SX8655IWLTRT Semtech, SX8655IWLTRT Datasheet - Page 16

• Manufacturer Part Number: SX8655IWLTRT
• Description: 58T7566
• Manufacturer: Semtech

Specifications of SX8655IWLTRT

• Controller Type: Resistive Touch Screen
• Ic Interface Type: I2C, Serial
• Supply Voltage Range: 2.3V To 3.6V
• Operating Temperature Range: -40°C To +85°C
• Resolution (bits): 12bit

ADVANCED COMMUNICATIONS & SENSING
Re-arranging z1 and z2 gives:
This finally results in:
The touch resistance calculation above hence requires three channel measurements (Ypos, z2 and z1) and one
specification data (Rytot).
An alternative calculation method is using Xpos, Ypos, one z channel and both Rxtot and Rytot as shown in the
next calculations.
R1 is inversely proportional to the X coordinate:
Substituting R1 and R4 into z1 and rearranging terms gives:
Please note that the chip only outputs z1, z2, etc. The calculation of R
performed by the host.
4.3.1
In order to perform correct measurements, some time must be given for the touch screen to reach a proper Vref
bias level before the conversion is actually performed. It is a function of the PCB trace resistance connecting the
chip to the touchscreen and also the capacitance of the touchscreen. If tau is this RC time constant, then
POWDLY duration must be programmed to 10 tau to reach 12 bits accuracy.
Adding a capacitor from the touch screen electrodes to ground may also be used to minimize external noise (if
the touchscreen is used as the proximity sensor, make sure you do not exceed the maximum capacitive load for
required for proper proximity sensing operation). The low-pass filter created with the capacitor may increase
settling time requirement. Therefore, POWDLY can be used to stretch the acquisition period and delay
conversion appropriately.
POWDLY can be estimated by the following formula:
4.4
The pen detection circuitry is used both to detect a user action and generate an interrupt or start an acquisition in
PENDET and PENTRG mode respectively. Doing pen detection prior to conversion avoids feeding the host with
dummy data and saves power. Pen detection is also used to disable and resume proximity sensing. For more
details on pen detection usage please refer to § 4.6.
A 4-wire touchscreen will be powered between X+ and Y- through a resistor RPNDT so no current will flow as
long as no pressure is applied to the surface (see figure below). When a touch occurs, a current path is created
bringing X+ to the level defined by the resistive divider determined by RPNDT and the sum of R1, RT and R4.
Rev 1 – 25
Pen Detection
Bias Time (POWDLY)
th
July 2011
R
T
=
Rytot Ypos
-------------------------------- -
PowD ly
4095
R
T
R1
=
Rytot Ypos
=
R
Rxtot
Rytot
R4
=
T
Rxtot
10
=
=
4095
----------- - 1
Resistive Touchscreen Controller with Proximity Sensing
R4
Rytot
z1
=
=
R1
R3
16
Rtouch
------------ -
4095
1
–
z2
z2
----- - 1
z1
+
+
–
Ypos
------------ -
4095
R2
R4
–
Xpos
------------ -
4095
–
SX8654/SX8655/SX8656
Rxtot
z2
----- - 1
z1
Ctouch
T
–
itself with the formulas above must be
1
–
Haptics Enabled 4/5-Wire
Xpos
------------ -
4095
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