Capacitive Touch Sense Controller - Documentation

Capacitive Touch Sense Controller
October 2012
Reference Design RD1136
Introduction
Capacitive sensing is a technology based on capacitive coupling which takes human body capacitance as input.
Capacitive touch sensors are used in many devices such as laptop trackpads, digital audio players, computer displays, mobile phones, mobile devices, tablets and more. Capacitive sensors are preferred for their versatility, reliability and robustness, unique human-device interface and cost reduction over mechanical switches.
The Capacitive Touch Sense Controller reference design demonstrates touch sensing and control using the
iCE40™ FPGA. This design implements four touch sense buttons with outputs to LEDs.
This reference design is implemented in Verilog. The Lattice iCEcube2™ Place and Route tool integrated with the
Synplify Pro synthesis tool is used for implementation of the design. The design can also be targeted to other
iCE40 family members.
Features
• Four capacitive touch sensitive user buttons
• Up to four LED toggle control outputs
• 3.3 MHz frequency operation
• Touch sensitive resolution up to 300ns
Functional Description
Figure 1. Block Diagram
Capacitive
Touch
Sense
Button
BTN1
LED1
BTN2
LED2
BTN3
Capacitive
Touch
Sense
Controller
BTN4
LED3
LED
Block
LED4
CLK_3P3_MHz
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rd1136_01.0
Capacitive Touch Sense Controller
Signal Descriptions
Table 1. Signal Descriptions
Signal
Type
Description
BTN1
Bi-directional
Capacitive Touch Sense input
BTN2
Bi-directional
Capacitive Touch Sense input
BTN3
Bi-directional
Capacitive Touch Sense input
BTN4
Bi-directional
Capacitive Touch Sense input
LED1
Output
LED output
LED2
Output
LED output
LED3
Output
LED output
LED4
Output
LED output
Input
Clock Input
CLK_3P3_MHZ
Design Module
Figure 2. Design Module Block Diagram
Capsense Sample and
Button Controller
Timeout
Counter
Display
Mode
Selection
Rotate LED
Capacitive Touch Sense Controller
The design module consists of the blocks shown in Figure 2 and described below.
Timeout Counter
When enabled, this block begins counting until the timeout interval period is reached. Once the timeout period is
reached, it helps to trigger the scroll mode. If a touch occurs before the timeout, the counter is reset.
Display Mode Selection
This block sets the LED output display mode of operation depending on the timeout or a touch occurrence. When a
timeout signal is asserted, the block selects the LED scroll mode. Otherwise, if buttons are pressed, toggle mode is
selected.
Capsense Sample and Button Controller
This block constantly checks for a touch occurrence at any of the capacitive buttons. It controls the buttons by constantly sampling the button data to detect a touch input at any of the four buttons. This block drives the buttons to
go to the high impedance state during the charging time for the Capsense capacitor. When a touch is sensed, the
block sends signals to mode selection to toggle the respective LED.
Rotate LED
Once instantiated, this block rotates the LED blinking in scroll mode.
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Capacitive Touch Sense Controller
Operation
The design includes a set of four touch sensitive buttons that drive the signals to the FPGA input when a touch
occurs. By default, the controller keeps the LED signals toggling in scroll-up mode until the touch interrupt occurs.
The sample block constantly monitors for the occurrence of a touch at any button. Depending on the touch
sequence and timing, the output LED control signal is toggled. The status block stores data on previous touch
occurrences.
Figure 3 describes the capacitive touch button flow.
Figure 3. Capacitive Touch Button Flow Diagram
START
Drive pin connected to
capacitive button Low long
enough to guarantee that the pin
is at GND, despite the attached
RC network.
Force pin to Hi-Z. The
external pull-up resistor pulls
the pin High and charges the
capacitor.
No
Did any pin go
High?
Yes
Wait one 3.3 MHz clock
Sample all pin values
Yes
Are all buttons
High?
No
No
Did pin value
change from last
sample go high?
Yes
Toggle pin value
3
Capacitive Touch Sense Controller
The sampling signal drives the voltage on the capacitive touch button to ground to bleed any residual charge. After
a period of time, depending on the button sample frequency, the button is allowed to float High. Once the FPGA
output goes to Hi-Z (high-impedance, floating, tri-state), the pull-up resistor charges the capacitor of the touch button. After about an RC time constant (tau), the voltage on the pad exceeds the input switching threshold of the
FPGA. A finger pressed against the capacitive touch button adds about another 5 pF of capacitance, increasing the
RC constant and delaying the Low-to-High transition for a pressed button.
Figure 4. Capacitive Touch Timing Example
Sampling
Signal
Allow Pad to
Float High
Drive Pad to ground
Button
No Finger
Switching
Threshold
Button Value
No Finger
τ
Time delta between pressed and
unpressed button
~300-500 ns
Button
Finger Press
Switching
Threshold
τ = R(CBUTTON + CFINGER)
Button Value
Finger Press
The switching time difference between an unpressed and one or more pressed buttons is roughly 300 to 500ns.
Using the 3.33 MHz input, this amounts to a one clock delay difference between an unpressed and pressed buttons. The design detects simultaneous button presses on up to three of the capacitive touch buttons. Pressing all
four buttons is the same as pressing no buttons.
Simulation Waveforms
Figure 5. Simulation Waveforms
Implementation
This design is implemented in Verilog. When using this design in a different device, density, speed or grade, performance and utilization may vary.
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Capacitive Touch Sense Controller
Table 2. Performance and Resource Utilization
Device Family
Language
Utilization (LUTs)
fMAX (MHz)
I/Os
Architectual
Resources
Verilog
83
>170
9
N/A
iCE401
1. Performance utilization characteristics are generated using iCE-40LP1K-CM121 with iCEcube2 design software.
References
• iCE40 Family Handbook
Technical Support Assistance
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+1-503-268-8001 (Outside North America)
e-mail: [email protected]
Internet: www.latticesemi.com
Revision History
Date
Version
October 2012
01.0
Change Summary
Initial release.
5
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