AN3516: E-field Keyboard Designs (pdf)

Freescale Semiconductor
Application Note
Document Number: AN3516
Rev. 0, 09/2007
E-field Keyboard Designs
by: Michael Steffen
Freescale Semiconductor
1
Introduction
This application note provides the fundamentals for
designing keyboards with electric field (E-field) devices
MC33794, MC34940, and MC33941. It describes the
E-field basic operation and single and multiplexed
electrodes. It also provides example keyboards you can
use in your designs.
2
E-field Keyboards
© Freescale Semiconductor, Inc., 2007. All rights reserved.
Contents
1
2
3
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E-field Keyboards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1 E-field Device Basic Operation . . . . . . . . . . . . . . . .
2.2 Human Interface Detection to Keyboards . . . . . . . .
2.3 Basic Keyboard Designs . . . . . . . . . . . . . . . . . . . . .
2.4 Multiplexed Electrode Design . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
1
2
2
3
4
5
E-field Keyboards
2.1
E-field Device Basic Operation
The E-field device uses sine wave generation to derive the electric field used to sense a person’s finger or
any other conductive object. Inside the device, the electric field is derived by the oscillator circuitry. The
oscillator circuitry generates a high purity, low frequency, 5 V peak-to-peak sine wave. This AC signal is
fed through an internal 22 kΩ resistor to a multiplexer that directs the signal to the selected electrode or
reference pin or to an internal measurement node. Unselected electrodes are automatically connected to
the circuit ground by the IC.
The point where each electrode is connected back to the IC forms an AC voltage divider. The top leg of
the divider is the 22 kΩ resistor; the bottom leg of the divider is a capacitor. Because the divider is fed with
an AC sine wave, the bottom leg is controlled by capacitive reactance. All of the variables drop out of the
equation:
Xc = 1 / (2 × Pi × F × C)
Eqn. 1
except Pi and F, which are constant. So, all that remains is 1/C, where C is the capacitance formed by the
finger or other stimulus approaching the electrode. After the divider, the signal is rectified and filtered and
becomes an analog voltage output on pin 12 of the device. As the finger or stimulus moves closer to the
electrode, the capacitance becomes larger. This results in a voltage drop across the internal 22 kΩ resistor.
The voltage drop causes a voltage change at the electrode input pin. An on-board rectifier in the IC
converts the AC signal to DC level. The DC level is then low-pass filtered using an internal series resistor
and an external parallel capacitor. This DC voltage is multiplied, offset, and sent to the LEVEL pin of the
IC.
2.2
Human Interface Detection to Keyboards
The E-field device can detect anything that is either conductive or has different dielectric properties than
the electrodes’ surroundings. Human beings are well suited for E-field imaging because the human body
is composed mainly of water that has a high dielectric constant and contains ionic matter. This makes
humans very conductive. The body also provides a good electrical coupling path to earth ground used for
return ground of the IC. Thus, when a finger is brought near to a metal electrode, an electrical path is
formed. This path produces a change in electric field current that is detected by the E-field device and is
translated to a different output voltage.
E-field Keyboard Designs, Rev. 0
2
Freescale Semiconductor
E-field Keyboards
2.3
2.3.1
Basic Keyboard Designs
Single Electrode Design
Figure 1. Single Electrode Design
In a single electrode keypad design, all keys are composed of a single plate or pad of copper. The keyboard
design is referred to as a single electrode keypad design. Each electrode or keypad is coupled back to the
E-field device through a series capacitor. The assumption is that the finger is capacitive coupled or
connected to virtual ground. As the finger approaches the single electrode, it provides a conductive path
from charged electrode to ground. Each time the key is selected by the E-field device, the keypad or
electrode will change the output voltage of the LEVEL pin. Single electrodes or keys are not limited to
keyboard design; single pads can also be used to detect proximity of a human hand or object. Figure 2
shows a few examples of single electrode designs.
Single key or electrode
Figure 2. MC33941 Demo Kit E-field Board
E-field Keyboard Designs, Rev. 0
Freescale Semiconductor
3
E-field Keyboards
2.4
Multiplexed Electrode Design
Figure 3. Multiplexed Electrode Design
In a multiplexed or dual electrode keypad design, all keys are composed of two plates or pads of copper.
This keyboard design is referred to as a multiplexed electrode keypad design. For dual sensors, one
electrode is charged and the other is at ground to increase sensitivity. This happens because inside the
E-field device, unselected electrodes are grounded. As the finger approaches the dual electrodes, it
provides a conductive path from charged electrode to ground through the finger and from charged
electrode to ground through the grounded electrode. In other words, the closer the finger gets to the
electrode, the greater the electrode loading. Each time a key is touched, two electrodes for each key will
change the output voltage of the LEVEL pin. Figure 4 shows an example of multiplexed electrode keys.
Each key is composed of two pads
positiioned side by side.
Figure 4. MC33941 Demo Kit E-field Board
E-field Keyboard Designs, Rev. 0
4
Freescale Semiconductor
References
3
References
Link to example keyboards in EXPRESSPCB format:
http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=DEMO1985MC34940E&fpsp=1&t
ab=Design_Tools_Tab
Touch Panel Application Note:
http://www.freescale.com/files/sensors/doc/app_note/AN1985.pdf
E-field Keyboard Designs, Rev. 0
Freescale Semiconductor
5
How to Reach Us:
Home Page:
www.freescale.com
Web Support:
http://www.freescale.com/support
USA/Europe or Locations Not Listed:
Freescale Semiconductor, Inc.
Technical Information Center, EL516
2100 East Elliot Road
Tempe, Arizona 85284
+1-800-521-6274 or +1-480-768-2130
www.freescale.com/support
Europe, Middle East, and Africa:
Freescale Halbleiter Deutschland GmbH
Technical Information Center
Schatzbogen 7
81829 Muenchen, Germany
+44 1296 380 456 (English)
+46 8 52200080 (English)
+49 89 92103 559 (German)
+33 1 69 35 48 48 (French)
www.freescale.com/support
Japan:
Freescale Semiconductor Japan Ltd.
Headquarters
ARCO Tower 15F
1-8-1, Shimo-Meguro, Meguro-ku,
Tokyo 153-0064
Japan
0120 191014 or +81 3 5437 9125
[email protected]
Asia/Pacific:
Freescale Semiconductor Hong Kong Ltd.
Technical Information Center
2 Dai King Street
Tai Po Industrial Estate
Tai Po, N.T., Hong Kong
+800 2666 8080
[email protected]
For Literature Requests Only:
Freescale Semiconductor Literature Distribution Center
P.O. Box 5405
Denver, Colorado 80217
1-800-441-2447 or 303-675-2140
Fax: 303-675-2150
[email protected]
Document Number: AN3516
Rev. 0
09/2007
Information in this document is provided solely to enable system and software
implementers to use Freescale Semiconductor products. There are no express or
implied copyright licenses granted hereunder to design or fabricate any integrated
circuits or integrated circuits based on the information in this document.
Freescale Semiconductor reserves the right to make changes without further notice to
any products herein. Freescale Semiconductor makes no warranty, representation or
guarantee regarding the suitability of its products for any particular purpose, nor does
Freescale Semiconductor assume any liability arising out of the application or use of any
product or circuit, and specifically disclaims any and all liability, including without
limitation consequential or incidental damages. “Typical” parameters that may be
provided in Freescale Semiconductor data sheets and/or specifications can and do vary
in different applications and actual performance may vary over time. All operating
parameters, including “Typicals”, must be validated for each customer application by
customer’s technical experts. Freescale Semiconductor does not convey any license
under its patent rights nor the rights of others. Freescale Semiconductor products are
not designed, intended, or authorized for use as components in systems intended for
surgical implant into the body, or other applications intended to support or sustain life,
or for any other application in which the failure of the Freescale Semiconductor product
could create a situation where personal injury or death may occur. Should Buyer
purchase or use Freescale Semiconductor products for any such unintended or
unauthorized application, Buyer shall indemnify and hold Freescale Semiconductor and
its officers, employees, subsidiaries, affiliates, and distributors harmless against all
claims, costs, damages, and expenses, and reasonable attorney fees arising out of,
directly or indirectly, any claim of personal injury or death associated with such
unintended or unauthorized use, even if such claim alleges that Freescale
Semiconductor was negligent regarding the design or manufacture of the part.
RoHS-compliant and/or Pb-free versions of Freescale products have the functionality
and electrical characteristics as their non-RoHS-compliant and/or non-Pb-free
counterparts. For further information, see http://www.freescale.com or contact your
Freescale sales representative.
For information on Freescale’s Environmental Products program, go to
http://www.freescale.com/epp.
Freescale™ and the Freescale logo are trademarks of Freescale Semiconductor, Inc.
All other product or service names are the property of their respective owners.
© Freescale Semiconductor, Inc. 2007. All rights reserved.
Similar pages