SYB
SXR

XPWRS

RSVD1

VREF

RESET

VSS2

VDD

LB
XR2
YB2
YT2
XL2
HDAT
XDAT
XCLK

OCSIN

OSCOUT

VSS

XR1

YB1

YT1

XL1

HCLK

RSVD2

RSVD3

RSVD4

RSVD5

POWER_DOWN

SXL

SYT

· Portable systems
· Notebooks and Laptops

· Interactive Kiosks
· Touch-enabled monitors

The ScreenCoder

PS2 is a high-

performance IC that interfaces any
4 or 8-wire resistive touch screen
to the standard PS/2 mouse port.
Low-power and tiny, the single IC
is ideal for portable systems, web
phones and interactive kiosks. It
offers an advanced algorithm for
enhanced motion control as well as
an extra port for hot-plug
connection of an external PS/2
pointing device. Both the touch
screen and the external device can
be operated concurrently without
any penalty.

The UR7HCTS2-P840 is equipped
with a special command protocol
that enables the touch screen to
operate in either relative (mouse-
like) or absolute positioning mode.
Because the IC interfaces via the
PS/2 mouse port, it can utilize any
standard mouse driver when
operating in relative mode. For
absolute positioning, offers a high-
functionality driver which includes
inking capabilities.

Mice connected to the
ScreenCoder

PS2 external mouse

port will always act in relative
mode, even if the touch screen is
operating in the absolute mode.
The PS/2 communication channel
is bi-directional at 10 kbps.

The ScreenCoder

PS2 offers built-

in A/D. Touch detection and
motion are handled in firmware,
and no digitizer is required. The
cost-effective solution requires few
external components, allowing for
a simple and real estate saving
implementation.

· Embedded touch screen and

external pointing device can
operate concurrently with no
performance penalty

· Supports both absolute and

relative (mouse-like) modes of
operation

· For relative mode, no special

drivers are needed; high-
functionality driver available for
absolute mode

· Supports all commands defined

in the IBM PS/2 Mouse
Communication Protocol, as
well as Semtech's extended PS/2
communication protocol with
absolute positioning support

· Available in a low-profile 32-pin

LQFP package

· Few external components required

· Controller & digitizer in a single IC;

no need for external A/D

· Interfaces ANY 4- or 8-wire

resistive touch screen to standard
PS/2 mouse port; regardless of
size, material or vendor

· High-performance IC offers

accurate cursor control due to
advanced algorithms

· Low-power consumption, due to

sophisticated power management
states, ideal for battery-operated
systems

· Highly resistant to RF & other

noise sources

· Enables hot-plug connection of

an external pointing device

· Wheelmouse functionality is

supported in the external pointing
device

· Other interfaces available

APPLICATIONS

PIN ASSIGNMENTS

DESCRIPTION

FEATURES

ScreenCoder

UR7HCTS2-P840

High-Performance, Low Power PS/2

Touch Screen Controller/Digitizer

ScreenCoder and MouseCoder are trademarks of
Semtech Corporation. All other trademarks belong
to their respective companies.

www.semtech.com

HID & SYSTEM MANAGEMENT PRODUCTS, MOUSECODER

FAMILY

PIN DEFINITIONS

www.semtech.com

Mnemonic

Pin #

Type

Name and Function

VDD

Power Supply

VSS

Ground

VSS2

Ground

OSCIN

Oscillator Input: external clock input or one
side of the Ceramic Resonator with built-in Load
Capacitors

_OSCOUT

Oscillator Output: open for external clock
input or other side of the Ceramic Resonator with
built-in Load Capacitors

_RESET

Reset: apply 0V to provide orderly start-up

HDAT

I/O (nd)

Mouse Data: connects to Host's data line

HCLK

I/O (nd)

Mouse Clock: connects to Host's clock line

XCLK

I/O (nd)

External Mouse Clock: PS/2 clock signal from
external mouse; keep open if unused

XDAT

I/O (nd)

External Mouse Data: PS/2 data signal from
external mouse; keep open if unused

_LB

I/O (nd)

Left Button: active low, strobed sampling

_RB

I/O (nd)

Right Button: active low, strobed sampling

XR1

I/O

Sensor's Excitation Driver: connect to X Right

YB1

I/O

Sensor's Excitation Driver: connect to Y Bottom

YT1

I/O

Sensor's Excitation Driver: connect to Y Top

XL1

I/O

Sensor's Excitation Driver: connect to X Left

XL2

I/O

Sensor's Excitation Driver: connect to X Left

YT2

I/O

Sensor's Excitation Driver: connect to Y Top

YB2

I/O

Sensor's Excitation Driver: connect to Y Bottom

XR2

I/O

Sensor's Excitation Driver: connect to X Right

VREF

Reference Voltage for built-in A/D

SXL

Sense line: for X Left

SYT

Sense line: for Y Top

SYB

Sense line: for Y Bottom

SXR

Sense line: for X Right

_POWER_

Hardware Power Down: tie high if unused

DOWN

XPWRS

External Mouse Power Sense: tie high if unused
and external mouse port is used; tie low if external
mouse port is unused

RSVD1-5 4, 26

I/O

Reserved

27-29

Note: An underscore before a pin mnemonic denotes an active low signal.
Pin Types Legend: AI=Analog Input; I=Input; O=Output; I/O=Input or Output;
I/O (nd)=Input or Output with N-channel Open Drain driver

FUNCTIONAL DESCRIPTION

www.semtech.com

The ScreenCoder

UR7HCTS2-P840 consists functionally of six major sections (see the Functional Diagram on

page 2). These are the Sensor Interface, Power Management section, the 16-Bit Timer, the Oscillator Circuit, the
PS/2 Communication Port and the 8042 Emulation Port. All sections communicate with each other and
operate concurrently.

To obtain position information from
the Resistive Touch Screen Sensor,
the ScreenCoder

PS2 uses four

internal drivers (two pins for each
driver) and four sensing lines.
During sampling, first the drivers
and sensing lines for X axis are
activated, by setting one X driver
high and the other X driver low;
the drivers for Y axis are set
floating. This action produces a
voltage gradient across the touch
screen's surface in the X direction.

The internal A/D measures both the
voltage across the activated X
plane and the voltage potential
between the planes. Next, the
drivers for the Y axis are activated,
while the drivers for the X axis are
set floating. Again, the internal A/D
measures both voltage across the
activated plane and potential
between the planes. The X and Y
absolute position information is
calculated from these four A/D
measurements.

Before the measurement of X
and Y positions, the ScreenCoder

PS2 checks if there is any touch
pressure applied to the sensor.
Proprietary touch detection
algorithm performs this test very
quickly, accommodating sensors
with various plate-to-plate
capacitances. Actual
measurements are somewhat more
complex and are covered by a US
and international patent currently
pending.

This IC has a built-in Oscillator circuit capable of operations with an
external 4.00 MHz Clock source, or a Ceramic Resonator (preferably with
built-in Load Capacitors). Note that the Crystals can NOT be used. The
ScreenCoder

PS2 frequently switches its Oscillator OFF and ON in order

to operate with the least amount of power consumption. Due to their very
high Q, the Crystal-based oscillators have exceedingly long Start-up times
and can NOT be used with the ScreenCoder

PS2.

SENSOR INTERFACE

OSCILLATOR

At start-up, or upon receiving a reset command, the UR7HCTS2-P840 will
wait between 300 and 500 milliseconds before sending an AAh to the Host
followed by a device ID of 00h. The IC will then set itself to its default
values (i.e., Incremental Stream Mode with 1:1 scaling, and a report
rate of 100 Hz). The device will then disable itself until an Enable (F4h)
command is sent from the Host.

PS/2 COMMUNICATION

For every correct command or parameter received from the Host, the
UR7HCTS2-P840 sends an Acknowledge (FAh). If an invalid command or
parameter is received, the UR7HCTS2-P840 issues a Resend Request
(FEh). If an invalid input is again received, the device transmits an Error
Code (FCh) to the Host.

Both Error and Resend Request responses are sent by the device within 25
milliseconds. Host may not issue any new commands until either the
ScreenCoder

PS2 has responded or until 25 milliseconds have elapsed.

ERROR HANDLING

POWER MANAGEMENT

The ScreenCoder

PS2 implements two power management methods:

Self-Power Management

and System-coordinated Power Management.

Self-Power Management
Self-Power Management

of the ScreenCoder

PS2 permits,

independently of any system intervention, the lowest power consumption
possible within the present parameters and conditions of operation.
Through Self-Power Management

, the ScreenCoder

PS2 is capable of

operating - most of the time - at only 1uA, independently of the state of the
system.

POWER MANAGEMENT (CON'T)

www.semtech.com

During the "Critical suspend", the ScreenCoder

will shut down all of the

pointing activities. However, the communications with the Host are still
enabled.

External PS/2 power down

The ScreenCoder

PS2 monitors the power state of the external PS/2

pointing device through the XPWRS pin. If the IC senses that the external
PS/2 device has been powered-down by the host, it will actively eliminate
shot-through current in the input buffers for the clock and data lines by
driving both low.

The ScreenCoder

UR7HCTS2-P840 will re-initialize the external PS/2

pointing device when the power to the external PS/2 port is restored.

Figure 1: Semtech's Self-Power Management

State

RUN

STOP

System Request To Send,

Mouse hot plug or Data Packet,

Touch Screen activity,

Button press

1s of inactivity

Critical Suspend

Ext Mouse

power down

Normal State

_PWR_DOWN=0

_PWR_DOWN=1
and XPWRS=1

_PWR_DOWN=0

_PWR_DOWN=1
and XPWRS=1

_PWR_DOWN=1
and XPWRS=0

Figure 2: Semtech's System-coordinated Power Management

The "Stop" mode is the lowest
power consumption mode. In this
mode, the oscillator is stopped and
the IC consumes only the leakage
current. This is the default mode to
which the IC will revert when it is
idle. An event or signal conditions
wake-up the IC. The
ScreenCoder

PS2 can still

operate most of the time at only
1uA, even when the host is in the
active state, and with active
external PS/2 device attached to
the ScreenCoder

PS2. If an

external PS/2 device sends a data
packet, the ScreenCoder

PS2 will

exit the "Stop" mode for as long as
it takes to process the message
and relay the information to the
system. This operation is done
transparently to the host, without
any data loss or any response
delay from the input device.

System-coordinated Power
Management

Normal Operation State

In normal operation state, both
_PWR_DOWN and XPWRS pins are
in high state. ScreenCoder

PS2

operations are controlled by Self-
Power Management

Critical Suspend

At any time, the Host may negate
the _PWR_DOWN pin in order to
force the ScreenCoder

PS2 into a

"Critical suspend" mode. This
signal can be activated (driven
low) due to specific conditions of
the Host's operations (for example,
a discharged battery), or due to
actions of the Operating System or
BIOS.

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