th July 2011 15 www.semtech.com SX8657/SX8658 Haptics " />
參數(shù)資料
型號(hào)： SX8658IWLTRT
廠商： Semtech
文件頁(yè)數(shù)： 7/61頁(yè)
文件大小： 0K
描述： IC TOUCH SCREEN 12BIT 20QFN
標(biāo)準(zhǔn)包裝： 1
類型： 電阻式，觸感驅(qū)動(dòng)器
觸摸面板接口： 4，5 線
輸入數(shù)/鍵： 1 TSC，雨刷，鑰匙
分辨率（位）： 12 b
評(píng)估套件： *
數(shù)據(jù)接口： I²C，串行
電壓基準(zhǔn)： 內(nèi)部
電源電壓： 2.3 V ~ 3.6 V
工作溫度： -40°C ~ 85°C
安裝類型： 表面貼裝
封裝/外殼： 20-WFQFN 裸露焊盤
供應(yīng)商設(shè)備封裝： 20-QFN（4x4）
包裝： 標(biāo)準(zhǔn)包裝
其它名稱： SX8658IWLDKR
ADVANCED COMMUNICATIONS & SENSING
Rev 1 – 25
th July 2011
15
www.semtech.com
SX8657/SX8658
Haptics Enabled 4/5-Wire
Resistive Touchscreen Controller with Proximity Sensing
Rxtot
R1
R2
+
=
Rytot
R3
R4
+
=
R4
Rytot
Ypos
4095
-------------
=
z2
Re-arranging z1 and z2 gives:
RT
R4
z2
z1
------
1
–
=
This finally results in:
RT
Rytot
Ypos
4095
-------------
z2
z1
------
1
–
=
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:
R1
Rxtot
1
Xpos
4095
-------------
–
=
Substituting R1 and R4 into z1 and rearranging terms gives:
RT
Rytot Y
pos
4095
---------------------------------
4095
z1
------------
1
–
Rxtot
1
Xpos
4095
-------------
–
–
=
Please note that the chip only outputs z1, z2, etc. The calculation of RT itself with the formulas above must be
performed by the host.
4.3.1
Bias Time (POWDLY)
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:
PowD ly
10
Rtouch
Ctouch
×
=
4.4
Pen Detection
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.
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