Revision V1.5/December 2010
©2010 Semtech Corp.
SX8651
www.semtech.com
Page 1
Multitouch 4-wire Resistive Touchscreen
Controller with 15kV ESD Protection
ADVANCED COMMUNICATIONS & SENSING
DATASHEET
4
The SX8651 is a multitouch controller that enables a
completely different user interaction with 4-wire resistive
touchscreen. It enables detection of 2 fingers on the
touchscreen and several gestures like rotation and pinch/
stretch.
This ultra low power touchscreen controller has been
optimized for portable equipment where power and board-
space are at a premium.
It incorporates a highly accurate 12-bit ADC for data
conversion and operates from a single 1.65 to 3.7V supply
voltage.
The SX8651 features a built-in preprocessing algorithm for
data measurements, which greatly reduces the host
processing overhead and bus activity. This complete
touchscreen solution includes four user-selectable
operation modes which offer programmability on different
configurations such as conversion rate and settling time,
thus enable optimization in throughput and power
consumption for a wide range of touch sensing applications.
The touch screen inputs have been specially designed to
provide robust on-chip ESD protection of up to ±15kV in
both HBM and Contact Discharge, and eliminates the need
for external protection devices.
The SX8651 is offered in two tiny packages: 3.0 mm x
3.0 mm DFN and a 1.5 mm x 2.0 mm wafer-level chip-scale
package (WLCSP).
Portable Equipment
Mobile Communication Devices
Cell phone, PDA, MP3, GPS, DSC
Touch Screen Monitors
Extremely Low Power Consumption: 23uA@1.8V 8kSPS
Superior On-chip ESD Protection
±15kV HBM (X+,X-,Y+,Y-)
±2kV CDM
±25kV Air Gap Discharge
±15kV Contact Discharge
±300V MM
Compatible with a Wide Range of 4-wire Resistive
Touchscreen for Single/Multi-touch Operation
Integrated Preprocessing Block to Reduce Host Loading
and Bus Activity
Four User Programmable Operation Modes provides
Flexibility to address Different Application Needs
Manual, Automatic, Pen Detect, Pen Trigger
High Precision 12-bit Resolution
Low Noise Ratiometric Conversion
Selectable Polling or Interrupt Modes
Touch Pressure Measurement
400kHz Fast-Mode I²C Interface
Hardware Reset & I²C Software Reset
-40°C to 85°C Operation
Pin-compatible with SX8650
Android and Linux Driver Support Available
12-LD DFN Package / 12-Ball WLCSP Package
RoHS, WEEE, HF and pb free
GENERAL DESCRIPTION
APPLICATIONS
Block Diagram
KEY PRODUCT FEATURES
ORDERING INFORMATION
Part Number Package
(Dimension in mm)
Marking
SX8651ICSTRT
1
1. 3000 Units / reel
12 - Ball WLCSP
(1.5 x 2.0)
FD77
SX8651IWLTRT
1
12 - Lead DFN
(3.0 x 3.0)
FD77
Touch
Screen
Interface
SX8651
VD D AUX
X+
Y+
X-
Y-
A0
NIRQ
NRST
SC L
SDA
GND
HOST
Control
I2C
Digital
Filter
ref+
ref- AD Cin out
OSCPO R
Vref
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Section Page
ADVANCED COMMUNICATIONS & SENSING
DATASHEET
Table of contents
1. General Description................................................................................................................................................. 4
1.1. DFN Pinout Diagram and Marking Information (Top View).............................................................................. 4
1.2. WLCSP Pinout Diagram and Marking Information (Top View) ........................................................................ 4
1.3. Pin Description................................................................................................................................................. 5
1.4. Simplified Block Diagram................................................................................................................................. 5
2. Electrical Characteristics ......................................................................................................................................... 6
2.1. Recommended Operating Conditions.............................................................................................................. 6
2.2. Thermal Characteristics................................................................................................................................... 6
2.3. Electrical Specifications................................................................................................................................... 7
2.4. Host Interface Specifications ........................................................................................................................... 9
2.5. Host Interface Timing Waveforms.................................................................................................................. 10
2.6. Typical Operating Characteristics.................................................................................................................. 11
3. Functional Description........................................................................................................................................... 13
3.1. General Introduction ..................................................................................................................................... 13
3.2. Channel Pins.................................................................................................................................................. 13
3.2.1. X+, X-, Y+. Y-.......................................................................................................................................... 13
3.2.2. AUX......................................................................................................................................................... 13
3.3. Host Interface and Control Pins..................................................................................................................... 14
3.3.1. NIRQ ....................................................................................................................................................... 14
3.3.2. SCL ......................................................................................................................................................... 14
3.3.3. SDA......................................................................................................................................................... 14
3.3.4. A0............................................................................................................................................................ 15
3.3.5. NRST ...................................................................................................................................................... 15
3.4. Power Management Pins............................................................................................................................... 15
3.4.1. VDD......................................................................................................................................................... 15
3.4.2. GND ........................................................................................................................................................ 15
4. Detailed Description............................................................................................................................................... 16
4.1. Touch Screen Operation................................................................................................................................ 16
4.2. Coordinates Measurement............................................................................................................................. 17
4.3. Pressure Measurement.................................................................................................................................. 17
4.4. Pen Detection ................................................................................................................................................ 18
4.5. Double touch measurement........................................................................................................................... 19
5. Data Processing .................................................................................................................................................... 20
5.1. Host Interface and Control............................................................................................................................. 20
5.1.1. I2C Address ............................................................................................................................................ 20
5.1.2. I2C Write Registers................................................................................................................................. 21
5.1.3. I2C Read Registers................................................................................................................................. 21
5.1.4. I2C Host Commands............................................................................................................................... 22
5.1.5. I2C Read Channels................................................................................................................................ 23
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Section Page
ADVANCED COMMUNICATIONS & SENSING
DATASHEET
Table of contents
5.1.6. Data Channel Format............................................................................................................................. 24
5.1.7. Invalid Qualified Data.............................................................................................................................. 24
5.2. I2C Register Map.......................................................................................................................................... 24
5.3. Host Control Writing....................................................................................................................................... 25
5.4. Host Commands............................................................................................................................................ 27
5.5. Power-Up....................................................................................................................................................... 28
5.6. Reset.............................................................................................................................................................. 28
6. Modes of Operation.............................................................................................................................................. 29
6.1. Manual Mode ................................................................................................................................................. 29
6.2. Automatic mode............................................................................................................................................. 30
6.3. PENDET Mode .............................................................................................................................................. 31
6.4. PENTRIG Mode............................................................................................................................................. 31
7. Application Information.......................................................................................................................................... 32
7.1. Acquisition Setup........................................................................................................................................... 32
7.2. Channel Selection.......................................................................................................................................... 32
7.3. Noise Reduction............................................................................................................................................. 32
7.3.1. POWDLY................................................................................................................................................. 32
7.3.2. SETDLY .................................................................................................................................................. 32
7.3.3. AUX Input................................................................................................................................................ 33
7.4. Interrupt Generation....................................................................................................................................... 33
7.5. Coordinate Throughput Rate ......................................................................................................................... 33
7.5.1. I2C Communication Time........................................................................................................................ 33
7.5.2. Conversion Time..................................................................................................................................... 33
7.5.3. AUTO MODE .......................................................................................................................................... 34
7.6. ESD event...................................................................................................................................................... 34
7.7. Application Schematic.................................................................................................................................... 35
8. Multi-Touch Gestures ............................................................................................................................................ 35
8.1. Zoom Gesture................................................................................................................................................ 35
8.2. Rotate Gesture............................................................................................................................................... 35
9. Packaging Information........................................................................................................................................... 36
9.1. DFN Package................................................................................................................................................. 36
9.2. WLCSP Package........................................................................................................................................... 38
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ADVANCED COMMUNICATIONS & SENSING
DATASHEET
Multitouch 4-wire Resistive Touchscreen
Controller with 15kV ESD Protection
1. General Description
1.1. DFN Pinout Diagram and Marking Information (Top View)
Figure 1. SX8651 DFN Top View, Pad on Bottom Side
The Device marking and
YYWW: Date Code
XXXXX: Lot Number
1.2. WLCSP Pinout Diagram and Marking Information (Top View)
Figure 2. SX8651 WLCSP Top View, Solder Bumps on Bottom Side
YYWW: Date Code
XXXXX: Lot Number
1
2
3
4
5
6
12
11
10
9
8
7
VDD AUX
X+
Y+
X-
Y-
A0
NIRQ
NRST
SCL
SDA
GND
SX8651
TOP VIEW
13
FD77
YYWW
XXXX
PIN 1
IDENTIFIER
SX8651 TOP VIEW
solder bumps on bottom side
VDD
AUX
X+ Y+
A
A0
NIRQ
NRST
SCLSDA
GND
B C D
3
2
1
X- Y-
BALL A1
IDENTIFIER
FD77
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Multitouch 4-wire Resistive Touchscreen
Controller with 15kV ESD Protection
1.3. Pin Description
Table 1. Pin description
1.4. Simplified Block Diagram
The SX8651 simplified block diagram is shown in Figure 3.
Figure 3. Simplified block diagram of the SX8651
Pin Number #Name Type Description
DFN WLCSP
1 A2 VDD Power Input power supply connect to a 0.1uF capacitor to GND
2 A3 X+ Analog X+ channel input
3 B3 Y+ Analog Y+ channel input
4 C3 X- Analog X- channel input
5 D3 Y- Analog Y- channel input
6 D2 GND Ground Ground
7 B1 NIRQ Digital Output / Open Drain Output Interrupt output, active low. Need external pull-up resistor
8 C1 SDA Digital Input / Open Drain Output I2C data input/output
9 D1 SCL Digital Input / Open Drain Output I2C clock, input/output
10 C2 NRST Digital Input / Output Reset Input, active low. Need external 50k pull-up resistor
11 B2 A0 Digital Input I2C slave address selection input
12 A1 AUX Digital Input/Analog Input Analog auxiliary input or conversion synchronization
13 GND Ground Die attach paddle, connect to Ground
Touch
Screen
Interface
SX8651
VDD AUX
X+
Y+
X-
Y-
A0
NIRQ
NRST
SCL
SDA
GND
Control
I2C
Digital
Filter
ref+
ref- ADCin out
OSCPOR
Vref
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Multitouch 4-wire Resistive Touchscreen
Controller with 15kV ESD Protection
2. Electrical Characteristics
Stresses above the values listed in “Absolute Maximum Ratings” may cause permanent damage to the device.
This is a stress rating only and functional operation of the device at these, or any other conditions beyond the “Recommended Operating
Conditions”, is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability
.
(i) Tested to TLP (10A)
(ii) Tested to JEDEC standard JESD22-A114
(iii) Tested to JEDEC standard JESD78
2.1. Recommended Operating Conditions
2.2. Thermal Characteristics
(
iii) θ
JA
is calculated from a package in still air, mounted to 3" x 4.5", 4 layer FR4 PCB with thermal vias under exposed pad (if applicable)
per JESD51 standards.
Parameter Symbol Min. Max. Unit
Supply Voltage V
DDABS
-0.5 3.9 V
Input voltage (non-supply pins) V
IN
-0.5 3.9 V
Input current (non-supply pins) I
IN
10 mA
Operating Junction Temperature T
JCT
125 °C
Reflow temperature T
RE
260 °C
Storage temperature T
STOR
-50 150 °C
ESD HBM
(Human Body Model)
High ESD pins: X+, X-,Y+,Y- ESD
HBM1
± 15
(i)
kV
± 8
(ii)
kV
All pins except high ESD pins:
AUX,A0,NRST,NIRQ,SDA,SCL ESD
HBM2
± 2 kV
ESD (Contact Discharge) High ESD pins: X+, X-,Y+,Y- ESD
CD
± 15 kV
Latchup I
LU
± 100
(iii)
mA
Table 2. Absolute Maximum Ratings
Parameter
Symbol Min. Max Unit
Supply Voltage V
DD
1.65V 3.7 V
Ambient Temperature Range T
A
-40 85 °C
Parameter
Symbol Min. Max Unit
Thermal Resistance with DFN package - Junction to Ambient
(i)
θ
JA
39 °C/W
Thermal Resistance with WLCSP package - Junction to Ambient
(i)
θ
JA
65 °C/W
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Multitouch 4-wire Resistive Touchscreen
Controller with 15kV ESD Protection
2.3. Electrical Specifications
Parameter
Symbol Conditions Min. Typ Max Unit
Current consumption
Manual I
pwd
Manual (converter stopped, pen
detection off, I2C listening, OSC
stopped)
0.4 0.75 uA
Pen Detect I
pndt
Pen detect mode (converter
stopped, pen detection activated,
device will generate interrupt upon
detection, I2C listening, OSC
stopped).
0.4 0.75 uA
Pen Trigger I
pntr
Pen trigger mode (converter
stopped, pen detection activated,
device will start conversion upon
pen detection. I2C listening, OSC
stopped
0.4 0.75 uA
Automatic I
wt
Automatic (converter stopped, pen
detection off, I2C listening, OSC and
timer on, device is waiting for timer
expiry)
1.5 uA
Operation @8kSPS, VDD=1.8V I
opl
X,Y Conv. RATE=4kSPS, N
filt
=1
PowDly=0.5us, SetDly=0.5us
23 50 uA
Operation @42kSPS, VDD=3.3V I
oph
X,Y Conv. RATE=3kSPS, N
filt
=7
PowDly=0.5us, SetDly=0.5us
105 140 uA
Digital I/O
High-level input voltage
1
V
IH
0.7V
DD
V
DD
+0.5 V
Low-level input voltage V
IL
V
SS
-0.3 0.3V
DD
V
SDA / SCL Hysteresis of Schmitt
trigger inputs
VDD > 2 V
VDD < 2 V
V
hys
0.05V
DD
0.1V
DD
V
Low-level output voltage V
OL
I
OL
=3mA, V
DD
>2V
I
OL
=3mA, V
DD
<2V 0
00.4
0.2V
DD
V
Input leakage current L
I
CMOS input ±1 uA
AUX
Input voltage range V
IAUX
0 V
DD
V
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Multitouch 4-wire Resistive Touchscreen
Controller with 15kV ESD Protection
All values are valid within the recommended operating conditions unless otherwise specified.
Input capacitance
C
X+
,C
X-
,C
Y+
,
C
Y-
50 pF
C
AUX
5 pF
Input leakage current
I
IAUX
-1 1 uA
Startup
Power-up time t
por
Time between rising edge VDD and
rising NIRQ 1 ms
ADC
Resolution A
res
12 bits
Offset A
off
±1 LSB
Gain error A
ge
At full scale 0.5 LSB
Differential nonlinearity A
dnl
±1 LSB
Integral nonlinearity A
inl
±1.5 LSB
Resistors
X+, X-, Y+, Y- resistance R
chn
Touch Pad Biasing Resistance 5 Ohm
Pen detect resistance R
PNDT_00
R
PNDT
= 0 100 kOhm
R
PNDT_01
R
PNDT
= 1 200 kOhm
R
PNDT_10
R
PNDT
= 2 50 kOhm
R
PNDT_11
R
PNDT
= 3 25 kOhm
External components recommendations
Capacitor between VDD, GND C
vdd
Type 0402, tolerance +/-50% 0.1 uF
1. SCL, SDA, NRST and NIRQ can be pulled up to a potential higher than the chip VDD but must not exceed the maximun voltage of 3.7V.
Parameter
Symbol Conditions Min. Typ Max Unit
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Multitouch 4-wire Resistive Touchscreen
Controller with 15kV ESD Protection
2.4. Host Interface Specifications
Table 3. Host Interface Specifications
Notes:
(i) All timing specifications refer to voltage levels (V
IL
, V
IH
, V
OL
) defined in
Table 3
unless otherwise mentioned.
Parameter Symbol Condition Min Typ Max Unit
I2C TIMING SPECIFICATIONS
(i)
SCL clock frequency f
SCL
0 400 kHz
SCL low period t
LOW
1.3 us
SCL high period t
HIGH
0.6 us
Data setup time t
SU;DAT
100 ns
Data hold time t
HD;DAT
0 ns
Repeated start setup time t
SU;STA
0.6 us
Start condition hold time t
HD;STA
0.6 us
Stop condition setup time t
SU;STO
0.6 us
Bus free time between stop and start t
BUF
1.3 us
Data valid time t
VD;DAT
0.9 us
Data valid ack time t
VD;ACK
0.9 us
Pulse width of spikes that must be
suppressed by the input filter t
SP
50 ns
I2C BUS SPECIFICATIONS
Capacitive Load on each bus line SCL, SDA C
b
400 pF
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Multitouch 4-wire Resistive Touchscreen
Controller with 15kV ESD Protection
2.5. Host Interface Timing Waveforms
Figure 4. I2C Start and Stop timing
Figure 5. I2C Data timing
SDA
SCL
t
SU;STA
t
HD;STA
t
SU;STO
t
BUF
70%
30%
70%
SDA
SCL
t
LOW
t
HIGH
t
HD;DAT
t
SU;DAT
t
SP
30%
70%
30%
70%
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Multitouch 4-wire Resistive Touchscreen
Controller with 15kV ESD Protection
2.6. Typical Operating Characteristics
At Ta= -40°C to +85°C, VDD=1.7V to 3.7V, PowDly=0.5 us, SetDly=0.5us, Filt=1, Resistive touch screen sensor current not
taking in account, unless otherwise noted.
CURRENT IN PEN TRIGGER MODE
0
100
200
300
400
500
1.5 2 2.5 3 3.5
V
DD
(V)
Supply Current (nA)
TOUCH SENSOR
NOT ACTIVATED
SUPPLY CURRENT in MANUAL MODE
vs TEMPERATURE
0
100
200
300
400
500
600
700
-40 -20 0 20 40 60 80 100
Temperature (C)
Manual Mode Supply Current (nA)
V
DD
=3.3V
V
DD
=1.85V
SUPPLY CURRENT VS CONVERSION RATE
VDD=1.8V - X,Y CONVERSION
0
10
20
30
40
50
60
70
80
90
100
012345
Conversion Rate (kCPS)
Supply Current (uA)
Filt=7
Filt=5
Filt=3
Filt=1
SUPPLY CURRENT VS CONVERSION RATE
VDD=1.8V - X,Y, Z1, Z2 CONVERSION
0
10
20
30
40
50
60
70
80
90
100
110
120
130
012345
Conversion Rate (kCPS)
Supply Current (uA)
Filt=7 Filt=5
Filt=3
Filt=1
SUPPLY CURRENT vs SAMPLE RATE
0
100
200
300
400
500
0 1 2 3 4 5
Sample Rate (kCPS)
Supply Current (uA)
TOUCH SENSOR
X+ to X- =1000 Ohm
Y+ to Y- =1000 Ohm
V
DD
=3.3V
V
DD
=2.5V
V
DD
=1.65V
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Multitouch 4-wire Resistive Touchscreen
Controller with 15kV ESD Protection
Typical Operating Characteristics (continued)
At Ta= -40°C to +85°C, VDD=1.7V to 3.7V, PowDly=0.5 us, SetDly=0.5us, Filt=1, Resistive touch screen sensor current not
taking in account, unless otherwise noted.
CHANGE IN ADC GAIN vs. TEMPERATURE
-2
-1
0
1
2
-40 -20 0 20 40 60 80 100
Temperature (C)
Delta from +25C (LSB)
CHANGE IN ADC OFFSET vs. TEMPERATURE
-2
-1
0
1
2
-40 -20 0 20 40 60 80 100
Temperature (C)
Delta from +25C (LSB)
ADC INL @ VDD=3.3V
-1
-0.75
-0.5
-0.25
0
0.25
0.5
0.75
1
0 0.5 1 1.5 2 2.5 3
V
X+
(V)
Error (LSB)
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Multitouch 4-wire Resistive Touchscreen
Controller with 15kV ESD Protection
3. Functional Description
3.1. General Introduction
This section provides an overview of the SX8651 architecture, device pinout and a typical application.
The SX8651 is designed for 4-wire resistive touch screen applications (Figure 6).The touch screen or touch panel is the
resistive sensor and can be activated by either a finger or stylus. The touch screen coordinates and touch pressure are
converted into I2C format by the SX8651 for transfer to the host.
Figure 6. SX8651 with screen
3.2. Channel Pins
3.2.1. X+, X-, Y+. Y-
The SX8651's channel pins (X+, X-, Y+, Y-) directly
connect to standard touch screen X and Y resistive
layers. The SX8651 separately biases each of these
layers and converts the resistive values into (X,Y)
coordinates.
The channel pins are protected to VDD and GROUND.
Figure 7 shows the simplified diagram of the X+, X-, Y+,
Y- pins.
Figure 7. Simplified diagram of X+, X-, Y+, Y- pins
3.2.2. AUX
The SX8651 interface includes an AUX pin that serves
two functions: an ADC input; and a start of conversion
trigger. When used as an ADC, the single ended input
range is from GND to VDD, referred to GND. When
the AUX input is configured to start conversions, the
AUX input can be further configured as a rising and /
or falling edge trigger.
The AUX is protected to VDD and GROUND.
Figure 8 shows a simplified diagram of the AUX pin.
Figure 8. Simplified diagram of AUX
Touch
Screen
Interface
SX8651
VDD AUX
X+
Y+
X-
Y-
A0
NIRQ
NRST
SCL
SDA
GND
HOST
Control
I2C
Digital
Filter
ref+
ref- ADCin out
OSCPOR
Vref
VDD
X+
X-
Y+
Y-
Touch Screen
Interface
R
chn
AUX
ADC
Control
VDD
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Controller with 15kV ESD Protection
3.3. Host Interface and Control Pins
The SX8651 host and control interface consists of: NIRQ, I2C pins SCL and SDA, A0, and NRST.
3.3.1. NIRQ
The NIRQ pin is an active low, open drain output
to facilitate interfacing to different supply voltages
and thus requires an external pull-up resistor (1-
10 kOhm). The NIRQ pin does not have
protection to VDD.
The NIRQ function is designed to provide an
interrupt to the host processor. Interrupts may
occur when a pen is detected, or when channel
data is available.
Figure 9 shows a simplified diagram of the NIRQ
pin.
Figure 9. Simplified diagram of NIRQ
3.3.2. SCL
The SCL pin is a high-impedance input and open-
drain output pin. The SCL pin does not have
protection to VDD to conform to I2C slave
specifications. An external pull-up resistor (1-10
kOhm) is required.
Figure 10 shows the simplified diagram of the
SCL pin.
Figure 10. Simplified diagram of SCL
3.3.3. SDA
SDA is an I/O pin. It can be used as an open-drain
output (with external pull-up resistor) or as an
input. An external pull-up resistor (1-10 kOhm) is
required.
The SDA I/O pin does not have protection to VDD
to conform to I2C slave specifications.
Figure 11 shows a simplified diagram of the SDA
pin.
Figure 11. Simplified diagram of SDA
NIRQ Control
HOST
VDD
IRQ
SCL
HOST
VDD
IN
OUT
SCL
I2C
SDA
HOST
VDD
IN
OUT
SDA
I2C
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Multitouch 4-wire Resistive Touchscreen
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3.3.4. A0
The A0 pin is connected to the I2C address select control
circuitry and is used to modify the device I2C address.
The A0 pin is protected to GROUND.
Figure 12 shows a simplified diagram of the A0 pin.
Figure 12. Simplified diagram of A0
3.3.5. NRST
The NRST pin is an active low input that provides a
hardware reset of the SX8651's control circuitry.
The NRST pin is protected GROUND to enable
interfacing with devices at a different supply
voltages.
Figure 13 shows a simplified diagram of the NRST
pin.
Figure 13. Simplified diagram of NRST
3.4. Power Management Pins
The SX8651's power management input consists of the following Power and Ground pins.
3.4.1. VDD
The VDD is a power pin and is the power supply for the SX8651.
The VDD has ESD protection to GROUND.
Figure 14 shows a simplified diagram of the VDD pin.
Figure 14. Simplified diagram of VDD
3.4.2. GND
The SX8651 has one power management ground pin, GND.
(The die attach paddle on DFN is also connected to GND.)
The GND has ESD protection to VDD.
Figure 15 shows a simplified diagram of the GND pin.
Figure 15. Simplified diagram of GND
A0
I2C
NRST Control
HOST
VDD
VDD
VDD
GND
VDD
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4. Detailed Description
4.1. Touch Screen Operation
A resistive touch screen consists of two (resistive) conductive sheets separated by an insulator when not pressed. Each
sheet is connected through 2 electrodes at the border of the sheet (Figure 16). When a pressure is applied on the top
sheet, a connection with the lower sheet is established. Figure 17 shows how the Y coordinate can be measured. The
electrode plates are connected through terminals X+, X- and Y+, Y- to an analog to digital converter (ADC) and a reference
voltage. The resistance between the terminals X+ and X- is defined by Rxtot. Rxtot will be split in 2 resistors, R1 and R2, in
case the screen is touched. The resistance between the terminals Y+ and Y- is represented by R3 and R4. The connection
between the top and bottom sheet is represented by the touch resistance (R
T
).
Figure 16. 4-wire Touch Screen
Figure 17. Touch Screen Operation ordinate measurement (Y)
X-
Y+
Y- X+
Top conductive sheet
Bottom conductive sheet
Y electrodes
X electrodes
X- X+
Y-
Y+
R3
R4
R2 R1 +
-ADC Ypos
Vref
+
-
R
T
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4.2. Coordinates Measurement
The top resistive sheet (Y) is biased with a voltage source. Resistors R3 and R4 determine a voltage divider proportional to
the Y position of the contact point. Since the converter has a high input impedance, no current flows through R1 so that the
voltage X+ at the converter input is given by the voltage divider created by R3 and R4.
The X coordinate is measured in a similar fashion with the bottom resistive sheet (X) biased to create a voltage divider by
R1 and R2, while the voltage on the top sheet is measured through R3. Figure 18 shows the coordinates measurement
setup. The resistance R
T
is the resistance obtained when a pressure is applied on the screen. R
T
is created by the contact
area of the X and Y resistive sheet and varies with the applied pressure.
Figure 18. Ordinate (Y) and abscissa (X) coordinates measurement setup
The X and Y position are found by:
4.3. Pressure Measurement
The pressure measurement consists of two additional setups: z1 and z2 (see Figure 19).
Figure 19. z1 and z2 pressure measurement setup
X-
X+
R2
R1
Vref
+
-
R
T
Y-
Y+
R4
R3
Ypos
X-
X+
R2
R1
Vref
+
-R
T
Y-
Y+
R4
R3
Xpos
Xpos 4095 R2
R1R2+
--------------------
⋅=Ypos 4095 R4
R3R4+
--------------------
⋅=
X-
X+
R2
R1 R
T
Y-
Y+
R4
R3
z1
X-
X+
R2
R1
Vref
+
-
Y-
Y+
R4
R3
z2
Vref
+
-R
T
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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
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
PNDT
, no
current will flow so long as pressure is not applied to the surface (see
Figure 20). When some pressure is applied, a current path is created and brings
X+ to the level defined by the resistive divider determined by R
PNDT
and the
sum of R1, R
T
and R4. Due to the capacitive loading of the touchscreen, the
bias delay is of 0.25 x POWDLY.
The level is detected by a comparator.
Figure 20. Pen detection
The resistor R
PNDT
can be configured to 4 different values (see Table 7) to accommodate different screen resistive values.
R
PNDT
should be set to a value greater than 7x(Rxtot + Rytot).
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.
z1 4095 R4
R1R4R
T
+ +
---------------------------------
⋅=z2 4095 R4Rt+
R1R4R
T
+ +
---------------------------------
⋅=
Rxtot R1R2+=
Rytot R3R4+=
R4Rytot Ypos
4095
------------
⋅=
R
T
R4z2
z1
----- 1–⋅=
R
T
Rytot Ypos
4095
------------ z2
z1
----- 1–⋅ ⋅=
R1Rxtot 1Xpos
4095
-------------
–⋅=
R
T
Rytot Y⋅pos
4095
------------------------------- 4095
z1
------------ 1– Rxtot 1Xpos
4095
-------------
–⋅–⋅=
X-
X+
R2
R1
Vref
+
-R
T
Y-
Y+
R4
R3
R
PNDT
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4.5. Double touch measurement
The simplified model for double touch on the
touchscreen is given in Figure 21.
R1, R2 and R3 are the top plate resistances.
R4, R5 and R6 are the bottom plate
resistances.
The two contacts on the touchscreen made by the two fingers create Rt1
and Rt2 which are the touch resistances between the two layers of the
touchscreen.
The host retrieves the data from the SX8651. A S/W running in the host
enables the detection of the events described in section[8].
Figure 21. Touchscreen model for double touch
To get the best gesture detection, the resistor RmSelX and RmSelY should be set according to the panel resistance and
the Table 4.
Table 4. . RmSelX and RmSelY resistance selection
Y Panel resistance (Ohm)
RmSelY
X Panel resistance (Ohm)
RmSelX
100 to 187 000 100 to 187 000
188 to 312 001 188 to 312 001
313 to 938 010 313 to 938 010
939 to 1875 011 939 to 1875 011
1876 to 4375 100 1876 to 4375 100
4376 to 9375 101 4376 to 9375 101
9376 to 18780 110 9376 to 18780 110
Larger than 18780 111 Larger than 18780 111
R1
R2
R3
R4
R5
R6
X+ Y+
X- Y-
Rt1
Rt2
Rxtot R1R2R3+ +=
Rytot R4R5R6+ +=
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5. Data Processing
The SX8651 offers 4 types of data processing
which allows the user to make trade-offs between
data throughput, power consumption and noise
rejection.
The parameter FILT is used to select the filter
order N
filt
. The noise rejection will be improved
with a high order to the detriment of the power
consumption. The K coefficient in Table 5 is a filter
constant. Its value is K=4079/4095.
Figure 22. Filter structure
.
Table 5. Filter order
5.1. Host Interface and Control
The host interface consists of I2C (SCL and SDA) and the NIRQ, A0, NRST signals.
The I2C implemented on the SX8651 is compliant with:
Standard Mode (100 kbit/s) & Fast Mode (400 kbit/s)
Slave mode
7 bit slave address
5.1.1. I2C Address
Pin A0 defines the LSB of the I2C address. It is shown on Figure 23.
.
Figure 23. I2C slave address
FILT N
filt
Explanation Processing
0 1 No average
1 3 3 ADC samples are averaged
2 5 5 ADC samples are averaged
3 7 7 ADC samples are sorted and
the 3 center samples are
averaged
ADC
c
n
,c
n-1
,c
n-2
,...
I2C
∑
−
=−
⋅=
1
0
1
N
iinn
c
N
s
Sort: .>.>.>.>.
Preprocessing
s
n
N
FILT
sncn
=
sn1
3
---
K
cncn1– cn2–
+ +( )=
sn1
5
---
K
cncn1– cn2– cn3– cn4–
+ + + +( )=
cmax1cmax2cacbcccmin1cmin2
≥ ≥ ≥ ≥ ≥ ≥
sn1
3
---
K
cacbcc
+ +( )=
1001000 with pin A0 connected to ground
SX8651 Slave Address(7:1) = 1001001 with pin A0 connected to VDD
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Upon request of the customer, a custom I2C address can be burned in the NVM.
The host uses the I2C to read and write data and commands to the configuration and status registers. During a conversion,
the I2C clock can be stretched until the end of the processing.
Channel data read is done by I2C throughput optimized formats.
The supported I2C access formats are described in the next sections:
I2C Write Registers
I2C Read Registers
I2C Host Commands
I2C Read Channels
5.1.2. I2C Write Registers
The format for I2C write is given in Figure 24.
After the start condition [S], the SX8651 slave address (SA) is sent, followed by an eighth bit (W=‘0’) indicating a Write.
The SX8651 then Acknowledges [A] that it is being addressed, and the host sends 8-bit Command and Register address
consisting of the command bits ‘000’ followed by the SX8651 Register Address (RA).
The SX8651 Acknowledges [A] and the host sends the appropriate 8-bit Data Byte (WD0) to be written.
Again the SX8651 Acknowledges [A].
In case the host needs to write more data, a succeeding 8-bit Data Byte will follow (WD1), acknowledged by the slave [A].
This sequence will be repeated until the host terminates the transfer with the Stop condition [P].
Figure 24. I2C write register
The register address increments automatically when successive register data (WD1...WDn) is supplied by the host.
The correct sampling of the screen by the SX8651 and the host I2C bus traffic are events that might occur simultaneously.
The SX8651 will synchronize these events by the use of clock stretching if that is required. The stretching occurs directly
after the last received command bit (see Figure 24).
5.1.3. I2C Read Registers
S SA W CRA A WD0 A WD1 A WDn A P
Optional Optional
S: Start condition
SA: SX8651 Slave Address(7:1)
W: '0'
A: Acknowledge
CR: '000' + Register Address(4:0)
WDn: Write Data byte(7:0), 0...n
P: Stop condition
From host to SX8651
From SX8651 to host
Clock stretching
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The format for incremental I2C read for registers is given in Figure 25. The read has to start with a write of the read
address.
After the start condition [S], the SX8651 Slave Address (SA) is sent, followed by an eighth bit (W=‘0’) indicating a Write.
The SX8651 then Acknowledges [A] that it is being addressed, and the host responds with a 8-bit CR Data consisting of
‘010’ followed by the Register Address (RA). The SX8651 responds with an Acknowledge [A] and the host sends the
Repeated Start Condition [Sr]. Once again, the SX8651 Slave Address (SA) is sent, followed by an eighth bit (R=‘1’)
indicating a Read.
The SX8651 responds with an Acknowledge [A] and the read Data byte (RD0). If the host needs to read more data it will
acknowledge [A] and the SX8651 will send the next read byte (RD1). This sequence can be repeated until the host
terminates with a NACK [N] followed by a stop [P].
Figure 25. I2C read registers
The I2C read register format of Figure 25 is maintained until the Stop Condition. After the Stop Condition the SX8651 is
performing succeeding reads by the compact read format of the I2C read channels described in the next section.
No clock stretching will occur for the I2C read registers.
5.1.4. I2C Host Commands
The format for I2C commands is given in Figure 26.
After the start condition [S], the SX8651 Slave Address (SA) is sent, followed by an eighth bit (W=‘0’) indicating a Write.
The SX8651 then Acknowledges [A] that it is being addressed, and the host responds with an 8-bit Data consisting of a ‘1’
+ command(6:0). The SX8651 Acknowledges [A] and the host sends a stop [P].
The exact definition of command(6:0) can be found in Table 9.
W CRAS SA A Sr SA R A RD0 A RD1 A RDn N P
From Host to SX8651
From SX8651 to Host
Optional
S: Start Condition
Sr: Repeated Start Condition
SA: SX8651 Slave Address(7:1)
W: '0'
R: '1'
A: ACKnowledge
N: Not ACKnowledge (terminating read stream)
CR: '010' + Register Address(4:0)
RDn: Read Data byte(7:0), 0...n
P: Stop Condition
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Figure 26. I2C host command
The sampling of the screen by the SX8651 and the host I2C bus traffic are events that might occur simultaneously. The
SX8651 will synchronize these events by the use of clock stretching if that is required. The stretching occurs directly after
the last received command bit (see Figure 26).
5.1.5. I2C Read Channels
The host is able to read the channels with a high throughput, by the format shown in Figure 27.
After the start condition [S], the SX8651 Slave Address (SA) is sent, followed by an eighth bit (R=‘1’) indicating a read. The
SX8651 responds with an Acknowledge [A] and the Read Data byte (RD0). The host sends an Acknowledge [A] and the
SX8651 responds with the Read Data byte (RD1). If the host needs to read more data, it will acknowledge [A] and the
SX8651 will send the next read bytes. This sequence can be repeated until the host terminates with a NACK [N] followed
immediately by a stop [P]. The NACK [N] releases the NIRQ line. The stop [P] must occur before the end of the conversion.
The channel data that can be read is defined by the last conversion sequence.
A maximum number of 10 data bytes is passed when all channels (X, Y, z1, z2 and AUX) are activated in the
“I2CRegChanMsk”.
The channel data is sent with the following order: X, Y, Z1, Z2, AUX. The first byte of the data contains the channel
information as shown in Figure 28.
Typical applications require only X and Y coordinates, thus only 4 bytes of data will be read.
Figure 27. I2C read channels
S SA W CRA A P
S: Start condition
SA: SX8651 Slave Address(7:1)
W: '0'
A: Acknowledge
CR: '1' + Command(6:0)
P: Stop condition
From host to SX8651
From SX8651 to host
Clock stretching
S SA R A N
S: Start condition
SA: SX8651 Slave Address(7:1)
R: '1'
A: Acknowledge
N: Not Acknowledge (terminating read stream)
RDn: Read Data byte(7:0), 0...n
P: Stop condition
From host to SX8651
From SX8651 to host
RD0 A RD1 P
Channel (i+1)
RDn-1 A RDnA
Channel (i)
Clock stretching
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The sampling of the screen by the SX8651 and the host I2C bus traffic are events that might occur simultaneously. The
SX8651 will synchronize these events by the use of clock stretching if that is required. The stretching occurs directly after
the address and read bit have been sent for the I2C read channels command (see Figure 27).
5.1.6. Data Channel Format
Channel data is coded on 16 bits as shown in Figure 28
Figure 28. data channel format
The 3 bits CHAN(2:0) are defined in Table 10 and show which channel data is referenced. The channel data D(11:0) is of
unsigned format and corresponds to a value between 0 and 4095.
5.1.7. Invalid Qualified Data
The SX8651 will return 0xFFFF data in case of invalid qualified data.
This occurs:
when the SX8651 converted channels and the host channel readings do not correspond. E.g. the host converts X and Y
and the host tries to read X, Y and z1 and z2.
when a conversion is done without a pen being detected.
5.2. I2C Register Map
The details of the registers are described in the next sections.
I2C register address RA(4:0)
Register Description
0 0000 I2CRegCtrl0 Write, Read
0 0001 I2CRegCtrl1 Write, Read
0 0010 I2CRegCtrl2 Write, Read
0 0011 I2CRegCtrl3 Write, Read
0 0100 I2CRegChanMsk Write, Read
0 0101 I2CRegStat Read
1 1111 I2CRegSoftReset Write
Table 6. I2C Register address
CHAN(2:0)
RD0
D(11:8) D(7:0)
RD1
0
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5.3. Host Control Writing
The host control writing allows the host to change SX8651 settings. The control data goes from the host towards the
SX8651 and may be read back for verification.
register bits default description
I2CRegCtrl0
7:4 0000 RATE
Set rate in coordinates per sec (cps) (± 20%)
If RATE equals zero then Manual mode.
if RATE is larger than zero then Automatic mode
0000: Timer disabled -Manual mode
0001: 10 cps
0010: 20 cps
0011: 40 cps
0100: 60 cps
0101: 80 cps
0110: 100 cps
0111: 200 cps
1000: 300 cps
1001: 400 cps
1010: 500 cps
1011: 1k cps
1100: 2k cps
1101: 3k cps
1110: 4k cps
1111: 5k cps
3:0 0000 POWDLY
Settling time (± 10%): The channel will be biased for a time of POWDLY
before each channel conversion
0000: Immediate (0.5 us)
0001: 1.1 us
0010: 2.2 us
0011: 4.4 us
0100: 8.9 us
0101: 17.8 us
0110: 35.5 us
0111: 71.0 us
1000: 0.14 ms
1001: 0.28 ms
1010: 0.57 ms
1011: 1.14 ms
1100: 2.27 ms
1101: 4.55 ms
1110: 9.09 ms
1111: 18.19 ms
I2CRegCtrl1
7:6 00 AUXAQC
00: AUX is used as an analog input
01: On rising AUX edge, wait
POWDLY and start acquisition
10: On falling AUX edge, wait
POWDLY and start acquisition
11: On rising and falling AUX
edges, wait POWDLY and start
acquisition
The AUX trigger requires the manual mode.
5 1 CONDIRQ
Enable conditional interrupts
0: interrupt always generated at end of conversion cycle. If no pen is
detected the data is set to ‘invalid qualified’.
1: interrupt generated when pen detect is successful
4 0 reserved
3:2 00
RPDNT
Select the Pen Detect Resistor
00: 100 KOhm
01: 200 KOhm
10: 50 KOhm
11: 25 KOhm
1:0 00 FILT
Digital filter control
00: Disable
01: 3 sample averaging
10: 5 sample averaging
11: 7 sample acquisition, sort, average 3 middle samples
Table 7. I2C registers
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I2CRegCtrl2
7:4 0 reserved
3:0 0000 SETDLY
Settling time while filtering (± 10%)
When filtering is enabled, the channel will initially bias for a time of
POWDLY for the first conversion, and for a time of SETDLY for each
subsequent conversion in a filter set.
0000: Immediate (0.5 us)
0001: 1.1 us
0010: 2.2 us
0011: 4.4 us
0100: 8.9 us
0101: 17.8 us
0110: 35.5 us
0111: 71.0 us
1000: 0.14 ms
1001: 0.28 ms
1010: 0.57 ms
1011: 1.14 ms
1100: 2.27 ms
1101: 4.55 ms
1110: 9.09 ms
1111: 18.19 ms
I2CRegCtrl3
7:6 0 reserved
5:3 RmSelY 000 Check Table 4
2:0 RmSelX 000 Check Table 4
I2CRegChanMsk
7 1 XCONV 0: no sample
1: sample, report X channel
6 1 YCONV 0: no sample
1: sample, report Y channel
5 0 Z1CONV 0: no sample
1: sample, report Z1 channel
4 0 Z2CONV 0: no sample
1:sample, report Z2 channel
3 0 AUXCONV 0: no sample
1: sample, report AUX channel
2 0 RXCONV Sample RX channel
1 0 RYCONV Sample RY channel
0 0 reserved
I2CRegStat
The host status reading allows the host to read the status of the SX8651. The data goes from the SX8651
towards the host. Host writing to this register is ignored.
7 0 CONVIRQ 0: no IRQ pending
1: End of conversion sequence IRQ pending
IRQ is cleared by the I2C channel reading
6 0 PENIRQ operational in pen detect mode
0: no IRQ pending
1: Pen detected IRQ pending
IRQ is cleared by the I2C status reading
5:0 000000 reserved
register bits default description
Table 7. I2C registers
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5.4. Host Commands
The host can write to and read from registers of the SX8651 by the write and read commands as defined in Table 8.
.The host can issue commands to change the operation mode or perform manual actions as defined in Table 9.
I2CRegSoftReset
7:0 0x00 If the host writes the value 0xDE to this register, then the SX8651 will be reset.
Any other data will not affect the SX8651
W/R command name CR(7:0) Function
7 6 5 4 3 2 1 0
WRITE(RA) 0 0 0 RA(4:0) Write register (see
Table 6
for RA)
READ(RA) 0 1 0 RA(4:0) Read register (see
Table 6
for RA)
Table 8. I2C W/R commands
command name CR(7:0) Function
7 6 5 4 3 2 1 0
SELECT(CHAN) 1 0 0 0 x CHAN(2:0) Bias channel (see Table 10 for CHAN)
CONVERT(CHAN) 1 0 0 1 x CHAN(2:0) Bias channel (see Table 10 for CHAN)
Wait POWDLY settling time
Run conversion
MANAUTO 1 0 1 1 x x x x Enter manual or automatic mode.
PENDET 1 1 0 0 x x x x Enter pen detect mode.
PENTRG 1 1 1 0 x x x x Enter pen trigger mode.
Table 9. I2C commands
register bits default description
Table 7. I2C registers
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The channels are defined in Table 10.
5.5. Power-Up
The NIRQ pin is kept low during SX8651 power-up.
During power-up, the SX8651 is not accessible and I2C
communications are ignored.
As soon as NIRQ rises, the SX8651 is ready for I2C
communication.
Figure 29. Power-up, NIRQ
5.6. Reset
The POR of the SX8651 will reset all registers and states of the SX8651 at power-up.
Additionally the host can reset the SX8651 by asserting the NRST pin (active low) and also via the I2C bus.
If NRST is driven LOW, then NIRQ will be driven low by the SX8651. When NRST is released (or set to high) then NIRQ
will be released by the SX8651.
The circuit has also a soft reset capability. When writing the code 0xDE to the register RegSoftReset, the circuit will be
reset.
Channel CHAN(2:0) Function
2 1 0
X 0 0 0 X channel
Y 0 0 1 Y channel
Z1 0 1 0 First channel for pressure measurement
Z2 0 1 1 Second channel for pressure measurement
AUX 1 0 0 Auxiliary channel
RX 1 0 1
Double touch RX measurement
RY 1 1 0
Double touch RY measurement
SEQ 1 1 1 Channel sequentially selected from
I2CRegChanMsk register, (see Table 8)
Table 10. Channel definition
voltage
time
voltage
time
VDD
NIRQ
t
POR
VDD/2
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6. Modes of Operation
The SX8651 has four operation modes that are configured using the I2C commands as defined in Table 9 and Table 7.
These 4 modes are:
manual (command ‘MANAUTO’ and RATE=0),
automatic (command ‘MANAUTO’ and RATE>0),
pen detect (command ‘PENDET’),
pen trigger mode (command ‘PENTRG’).
At startup the SX8651 is set in manual mode.
In the manual mode the SX8651 is entirely stopped except for the I2C peripheral which accepts host commands. This
mode requires RATE equal to be zero (RATE = 0, see Table 7).
In the automatic mode the SX8651 will sequence automatic channel conversions. This mode requires RATE to be larger
than zero (RATE > 0, see Table 7).
In the PENDET mode the pen detection is activated. The SX8651 will generate an interrupt (NIRQ) upon pen detection and
set the PENIRQ bit in the I2C status register. To quit the PENDET mode the host needs to configure the manual mode.
In the PENTRG mode the pen detection is activated and a channel conversion will start after the detection of a pen. The
SX8651 will generate an interrupt (NIRQ) upon pen detection and set the CONVIRQ bit in the I2C status register. To quit
the PENTRIG mode the host needs to configure the manual mode. The PENTRG mode offers the best compromise
between power consumption and coordinate throughput.
6.1. Manual Mode
In manual mode (RATE=0) single actions are triggered by the I2C commands described in Table 11.
When a command is received, the SX8651 executes the associated task and waits for the next command. It is up to the
host to sequence all actions.
Table 11. CONVERT and SELECT command
The channel can be biased for an arbitrary amount of time by first sending a SELECT command and then a CONVERT
command once the settling time requirement is met.
The SELECT command can be omitted if the large range of POWDLY settings cover the requirements. In the latter case,
the CONVERT command alone is enough to perform an acquisition.
With CHAN=SEQ, multiple channels are sampled. This requires programming the POWDLY field in register RegCTRL0.
The selected channel will be powered during POWDLY before a conversion is started. The channel bias is automatically
removed after the conversion has completed.
Command
Action
CONVERT(CHAN)
Select and bias a channel
Wait for the programmed settling time (POWDLY)
Start conversion
SELECT(CHAN)
Select and bias a channel
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6.2. Automatic mode
In automatic mode (RATE > 0), SX8651 will automatically decide when to start
acquisition, sequence all the acquisitions and alerts the host if data is available
for download with a NIRQ. The host will read the channels and the SX8651 will
start again with the next conversion cycle.
The fastest coordinate rate is obtained if the host reads the channels
immediately after the NIRQ.
To not loose data, the SX8651 will not begin conversion before the host read the
channels. If after the NIRQ a delay superior to the sampling period is made by
the host to read the channels a slower coordinate rate is obtained.
When the control CONDIRQ bit (see register I2CRegStat Table 7) is set to ‘1’
then the interrupts will only be generated if the pen detect occurred. This result in
a regular interrupt stream, as long as the host performs the read channel
commands, and the screen is touched. When the screen is not touched,
interrupts does not occur.
If the control CONDIRQ bit is cleared to ‘0’, the interrupts will always be
generated. In case there is no pen detected on the screen then the coordinate
data will be qualified as invalid, see section [5.1.7]. This result in a regular
interrupt stream, as long as the host performs the read channel commands,
independent of the screen being touched or not.
This working is illustrated in Figure 30.
Figure 30. AUTO Mode Flowchart
Figure 31 shows the I2C working in automatic mode. After the first sentence send through the I2C to make the initialization,
traffic is reduced as only reads are required.
The processing time is the
necessary time for the SX8651 to
makes the pen detection, the
settling time (POWDLY) and the
conversion. This time increases
with the number of channel
selected and the filter used.All
succeeding conversions notifies
the host by an interrupt signal and
the host only needs to issue the
I2C read command.
The reads occur at the RATE
interval.
Figure 31. I2C working in AUTO mode
CONDIRQ=1 ?
Touch Detected ?
Set timer=RATE
Start timer
Start channel conversion
Set interrupt
NIRQ=0
Release Interrupt
NIRQ=1
yes
Timer expire
All channel
data read
All conversion
finished
yes
no
AUTO MODE
TOUCH
NIRQ
I2C Read Channels Conversion time
Time is 1/RATE
NAK
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6.3. PENDET Mode
The PENDET mode can be used if the host only needs to know if the screen has been
touched or not and take from that information further actions. When pen detect circuitry is
triggered the interrupt signal NIRQ will be generated and the status register bit ‘PENIRQ’ will
be set. The bit is cleared by reading the status register RegStat.
Figure 32. PENDET Mode Flowchart
6.4. PENTRIG Mode
The PENTRIG mode offers the best compromise between power consumption and coordinate throughput.
In this mode the SX8651 will wait until a pen is detected on the screen and then starts the coordinate conversions. The
host will be signalled only when the screen is touched and coordinates are available.
The coordinate rate in pen trigger mode is determined by the speed of the host reading the channels and the conversion
times of the channels. The host performs the minimum number of I2C commands in this mode.
The host has to wait for the NIRQ interrupt to make the acquisition of the data.
The flowchart and the I2C working is illustrated in Figure 33.
Figure 33. PENTRIG Mode Flowchart and I2C working in PENTRIG mode
Touch Detected ?
Set interrupt
NIRQ=0
Release Interrupt
NIRQ=1
RegStat read
yes
no
PENDET MODE
Touch Detected ?
Start channel conversion
Set interrupt
NIRQ=0
Release Interrupt
NIRQ=1
All channel
data read
All conversion
finished
yes
no
PENTRIG MODE
I2C bus
TOUCH
NIRQ
I2C Read Channels Conversion time
NAK
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7. Application Information
This section describes in more detail application oriented data.
7.1. Acquisition Setup
Prior to an acquisition, the SX8651 can be setup by writing the control registers with a register write command. They can
be read by issuing the read command. Please refer to the section [5.3]. After power-up, the circuit is in manual mode.
7.2. Channel Selection
The SX8651 can be setup to start a single channel conversion or to convert several channels in sequence. For a single
conversion, the channel to be converted is determined from the CHAN(2:0) field in the command word (defined in
Table 10).
Several channels can be acquired sequentially by setting the CHAN(2:0) field to SEQ. The channels will be sampled in the
order defined by register RegChanMsk from MSB to LSB.
If a “one” is written in a channel mask, the corresponding channel will be sampled, in the opposite case, it is ignored and
the next selected channel is chosen.
7.3. Noise Reduction
A noisy environment can decrease the performance of the controller. For example, an LCD display located just under the
touch screen can adds a lot of noise on the high impedance A/D converter inputs.
7.3.1. POWDLY
In order to perform correct coordinates acquisition properly, some time must be given for the touch screen to reach a
proper level. It is a function of the PCB trace resistance connecting the SX8651 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 bit accuracy.
Adding a capacitor from the touch screen drivers to ground is a solution to minimize external noise. A low-pass filter
created by the capacitor may increase settling time. Therefore, use POWDLY to stretch the acquisition period. POWDLY
can be estimated by the following formula:
Rtouch is the sum of the panel resistances plus any significant series input resistance, Rxtot + Rytot + Ri.
Ctouch is the sum of the touch panel capacitance plus any noise filtering and routing capacitances.
7.3.2. SETDLY
A second method of noise filtering uses an
averaging filter as described in section [5]
(Data processing). In this case, the chip
will sequence up to 7 conversions on each
channel. The parameter SETDLY sets the
settling time between the consecutive
conversions.
In most applications, SETDLY can be set
to 0. In some particular applications, where
accuracy of 1LSB is required and Ctouch
is less than 100nF a specific value should
be determined.
Figure 34. POWDLY and SETDLY timing with FILT=2
PowDly 10 Rtouch Ctouch××=
POWDLY SETDLY
Start of the
conversion
time
X+
5 successive
conversions
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7.3.3. AUX Input
An alternate conversion trigger method can be used if the host system provides additional digital signals that indicate noisy
or noise-free periods. The SX8651 can be set up to start conversions triggered by the AUX pin. A rising edge, a falling
edge or both can trigger the conversion. To enter this mode, AUXACQ must be set to a different value than '00' as defined
in Table 7. The AUX edge will first trigger the bias delay (POWDLY). Following the programmed delay, the channel
acquisition takes place.
7.4. Interrupt Generation
An interrupt (NIRQ=0) will be generated:
During the power-up phase or after a reset
After completion of a conversion in MANUAL, PENTRIG or AUTO mode. CONVIRQ (bit [7] of RegStat) will be set at the
same time.
After a touch on the panel is detected in PENDET mode. PENIRQ (bit [6] of RegStat) will be set at the same time.
The NIRQ will be released and pulled high(NIRQ=1) by the external pull-up resistor:
When the power-up phase is finished
When the host read all channels data that were previously converted by the SX8651 in MANUAL, PENTRIG or AUTO
mode. CONVIRQ will be cleared at the same time.
When the host read the status register in PENDET mode. PENIRQ, will be cleared at the same time.
An active NIRQ (low) needs to be cleared before any new conversions will occur.
7.5. Coordinate Throughput Rate
The coordinate throughput rate depends on the following factors:
The I2C communication time: T
com
The conversion time: T
conv
The coordinate rate is the frequency to get the X, Y, Z1 and Z2 coordinate:
7.5.1. I2C Communication Time
The minimum time to read the channel data in PENTRIG mode is:
The highest throughput will be obtained with a I2C frequency of 5MHz when the host read the channel data as quickly as
possible after the NIRQ falling edge.
7.5.2. Conversion Time
The maximum possible throughput can be estimated with the following equation
with:
N
filt
= {1,3,5,7} based on the order defined for the filter FILT (see Figure 5).
N
chan
= {1,2,3,4,5} based on the number of channels defined in RegChanMsk
POWDLY = 0.5us to 18.19ms, settling time as defined in RegCtrl0
SETDLY = 0.5us to 18.19ms, settling time when filtering as defined in RegCtrl2
Tosc is the oscillator period (555ns +/- 15%)
CoordRate 1
T
com
T
conv
+
-------------------------------
=
T
com
8 16 N
chan
×+( ) T
SPI
×=
Tconv us( ) 47 Tosc⋅N+
chan
POWDLY N
filt
1–( ) SETDLY 21N
filt
1+( ) Tosc⋅+⋅+( )⋅=
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Table 12 gives some examples of Coordinate Rate and Sample Rate for various setting in PENTRIG mode.
Table 12. Coordinate throughput examples
7.5.3. AUTO MODE
In AUTO mode, the coordinate throughput rate is the RATE set in RegCtrl0 if the host retrieve channel data at this RATE.
The RATE set should be superior or equal to the CoordRate.
7.6. ESD event
In case of ESD event, the chip may reset to protect its internal circuitry. ESD event may trig the pen detection circuitry. In
this case wrong data will be send to the host. To detect this false coordinates on 4-wire touchscreen, Z1 and Z2 can be
read. The conditions Z1<LowThreshold and Z2>HighThreshold may indicate an ESD event. The values LowThreshold and
HighThreshold are given for indication only on the table below and should be fine tune according to the system.
Table 13. Threshold to detect false coordinates
Nch
[1..5 ] Nfilt
[1 3 5 7] PowDly
[uS] SetDly
[uS] Tconv
[uS] Tcomm
[uS] Total
[uS] CR
[kCPS] ECR
[kCPS] SR
[kSPS] ESR
[kSPS]
2.0
1.0 0.5 0.5
51.7 91.2 142.9 7.0 14.0 7.0 14.0
2.0
3.0 35.5 0.5
170.6 91.2 261.8 3.8 7.6 11.5 22.9
2.0
5.0 2.2 0.5
152.8 91.2 244.0 4.1 8.2 20.5 41.0
4.0
3.0 35.5 0.5
315.0 181.2 496.2 2.0 8.1 6.0 24.2
LowThreshold HighThreshold
10 4070
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7.7. Application Schematic
A typical application schematic is shown in Figure 35
Figure 35. Typical application
VDD_HOST can be higher than VDD but must not exceed the maximun voltage of 3.7V.
The host GPIO D0 output is connected to the SX8651 NRST input to allows SX8651 hardware reset.
The host D0 may be a totem pole output. In this configuration and if the host and the SX8651 are supply with the same
VDD, the R1 pull-up resistor is not required.
NIRQ pin is connected to a host interrupt pin. Once NIRQ event happens, the host read the data by a I2C read register.
8. Multi-Touch Gestures
8.1. Zoom Gesture
A simple thumb and forefinger “pinch” movement that enables a
user to enlarge objects onscreen (moving fingers away from each
other) or make them smaller (move them towards each other).
This intuitive zooming function replaces the standard point-and-
click functionality of a mouse and provides far greater accuracy to
the user.
8.2. Rotate Gesture
Rotate objects onscreen by making simple clockwise (right) or
counterclockwise (left) movements with the anchored thumb and
forefinger. This multi-touch function enables swift and accurate
positioning of objects without needing to point and click
repeatedly on a rotate left-right function button in order to achieve
the desired effect.
Touch
Screen
Interface
SX8651
VDD AUX
X+
Y+
X-
Y-
A0
NIRQ
NRST
SCL
SDA
GND
Control
I2C
Digital
Filter
ref+
ref- ADCin out
OSCPOR
Vref
HOST
INT
VDD_HOST
0.1 uF
2.2k
2.2k
2.2k
SDA
SCL
I2C
Interface
DO
TOUCH
SCREN
2.2k
R1 R2 R3 R4
VDD
VDD_HOST
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9. Packaging Information
9.1. DFN Package
Figure 36. DFN Package Outline Drawing
(LASER MARK)
INDICATOR
PIN 1
1
N
2
MIN
aaa
bbb
b
e
L
N
E1
D1
E
A1
A2
A
DIM MILLIMETERS
NOM
DIMENSIONS
MAXNOM
INCHES MIN MAX
.114 .118 .122 2.90 3.00 3.10
A1A2
LxN
E1
D1
e/2
bxN
D .114 .118 .122 2.90 3.00 3.10
D
E
E/2
D/2
A
NOTES:
CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES).
COPLANARITY APPLIES TO THE EXPOSED PAD AS WELL AS TERMINALS.2.
1.
.003
.006
.042
12
.008
.048
.000
.028
(.008)
0.08
0.20
12
.010
.052
0.15
1.06
.031
.002 0.00
0.70
1.31
0.25
1.21
0.05
0.80
(0.20)
.004 0.10
0.45 BSC.018 BSC 0.30.012 .020.016 0.40 0.50
aaa C SEATING
PLANE
A
bbb C A B
B
e
C
.074 .079 .083 1.87 2.02 2.12
0.02
0.75
.001
.030
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Figure 37. DFN Package Land Pattern
.087
.055 2.20
1.40
.150
.018
.010
.037 3.80
0.25
0.95
0.45
(.112)
.075 1.90
(2.85)
THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY.
CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR
COMPANY'S MANUFACTURING GUIDELINES ARE MET.
NOTES:
2.
THERMAL VIAS IN THE LAND PATTERN OF THE EXPOSED PAD
SHALL BE CONNECTED TO A SYSTEM GROUND PLANE.
FUNCTIONAL PERFORMANCE OF THE DEVICE.
FAILURE TO DO SO MAY COMPROMISE THE THERMAL AND/OR
3.
INCHES
DIMENSIONS
G
K
H
X
Y
P
Z
C
DIM MILLIMETERS
H
K
G
Y
Z
P
(C)
X
CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES).
1.
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9.2. WLCSP Package
Figure 38. WLCSP Package Outline Drawing
Figure 39. WLCSP Land Pattern
0.10 C
0.08 C
0.05 C A B
CONTROLLING DIMENSIONS ARE IN MILLIMETERS
NOTES:
1.
AB
C
A
B
C
INDEX AREA
A1 CORNER
0.25±0.02 SEATING
1 2 3
D
1.5±0.10
2.0±0.10
0.50
1.00
0.25
0.50
1.50
12X Ø0.315±0.03
PLANE
0.625 Max.
THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY.
CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR
NOTES:
2.
COMPANY'S MANUFACTURING GUIDELINES ARE MET.
1. CONTROLLING DIMENSIONS ARE IN MILLIMETERS
0.50
0.25 1.50
0.50
1.00
12X Ø0.25
Revision 1.0 / December 2010
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CERTIFIED
ADVANCED COMMUNICATIONS & SENSING
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