DATASHEET
TOUCH SCREEN CONTROLLER MK712
IDT™
TOUCH SCREEN CONTROLLER 1
MK712 REV N 051310
Description
The MK712 Touch Screen Controller IC provides all the
screen drive, A to D converter and control circuits to easily
interface to 4-wire analog resistive touch screens.
The IC continually monitors the screen waiting for a touch.
In this mode, the supply current is typically 0.5µA. When
the screen is touched, the IC performs analog to digital
conversions to determine the location of the touch, stores
the X and Y locations in the registers, and issues an
interrupt. This process is repeated up to 197 times per
second until no further screen touches are detected, at
which time the low current mode is resumed.
Block Diagram
Features
•Packaged in 28-pin SSOP (150 mil) or 28-pin SOIC (300
mil)
•Industrial (-30°C to +70°C) and commercial (0 to +70°C)
and automotive (-35°C to +85°C) temperature ranges
•Pb (lead) free
•Fully integrated, lowest power solution
•Low standby current of 0.5µA (typical) at 5.0 V
•Active current while converting less than 10 mA
•Resolves 350 dots/inch on 10.5” display
•Maximum speed of 197 points/second
•Ratio metric conversion may eliminate screen calibration
•Automatically wakes up and goes back to standby
•3.3 V or 5 V supply
•12 bit A/D converter
•Simplifies the writing of software for touch screens
•Extended temperature range available
•Capacitors are the only external components required
•Touch screen is directly driven - no external transistors
are required
•Programmable conversion rate
•A to D Converter is monotonic
•Parallel interface
Pin Assignment
18
7
17
8
16
9
15
XL
10
YH
11
YL
12
GND
13
GND
14
GND
D6
D7
CLKIN
WR A0
A2
RD A1
22
21
20
19
CS
VDD
5
6
VDD
XH D5
24
23
D4
3
4
TOUCH
VDD D3
26
25
D2
1
2
NC
INT D1
28
27
D0
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Pin Descriptions
Interrupts
Interrupts are controlled by register 6 (see Register Description). The INT pin (pin 2) goes high to signal an
interrupt. Interrupts are then cleared by reading any register. However, if the MK712 is in the process of generating
an interrupt during a read cycle, then the interrupt is not cleared and INT will stay high. This internal process may
take 100ns, and so to guarantee that the interrupt is cleared, two successive read cycles may be necessary.
Pin
Number
Pin
Name
Pin Type Pin Description
1 NC — No connect. Do not connect this pin to anything.
2 INT Output Interrupt. Goes high to signal an interrupt. Cleared on next read.
3 TOUCH Output Touch signal. Goes low when screen is touched. Goes high when not touched.
4 VDD Power Power Supply. Connect to +5 V or +3.3 V. (Connect all VDDs to same voltage)
5 VDD Power Power Supply. Connect to +3.3 V.
6 XH Touch Screen Pin Connect to X-high on touch screen. (Right side)
7 XL Touch Screen Pin Connect to X-low on touch screen. (Left side)
8 YH Touch Screen Pin Connect to Y-high on touch screen. (Top)
9 YL Touch Screen Pin Connect to Y-low on touch screen. (Bottom)
10 GND Power Connect to ground.
11 GND Power Connect to ground.
12 WR Input Write enable. Active low.
13 RD Input Read enable. Active low.
14 CS Input Chip select. Active low.
15 A2 Input Register address 2.
16 A1 Input Register address 1.
17 A0 Input Register address 0.
18 CLKIN Input Clock input. Typically 14.3 MHz. Best if driven by an IDT clock synthesizer.
19 D7 I/O Data bit 7.
20 D6 I/O Data bit 6.
21 GND Power Connect to ground.
22 VDD Power Power Supply. Connect to +5 V or +3.3 V. (Connect all VDDs to same voltage)
23 D5 I/O Data bit 5.
24 D4 I/O Data bit 4.
25 D3 I/O Data bit 3.
26 D2 I/O Data bit 2.
27 D1 I/O Data bit 1.
28 D0 I/O Data bit 0.
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IC Operation
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Register Description
The MK712 register set is designed to be compatible with the Gazelle pen digitizer registers allowing a software
driver for the Gazelle digitizer to be used with the MicroClock MK712. The eight 8-bit registers are defined below:
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Rate Register (Register 7) Programming
Calculating Points Per Second
The formula for determining P.P.S. is:
Power-On Reset
On application of power, an internal reset is generated that clears all bits in registers 0, 1, and 6. Register 7 is set to
32 giving a rate of 86 PPS. Note that clearing register 6 puts the IC into the power down condition.
Initializing the MK712
The interrupt on the MK712 can only be cleared by reading a register or by writing register 6 to the power down
condition. After a fault condition, initialize the MK712 by writing register 6 to zero and then writing register 6 to the
desired value. This will always clear pending interrupts.
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Recommended Circuit
Pen Bounce
When the screen is untouched, the Y plate is driven high and the X plate is driven low. When the screen is touched,
the X plate is pulled high, which is detected by the MK712. This initiates a conversion (as long as periodic
conversions are enabled). Some de-bounce is provided by the time constant of the screen decoupling capacitors
combined with the screen resistance. However, once conversions have started, pen bounce will not be detected
until after both the X and Y points have been taken.
If the pen is lifted during a conversion, this will also not be detected until the conversion is complete. This is typically
indicated by a point with the correct Y coordinates, but minimum (for that screen) X coordinates.
Screen Decoupling
The component values suggested should give excellent results. However, for screens with a high resistance
operating at a high conversion rate, extra interrupts may be issued. This is because the time constant of the screen
decoupling capacitors and screen resistance is high, resulting in the screen appearing to be untouched and then
touched between conversions. These extra interrupts can be eliminated by reducing the size of the screen
decoupling capacitors on pins XH, XL, YH, and YL. This will, however, slightly reduce the noise rejection, which
could decrease the accuracy of the conversion.
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Optimizing Performance when Reading and Writing Registers
Reading and writing the MK712 generates digital noise that may reduce the accuracy of the A to D converter. This
noise has several causes, including board layout, data bus loading, and power supply voltage. By appropriately
timing the register operations, the effects of this noise can be minimized.
After an interrupt is issued, the MK712 allows a minimum of 1 millisecond to elapse before initiating the conversion
cycle. This allows the screen drivers to settle. For best performance, complete all register operations within this 1ms
window after an interrupt.
Power Down/Stopping the Clock
If the screen is untouched, the supply current is under 3µA. If the input clock is stopped, the supply current is also
less than 3µA. When the screen is touched with no clock, TOUCH goes low and INT goes high, but no further
operations occur until the clock starts. A to D conversions are invalid until after the clock has stabilized at the correct
frequency and one full conversion cycle has been completed.
Input Clock
The MK712 is highly tolerant of different clock frequencies (from 12 to 20 MHz), rise times, and duty cycles.
However, any input clock undershoot or overshoot will cause the MK712 to draw extra current. This can be
eliminated by using a MicroClock clock synthesizer and properly terminating the clock driver. MicroClock makes low
power clock synthesizers with 14.3 MHz power down.
Resistive Touch Screen (4-wire)
Resistive touch screens consist of 2 resistive plates that are separated by a small gap. Each plate has an electrode
at each end and when the screen is touched, the two plates are shorted together at that point.
If a voltage is applied, for example, between XL and XH, then a voltage divider is formed on the X PLATE. When the
Y PLATE is touched to the X PLATE, a voltage will be developed on the Y PLATE that is proportional to distance of
the touch from XL and XH. By accurately measuring this voltage, the position of the touch can be determined.
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Recommended Method for Reading Points
If the user is pressing lightly on the touch screen, it is likely that the screen contact will be intermittent. The screen
will bounce, causing false points. This effect can be reduced by insuring that the screen is touched both before and
after a conversion. By doing this, the first and last point in a series of points are discarded. The following algorithm
checks that a point has valid touches both before and after a conversion, and it distinguishes between
change-in-touch interrupts and conversion complete interrupts. Register 6 bits 0, 1, and 2 must all be true (logical
1) in order to use this method.
Warning–When Using a Switching Regulator Power Supply
When using the MK712 in a system where the power is supplied by a switching regulator, do not perform screen
conversions when the regulator is operating in the power saving mode. Some switching regulators feature a low
power mode (for example, Linear Technology’s “Burst Mode”) where the output is turned on and off in order to save
power. The extra power supply noise generated when using this mode causes spurious data points to be returned
from the MK712, so it should be disabled when the MK712 is doing screen conversions.
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Analog to Digital Converter Option
The 12-bit ADC converts X and Y co-ordinates at a rate determined by registers 6 and 7. The converter uses a
ratiometric technique to give absolute co-ordinates on the screen, largely independent of variations in screen
resistance, temperature or power supply voltage. The total voltage applied across the screen is defined as full-scale
for the converter (i.e. 4095) and any point touched on the screen is proportional to this. For example, if the screen
is touched exactly in the center, the converter will read 2047. This feature may allow for the elimination of calibration
upon startup.
However, the full scale voltage is defined at the IC pins and so any parasitic interconnect resistance will be included
in full scale. In addition, the interconnect resistances on the screen also account for about 20% of the total
resistance. This means that approximately the bottom 10% and top 10% of full scale are inaccessible.
The converter is guaranteed to be monotonic, with no missing codes.
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Absolute Maximum Ratings
Stresses above the ratings listed below can cause permanent damage to the MK712. These ratings, which are
standard values for IDT commercially rated parts, are stress ratings only. Functional operation of the device at these
or any other conditions above those indicated in the operational sections of the specifications is not implied.
Exposure to absolute maximum rating conditions for extended periods can affect product reliability. Electrical
parameters are guaranteed only over the recommended operating temperature range.
Recommended Operation Conditions
Item Rating
Supply Voltage, VDD (referenced to ground) 7 V
All Inputs and Outputs (referenced to ground) -0.5 V to VDD+0.5 V
Ambient Operating Temperature (commercial) 0 to +70°C
Ambient Operating Temperature (industrial) -30 to +70°C
Ambient Operating Temperature (automotive) -35 to +85°C
Storage Temperature -65 to +150°C
Junction Temperature 125°C
Soldering Temperature (10 to 20 seconds max) 260°C
Parameter Min. Typ. Max. Units
Ambient Operating Temperature (commercial) 0 +70 °C
Ambient Operating Temperature (industrial) -30 +70 °C
Ambient Operating Temperature (automotive) -35 +85 °C
Power Supply Voltage (measured in respect to GND)
(commercial and industrial)
+3.0 +5.5 V
Power Supply Voltage (measured in respect to GND)
(automotive)
+3.135 +5.5 V
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DC Electrical Characteristics
Unless stated otherwise,VDD = 3.3 V or 5V, TA = 0 to +70°C (commercial), -30°C to +70°C (industrial), -35°C to
+85°C (automotive)
Parameter Symbol Conditions Min. Typ. Max. Units
Operating Voltage VDD Commercial and
industrial
3.0 5.5 V
Operating Voltage VDD Automotive 3.135 5.5 V
Input High Voltage VIH VDD=5 V 2 V
Input Low Voltage VIL VDD=5 V 0.8 V
Output High Voltage VOH IOH = -2 mA VDD-0.4 V
Output High Voltage VOH VDD=5 V,
IOH = -12 mA
2.4 V
Output Low Voltage VOL VDD=5 V,
IOL = 12 mA
0.4 V
Input High Voltage VIH VDD=3.3 V 1.9 V
Input Low Voltage VIL VDD=3.3 V 0.4 V
Output High Voltage VOH VDD=3.3 V,
IOH = -8 mA
2.4 V
Output Low Voltage VOL VDD=3.3 V,
IOL = 8 mA
0.4 V
Operating Supply Current IDD At VDD=5 V, 300Ω
screen, 100 PPS
5mA
At VDD=3.3 V, Note 3 3 mA
Standby, Clock, Note 4 0.5 3 µA
Standby, Note 5 0.5 3 µA
Power down, PD=0 or
PD=1, no clock
0.2 3 µA
Short Circuit Current IOS Each output ±50 mA
Input Capacitance CIN 7pF
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AC Electrical Characteristics
Unless stated otherwise,VDD = 3.3 V or 5 V, TA = 0 to +70°C (commercial), -30°C to +70°C (industrial), -35°C to
+85°C (automotive)
Note 1: Consult with IDT about the best device to use to generate the input clock.
Note 2: With no missing codes.
Note 3: Assumes no 14.3 MHz input clock overshoot, 300Ω screen, 100 PPS.
Note 4: Assumes no 14.3 MHz input clock overshoot, no touch.
Note 5: With no touch, no clock.
Parameter Symbol Conditions Min. Typ. Max. Units
Input Clock Frequency, pin 18 Note 1 12 14.32 20 MHz
Input Clock Duty Cycle, pin 18 At 1.5 V 25 75 %
Output Rise Time tOR 0.8 to 2.0 V, VDD=5 V 3 ns
Output Fall Time tOF 2.0 to 0.8 V, VDD=5 V 3 ns
Resolution 12 bits
Non-linearity Monotonic, Note 2 ±2 LSB
Touch Screen Resistance 100 2000 Ω
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Register Interface Timing
Notes:
1. RD edges may precede or follow CS edges.
2. WR edges may precede or follow CS edges.
3. Either TRDW or TRDWC specification must be met.
4. Either TRDHW or TRDHWC specification must be met.
5. It is not permitted to have both RD and WR low at the same time.
Number Symbol Description Min. Max. Units Note
Read and Write Cycles
1 TRASC Address Setup to CS Low 5 ns
2 TRAHC Address Hold from CS High 0 ns
3 TRCCY CS High to CS Low 40 ns
Read Cycle Only
4 TRSCR CS Low to RD Low 10 ns
5 TRRD RD Pulse Width 40 ns
6 TRCHR1 RD High to CS High 0 ns
7 TRCHR2 RD High to CS Low 40 ns
8 TRDC CS Low to Data 35 ns
9 TRDR RD Low to Data 35 ns
10 TRDHC CS High to Data Release 2 20 ns 1
11 TRDHR RD High to Data Release 2 20 ns 1
Write Cycle Only
12 TRCSW CS Low to WR Low 10 ns
13 TRWR WR Pulse Width 40 ns
14 TRCHW WR High to CS High 0 ns 2
15 TRWH WR High to CS Low 40 ns
16 TRWCY WR High to WR Low 40 ns
17 TRDW Data Setup to WR High 15 ns 3
18 TRDHW Data Hold from WR High 15 ns 4
19 TRDWC Data Setup to CS High 15 ns 3
20 TRDHWC Data Hold from CS High 15 ns 4
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Register Read
Register Write
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Touchscreen Resistance Graph 1
Touchscreen Resistance Graph 2
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Touchscreen Resistance Graph 3
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Suggested Layout
Notes:
1. The 14.318 MHz input clock on pin 18 should have a 33Ω series termination resistor at its source. Consult IDT for
the best way to generate this clock.
2. All digital signals should be kept well away from pins 4, 5, 6, 7, 8, 9, 10, and 11, and any traces connected to
those pins.
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Package Outline and Package Dimensions (28-pin SOIC, 300 Mil. Wide Body)
Package dimensions are kept current with JEDEC Publication No. 95
INDEX
AREA
1 2
28
D
E
SEATING
PLANE
A1
A
e
- C -
B
.10 (.004) C
C
L
H
h x 45
Millimeters Inches
Symbol Min Max Min Max
A — 2.65 — 0.104
A1 0.10 — 0.004 0.010
A2 -- 1.50 0.0040 —
B 0.33 0.51 0.013 0.020
C 0.18 0.32 0.007 0.013
D 17.70 18.40 0.697 0.724
E 7.40 7.60 0.291 0.299
e 1.27 Basic .050 Basic
H 10.00 10.65 0.394 0.419
h 0.25 0.75 0.010 0.029
L 0.40 1.27 0.016 0.050
α0°8°0°8°
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Package Outline and Package Dimensions (28-pin SSOP, 150 Mil. Wide Body)
Package dimensions are kept current with JEDEC Publication No. 95
INDEX
AREA
1 2
28
D
E1 E
SEATING
PLANE
A
1
A
A
2
e
- C -
b
aaa C
c
L
Millimeters Inches
Symbol Min Max Min Max
A 1.35 1.75 0.053 0.069
A1 0.10 0.25 0.004 0.010
A2 -- 1.50 -- .059
b 0.20 0.30 0.008 0.012
C 0.18 0.25 0.007 0.010
D 9.80 10.00 0.386 0.394
E 5.80 6.20 0.228 0.244
E1 3.80 4.00 0.150 0.157
e .635 Basic .025 Basic
L 0.40 1.27 0.016 0.050
α0°8°0°8°
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Ordering Information
“LF” suffix to the part number are the Pb-Free configuration and are RoHS compliant.
While the information presented herein has been checked for both accuracy and reliability, Integrated Device Technology (IDT) assumes
no responsibility for either its use or for the infringement of any patents or other rights of third parties, which would result from its use. No
other circuits, patents, or licenses are implied. This product is intended for use in normal commercial applications. Any other applications
such as those requiring extended temperature range, high reliability, or other extraordinary environmental requirements are not
recommended without additional processing by IDT. IDT reserves the right to change any circuitry or specifications without notice. IDT
does not authorize or warrant any IDT product for use in life support devices or critical medical instruments.
Part / Order Number Marking Shipping Packaging Package Temperature
MK712SLF MK712SL Tubes 28-pin SOIC 0 to +70° C
MK712SLFTR MK712SL Tape and Reel 28-pin SOIC 0 to +70° C
MK712SILF MK712SIL Tubes 28-pin SOIC -30 to +70° C
MK712SILFTR MK712SIL Tape and Reel 28-pin SOIC -30 to +70° C
MK712RLF MK712RLF Tubes 28-pin SSOP 0 to +70° C
MK712RLFTR MK712RLF Tape and Reel 28-pin SSOP 0 to +70° C
MK712RWLF MK712RWLF Tubes 28-pin SSOP -35 to +85° C
MK712RWLFTR MK712RWLF Tape and Reel 28-pin SSOP -35 to +85° C
© 2006 Integrated Device Technology, Inc. All rights reserved. Product specifications subject to change without notice. IDT and the IDT logo are trademarks of Integrated Device
Technology, Inc. Accelerated Thinking is a service mark of Integrated Device Technology, Inc. All other brands, product names and marks are or may be trademarks or registered
trademarks used to identify products or services of their respective owners.
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www.idt.com
For Sales
800-345-7015
408-284-8200
Fax: 408-284-2775
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www.idt.com/go/clockhelp
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MK712
TOUCH SCREEN CONTROLLER TOUCH SCREEN CONTROL
