ANALOG DEVICES AD7879 Service Manual

VCC/REF

Low Voltage Controller for Touch Screens

FEATURES

4-wire touch screen interface

1.6 V to 3.6 V operation Median and averaging filter to reduce noise Automatic conversion sequencer and timer User-programmable conversion parameters Auxiliary analog input/battery monitor (0.5 V to 5 V) 1 optional GPIO Interrupt outputs ( Touch-pressure measurement Wake-up on touch function Shutdown mode: 6 μA maximum 12-ball, 1.6 mm × 2 mm WLCSP 16-lead, 4 mm × 4 mm LFCSP

APPLICATIONS

Personal digital assistants Smart handheld devices Touch screen monitors Point-of-sale terminals Medical devices Cell phones

INT

PENIRQ

)

X+ X–

Y+ Y–

GND

AUX/VBAT/GPIO

AD7879/AD7889

FUNCTIONAL BLOCK DIAGRAM

X– Y– X+ Y+

REF–

TEMPERATURE

SENSOR

RESULT

REGISTERS

ADD0

REF–

12-BIT

SAR ADC

6-TO-1 M UX

AD7879/

AD7879-1/

AD7889/

AD7889-1

SERIAL PORT

DIN/

DOUT/

ADD1

SDA

Figure 1.

REF+

SCLCS/

FILTERING

RESULT

REGISTERS

CONTROL

REGISTERS

SEQUENCER

AND TIMER

PENIRQ/INT/DAV

07667-001

GENERAL DESCRIPTION

The AD7879/AD7889 are 12-bit successive approximation analog-to-digital converters (SAR ADCs) with a synchronous serial interface and low on-resistance switches for driving 4-wire resistive touch screens. The AD7879/AD7889 work with a very low power supply—a single 1.6 V to 3.6 V supply—and feature throughput rates of 105 kSPS. The devices include a shutdown mode that reduces current consumption to less than 6 μA.

To reduce the effects of noise from LCDs and other sources, the AD7879/AD7889 contain a preprocessing block. The preprocessing function consists of a median filter and an averaging filter. The combination of these two filters provides a more robust solution, discarding the spurious noise in the signal and keeping only the data of interest. The size of both filters is programmable. Other user-programmable conversion controls include variable acquisition time and first conversion delay; up to 16 averages can be taken per conversion. The AD7879/AD7889 can run in slave mode or standalone (master) mode, using an automatic conversion sequencer and timer.

Rev. C

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The AD7879/AD7889 have a programmable pin that can operate as an auxiliary input to the ADC, as a battery monitor, or as a GPIO. In addition, a programmable interrupt output can operate in three modes: as a general-purpose interrupt to signal when new data is available (

limits are exceeded ( screen is touched (

DAV

), as an interrupt to indicate when

INT

), or as a pen-down interrupt when the

PENIRQ

). The AD7879/AD7889 offer temp-

erature measurement and touch-pressure measurement.

The AD7879 is available in a 12-ball, 1.6 mm × 2 mm WLCSP and in a 16-lead, 4 mm × 4 mm LFCSP. The AD7889 is available in a backside-coated version of the WLCSP. Both parts support an SPI interface (AD7879/AD7889) or an I

C interface

(AD7879-1/AD7889-1).