SX8654ICSTRT SEMTECH [Semtech Corporation], SX8654ICSTRT Datasheet - Page 23

• Manufacturer Part Number: SX8654ICSTRT
• Description: Haptics Enabled 4/5-Wire Resistive Touchscreen Controller with Proximity Sensing
• Manufacturer: SEMTECH [Semtech Corporation]

ADVANCED COMMUNICATIONS & SENSING
5
5.1
The purpose of the proximity sensing interface is to detect when a conductive object (usually a body part i.e.
finger, palm, face, etc) is in the proximity of the system. This is commonly used in power-sensitive mobile
applications to turn the screen’s LCD ON/OFF depending on user’s finger/palm/face proximity.
The chip’s proximity sensing interface is based on capacitive sensing technology and shares the ADC with the
touchscreen interface (Cf § 5.4.2). An overview is given in figure below.
To save power since the proximity event is slow by nature, the block will be waken-up regularly at every
programmed scan period (PROXSCANPERIOD) to sense and then process a new proximity sample. The block
will be in idle mode most of the time. This is illustrated in figure below
Rev 1 – 25
P
ROXIMITY
Introduction
The sensor can be the top layer of the touchscreen or a simple copper area on the PCB (programmable
in PROXSENSORCON). Its capacitance (to ground) will vary when a conductive object is moving in its
proximity.
The optional shield can be the bottom layer of the touchscreen or a simple copper area on the PCB
(programmable in PROXSHIELDCON) below/under/around the sensor. It is used to protect the sensor
against potential surrounding noise sources and improve its global performance. It also brings directivity
to the sensing, for example sensing objects approaching from top only.
The analog front-end (AFE) performs the raw sensor’s capacitance measurement and converts it into a
12 bit digital code. It also controls the shield. See § 5.2 for more details.
The digital processing block computes the raw capacitance measurement from the AFE and extracts a
binary information PROXSTAT corresponding to the proximity status, i.e. object is “Far” or “Close”. It
also triggers AFE operations (compensation, etc). See § 5.3 for more details.
th
July 2011
S
ENSING
Sensor
Shield
I
NTERFACE
Figure 26 – Proximity Sensing Interface Overview
Finger, palm,
Figure 27 – Proximity Sensing Sequencing
face, etc
(SX8654/56)
SX8654/56
Front-End
Analog
(AFE)
Resistive Touchscreen Controller with Proximity Sensing
23
Processing
SX8654/SX8655/SX8656
Digital
PROXSTAT
0
Haptics Enabled 4/5-Wire
1
0
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