1mm...3mm).
The area of a sensor is typically one square centimeter which corresponds about to the area of a finger touching
the overlay material.
The capacitive sensors can be setup as ON/OFF buttons (see example Figure 9).
layer of four sensors for buttons
Figure 9 PCB top
(surrounded by a ground plane)
3.6 Button Information
3.6.1
Button Information
The buttons have two simple states (see Figure 10): ON (touched by finger) and OFF (released and no finger
press).
Buttons
Figure 10
A finger is detected as soon as the number of ticks from the ASI reaches a user-defined threshold plus a
hysteresis.
A release is detected if the ticks from the ASI go below the threshold minus a hysteresis. The hysteresis around
the threshold avoids rapid touch and release signaling during transients.
3.7 Analog Sensing Interface
The Analog Sensing Interface (ASI) converts the charge on the sensors into ticks which will be further digitally
processed. The basic principle of the ASI will be explained in this section.
The ASI consists of a multiplexer selecting the sensor, analog switches, a reference voltage, an ADC sigma delta
converter, an offset compensation DAC and an external integration capacitor (see Figure 11).
Revision 7_4, February 10
© 2010 Semtech Corp.
www.semtech.com
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