SX8674ICSTRT datasheet(28/64 Pages) SEMTECH

28 / 64 page

ADVANCED COMMUNICATIONS & SENSING

Rev 1 – 25

th July 2011

www.semtech.com

SX8674/SX8675/SX8676

Haptics Enabled Multitouch 4/5-Wire

Resistive Touchscreen Controller with Proximity Sensing

Block startup: a compensation is automatically requested when the proximity sensing is enabled via

PROXSCANPERIOD.

I2C: a compensation can be manually requested anytime by the host through I2C interface.

PROXAVG update: a compensation can be automatically requested if it is detected that CEnv has drifted

beyond a set level since the last compensation.

PROXCOMPPRD: a compensation can be automatically requested at a predefined rate programmed by

the host.

PROXSTUCK: a compensation can be automatically requested if it is detected that the proximity “Close”

state is lasting abnormally long.

Please note that the compensation request flag can be set anytime but the compensation itself is always done at

the beginning of a scan period to keep all parameters coherent (PROXRAW, PROXAVG, PROXDIFF), see

§5.3.2.

5.2.6

Analog-to-Digital Conversion (ADC)

A 12-bit ADC is used to convert the capacitance information into a digital 12-bit word PROXRAW.

The ADC is shared with the touchscreen interface using time multiplexing (see §5.4.2 for more details).

5.3

Digital Processing

5.3.1

Overview

The main purpose of the digital processing block is to convert the raw capacitance information coming from the

AFE (PROXRAW) into a robust and reliable digital flag (PROXSTAT) indicating if the user’s finger/hand/head is

close to the proximity sensor.

The offset compensation performed in the AFE is a one time measurement. However, the environment

capacitance CEnv may vary with time (temperature, nearby objects, etc). Hence, in order to get the best

estimation of CUser (PROXDIFF) it is needed to dynamically track and subtract CEnv variations. This is performed

by filtering PROXUSEFUL to extract its slow variations (PROXAVG).

PROXDIFF is then compared to user programmable threshold to extract PROXSTAT flag.

Figure 34 – Digital Processing Block Diagram

Digital processing sequencing is illustrated in figure below. At every scan period wake-up (defined by

PROXSCANPERIOD), the block updates sequentially PROXRAW, PROXUSEFUL, PROXAVG, PROXDIFF and

PROXSTAT before going back to Idle mode.

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