UR7HCTS2-P840 Datasheet, PDF (4/16 Pages) Semtech Corporation – High-Performance, Low Power PS/2 Touch Screen Controller/Digitizer

FUNCTIONAL DESCRIPTION
The ScreenCoder^TM 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.
SENSOR INTERFACE
To obtain position information from
the Resistive Touch Screen Sensor,
the ScreenCoder^TM 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^TM
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.
OSCILLATOR
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^TM 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^TM PS2.
PS/2 COMMUNICATION
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.
ERROR HANDLING
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^TM PS2 has responded or until 25 milliseconds have elapsed.
POWER MANAGEMENT
The ScreenCoder^TM PS2 implements two power management methods:
Self-Power Management^TM and System-coordinated Power Management.
Self-Power Management
Self-Power Management^TM of the ScreenCoder^TM PS2 permits,
independently of any system intervention, the lowest power consumption
possible within the present parameters and conditions of operation.
Through Self-Power Management^TM, the ScreenCoder^TM PS2 is capable of
operating - most of the time - at only 1uA, independently of the state of the
system.
Copyright ©1999-2001 Semtech Corporation
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DOC7-TS2-P840-DS-106
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