sx8652 Semtech Corporation, sx8652 Datasheet - Page 15

• Manufacturer Part Number: sx8652
• Description: 15kv Esd Low Power 4-wire / 5-wire Resistive Touchscreen Controller With Spi Interface
• Manufacturer: Semtech Corporation

The corresponding equations for the pressure:
The X and Y total sheet resistance (Rxtot, Rytot) are known from the touch screen
supplier.
R4 is proportional to the Y coordinate.
The R4 value is given by the total Y plate resistance multiplied by the fraction of the Y
position over the full coordinate range.
By re-arranging z1 and z2 one obtains
Which results in:
The touch resistance calculation above 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 shown in the next
calculations
R1 is inverse proportional to the X coordinate.
Substituting R1 and R4 into z1 and rearranging terms
gives:
4.4. Pen Detection
Figure 16. Pen detection
The resistor R
values.
R
The pen detection will set the PENIRQ bit of the RegStat register.
In PENDET mode, the pen detection will set NIRQ low. The PENIRQ bit will be cleared and the NIRQ will be de-asserted
as soon as the host reads the status register.
Revision V1.6/July 2010
©2010 Semtech Corp.
ADVANCED COMMUNICATIONS & SENSING
PNDT
Vref
+
-
should be set to a value greater than 7x(Rxtot + Rytot).
Y-
Y+
R3
R4
R
PNDT
T
X-
can be configured to 4 different values (see Table 13) to accommodate different screen resistive
X+
R1
R2
R
PNDT
Sb
Rb
PDIRQ
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 a pen detection prior to conversion avoids feeding
the host with dummy data and saves power.
If the touchscreen is powered between X+ and Y- through a resistor R
no current will flow so long as pressure is not applied to the surface (see
Figure 16). When some pressure is applied, a current path is created and
brings X+ to the level defined by the resistive divider determined by R
and the sum of R1, R
touchscreen, the bias delay is of 0.25 x POWDLY.
z1
=
4095
15kV ESD Low Power 4-Wire / 5-Wire Resistive
Page 15
R
-------------------------------- -
R1
T
Touchscreen Controller with SPI Interface
=
+
R4
R4
Rytot Ypos
------------------------------ -
T
+
4095
and R4. Due to the capacitive loading of the
R
T
z2
4095
----------- - 1
=
z1
R1
Rytot
Rxtot
R
R
R4
4095
T
T
–
=
=
=
=
Rxtot
Rytot
R4
=
=
Rytot
-------------------------------- -
R1
–
R3
R1
Rxtot
R4
+
www.semtech.com
R4
DATASHEET
+
z2
---- - 1
z1
+
+
Ypos
----------- -
4095
SX8652
Ypos
----------- -
4095
R4
R2
1
Rt
–
+
–
R
1
Xpos
------------ -
4095
T
–
Xpos
------------ -
4095
z2
---- - 1
z1
PNDT
PNDT
–
,