THIS SPEC IS OBSOLETE
Spec Number:
001-41442
Spec Title:
Capacitance Sensing - PC-Compatible USB
CapSense Matrix Keyboard
Sunset Owner:
ANBA
Replaced By:
NONE
Capacitance Sensing - PC-Compatible USB
CapSense Matrix Keyboard
AN2407
Author: Michael Macovetskyi, Ruslan Bachynskyy, Ryshtun Andrij
Associated Project: Yes
Associated Part Family: CY8C21434-24LKXI
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Software Version: PSoC Designer™ 4.3 SP2
Associated Application Notes: AN2233a, AN2292, AN2318, AN2352, AN2355
PSoC Application Notes Index
Application Note Abstract
This Application Note discusses construction, firmware communication, and considerations for a capacitive sensing matrixed
keyboard using PSoC® CapSense.
Table 1. Matrix Keyboard Specification
Introduction
Capacitive sensor keyboards are widely used in modern
compact devices. Keyboard sensors can be connected to
the microcontroller pin individually. However, such a
connection scheme limits the number of sensors to the
number of available sensing pins. For applications with
only a small number of sensors, this is not an issue. For
keyboards with many sensors, a new connection scheme
must be devised to maximize the number of sensors while
maintaining a small pin count. Creating a matrix of sensors
can drastically raise the number of sensors in an
application without increasing the pin overhead needed to
control them.
Characteristic
Total Keys Quantity
69 (64 matrix + 5 separated)
Matrix Structure
8x8
Sensor Dimension
11x11 mm
Communication Interface
I2C™
Overlay Thickness
1-6 mm
PC Interface
USB, via I2C-USB bridge
Power Supply Voltage
From USB 5 ± 0.25V
PC Implementation
Standard HID keyboard, no
additional GUI required
Figure 1. CapSense Matrix Keyboard Usage
CapSense Matrix
Keyboard
I2C
I2C-USB
Bridge
Keys Matrix Structure
USB
CY-3240-I2CUSB
Laptop or PC
This Application Note presents an example scheme for a
matrixed capacitive sensing keyboard. A system block
diagram is shown in Figure 1.
Technical specifications for the matrixed
presented here are listed in Table 1.
October 14, 2011
Value
keyboard
Keyboard keys are organized into two groups. The first
group is comprised of special function keys such as Shift,
Ctrl, Alt keys. These keys are connected to PSoC pins
individually and are scanned separately. The rest of the
keys are organized in a rectangular 8-row, 8-column
matrix. The 64 keys in this matrix represent alphanumeric
keys as well as special keys such as WIN and EXC.
Shift, Ctrl, Alt and some other keys can be activated at the
same time as other keys. Scanning these keys separately
allows correct detection of simultaneous sensor activation
of these and keys in the general matrix.
Document No. 001-41442 Rev. *A
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AN2407
The sensor design implemented on the demonstration
board increases the reliability of sensor activation
detection. Each key consists of four segments. Segments
opposite each other are connected to each other, creating
two interwoven yet electrically distinct sensors for each
key; there is a row sensor and column sensor for each
key. Figure 2 illustrates the row (gray) and column (white)
construction in two colors. This interwoven key structure
means that a key represents activation on a row and a
column at the same time.
Figure 3. Key Press/Sensor Activation Examples
Sensors
COL1
Keys
COL2
ROW1
ROW2
Figure 2. Matrix Keys Structure
COL1
COL2
COL3
Key1
Key2
Key3
Key4
Key1
Key2
Key3
Key4
Key1
Key2
Key3
Key4
Key1
Key2
Key3
Key4
a)
COL1
ROW1
COL2
ROW1
ROW2
ROW2
ROW3
b)
COL1
COL2
ROW1
Firmware for activation detection completed an analysis of
the row=column intersections. This method works well
when only one key (one row and one column) is pressed
at a time. If more than one key is pressed, false
activations can occur due to multiple row-column
activations.
Figures 3a shows the activation scheme for a single
pressed key. Figure 3b shows the activation scheme for a
simultaneous-key press that can be detected by the
controller. Figures 3c and 3d show different simultaneouskey presses that exhibit the same activation state and
cannot be distinguished. Two keys that do not show either
a row or a column sensor, when pressed, are viewed as
four active keys.
To avoid this phenomenon, keyboard design logic allows
only one key to register as pressed by selecting only one
row and column sensor at a time. The selected sensor is
determined by using a sorting algorithm for signals.
ROW2
c)
COL1
COL2
ROW1
ROW2
d)
Legend:
Activated Row Sensor
Detected as Not
Pressed
Activated Column Sensor
Non-activated Row Sensor
Detected as
Pressed
Non-activated Column Sensor
Touched Sensor
October 14, 2011
Document No. 001-41442 Rev. *A
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AN2407
Keyboard Schematic and PCB
Keyboard Firmware
The proposed keyboard uses minimal external
components and no mechanical keys. All the keys are
capacitive sensors implemented on the PCB as copper
pads. The proposed keyboard consists of a PSoC and
several resistors and capacitors. The keyboard schematic
is shown in Appendix A. Series resistors are placed on
each sensing trace and close to the PSoC. These
resistors decrease the influence of the RF noise caused
by cell phones, microwave ovens, etc. For more
information on this, see AN2292, “Layout Guidelines for
PSoC CapSense” and AN3218, “EMC Considerations for
PSoC CapSense.”
The PSoC Designer project for this Application Note uses
the CapSense Sigma Delta (CSD) User Module (UM) for
capacitive sensing and the EzI2C UM to transmit the
2
results of the capacitive scans to the host though an I C
interface. The firmware scans the individual and matrix
sensors and sends the key codes to the I2C-USB bridge.
Keys activation detection logic and sound generation is
implemented in the firmware of the PSoC, not in the host
or GUI. The CSD UM parameters are shown in Figure 4.
Figure 4. CSD UM Parameters
When multiple sensing elements are connected in parallel
(sensors in rows and columns) the parasitic capacitance
increases. A shield electrode is added behind the sensors
to reduce this parasitic capacitance. The shield electrode
is located on the bottom PCB layer.
Some sensors (Space, Enter) have much larger area than
other sensors. This difference causes large sensor-tosensor raw count variation, yielding different optimal
modulator feedback resistor, R32 values. To balance raw
counts for different sensors, resistor R27 is connected in
parallel to the modulator capacitor when large area
sensors are scanned. This connection is done in the
firmware. For details on the modulator resistor and its
impact on raw counts in CSD capacitive sensing, see
AN2233a, “Capacitive Sensing Switch Scan/Theory and
Operation.” For information on how to determine the value
of R32 for other applications, see AN2355, “Calibrating
CapSense Applications.”
Speaker LS1 is used to emulate the „click‟ of a mechanical
button. When key press is detected, PSoC generates a
click signal on port P1[7] using a pulse width modulator
(PWM). This signal is amplified through VT1 and played
via speaker LS1.
Connector J3 is used for ISSP and for I2C data transfer.
The following requirements should be applied to board
layout for optimal matrix keyboard performance.
Reduce spacing between row and column sensors for
each button (8-12 mill/0.2-0.3 mm).
Keep the size and spacing of sensors consistent
when possible.
Use the physical key structure to determine the
connection matrix structure for simplified layout.
The Enter and Space keys are not special function keys
as are Ctrl, Shift and Alt. But they are very large by the
mechanical design. Therefore, these keys have individual
PSoC pins and are scanned with individual threshold
parameters.
October 14, 2011
A sorting algorithm is used to detect which buttons are
really pressed when multiple keys are active. When a
finger is near a key, non-zero signals exist on several
neighboring key row and column elements at the same
time. The activation may be on a particular key (row and
column), but other rows and columns exhibit some signal
as well. The most upper element is used to detect which
element is really being pressed by the sensor when
multiple sensors show possible activation.
After the active row and column are determined, the
internal-use key code is derived using the following
formula
KeyCode=(RowMaxSenNum<<3)+ColMaxSenNum[15]-8;
For Shift, Ctrl, Alt, Space, and Return key activation
detection, CSD high-level APIs are used. After, the
scanning
key
state
is
read
from
the
CSD_baSnsOnMask[2] array.
Document No. 001-41442 Rev. *A
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AN2407
When finger is between two keys, false activation is
possible. To avoid this, an additional selection algorithm
and hysteresis are used. A key pressed event is detected
only when the signal from this key is two times or more
greater than the signal from neighboring keys. If it is not,
the key activation state is not changed.
Figure 7. HID Report Description
USB HID Device
To test and demonstrate the matrix keyboard unit, we use
special firmware that was developed for Cypress, I2C to
USB Bridge Kit CY3240-I2USB. The PC operating system
detects the I2C-USB bridge as an HID keyboard. The
bridge firmware converts internal key code to HID key
codes. We used two different PSoC devices for sensor
scanning (CY8C21434-24LKXI) and USB communication
(CY8C24894-24LFXI). It is possible to implement this
design using a single CY8C24784-24LKXI, which supports
both CSD capacitive sensing and USB communication.
When a user holds a finger on the button for a long time,
the PC operating system automatically runs character
auto-repeat. This is implemented by the operating system,
not the I2C-USB bridge.
The USB HID keyboard endpoint descriptors and HID
report description are shown in Figure 5 and Figure 7. The
Windows device manager screen is shown in Figure 6.
Figure 5. USB HID Keyboard Endpoint Descriptors
The sample text in Figure 8 was typed using the USB
CapSense Matrix Keyboard.
Figure 8. Sample Text
Figure 6. Windows Device Manager
The matrix keyboard PCB pictures are shown in Appendix
B. Gerber files are included in the project archive for
reference.
The CapSense matrix keyboard with I2C-USB bridge
photo is shown in Appendix C.
October 14, 2011
Document No. 001-41442 Rev. *A
4
AN2407
Appendix A. Keyboard Schematic
C_1
C_2
R16
200
C_3
R17
200
C_4
R18
200
C_5
R19
200
C_6
R20
200
C_7
R21
200
C_8
R23
200
R22
200
R8 200
1
!
1
2
K1
1
@
2
2
K2
1
#
3
2
K3
1
$
4
2
K4
1
%
5
2
K5
4
3
4
3
4
3
4
3
4
3
4
1
Esc
R10 200
3
4
3
4
3
R_1
1
^
6
2
K6
1
&
7
2
K7
2
K8
1
~
1
T
1
U
2
K13
4
3
1
Y
2
K12
4
3
4
1
R
2
K11
3
1
E
2
K10
4
3
4
1
W
2
K17
3
4
1
Q
2
R9 200
3
4
3
4
3
R_2
2
K14
2
K15
K16
1
TAB
1
G
1
J
2
K21
4
3
1
H
2
K20
4
3
4
1
F
2
K19
3
1
D
2
K18
4
3
4
1
S
2
K9
3
4
1
A
2
R12 200
3
4
3
4
3
R_3
2
K22
2
K23
K24
1
BACK
1
B
1
M
2
K29
4
3
1
N
2
K28
4
3
4
1
V
2
K27
3
1
C
2
K26
4
3
4
1
X
2
K41
3
4
1
Z
2
R14 200
3
4
3
4
3
R_4
2
K30
2
K31
K32
1
*
8
R11 200
1
(
9
2
K36
1
)
0
2
K37
1
__
_
2
K38
1
+
=
2
K39
1
PRINT
2
2
K40
1
Scroll Lock
2
K70
4
3
4
3
4
3
4
3
4
3
4
3
4
3
4
3
R_5
1
PAUSE
2
K71
K72
1
I
1
{
[
2
K45
K46
1
}
]
2
K47
1
Insert
1
2
K48
1
Home
2
4
3
4
3
4
3
4
3
4
1
P
2
K44
3
4
1
O
2
R13 200
3
4
3
4
3
R_6
PUp
2
K64
2
K65
K67
1
K
K53
2
K54
1
"
'
1
|
\
2
K55
1
DELETE
1
PDown
2
K56
1
End
2
4
3
4
3
4
3
4
3
4
1
:
;
2
K52
3
4
1
L
2
R24 200
3
4
3
4
3
R_7
2
K42
2
K66
K68
K60
K61
1
2
2
K73
1
1
<
2
2
K43
4
3
4
3
4
3
4
3
4
1
1
K49
1
>
>
K59
?
/
2
3
4
1
>
.
2
<
1
<
,
3
4
3
4
3
R_8
2
2
K50
K51
ENTER
SPACE
SHIFT
VCC
R5 200
ALT1
ALT2
ALT
R6
200
C3
0.1uF
+
C4
4.7uF
SHIFT
SHIFT1
SHIFT2
R7
200
R15
200
R26
200
SPACE
SPACE
ENTER
CTRL
Vss
P0[3]
P0[5]
P0[7]
Vdd
P0[6]
P0[4]
P0[2]
CTRL1
CTRL2
32
31
30
29
28
27
26
25
ENTER
C_7
C_8
C_6
C_5
C_4
SHILD
SPK
LS1
C2 4.7uF
R25
VT1
BC807-40 T/R
SPK
U1
CY 8C21434
P0[0]
P2[6]
P2[4]
P2[2]
P2[0]
P3[2]
P3[0]
XRES
24
23
22
21
20
19
18
17
R_1
R_2
R_3
R_4
R_5
R_6
R_7
R_8
CTRL
ALT
C_1
C_2
C_3
1k
9
10
11
12
13
14
15
16
R1
5K1
VCC
P0[1]
P2[7]
P2[5]
P2[3]
P2[1]
P3[3]
P3[1]
P1[7]
P1[5]
P1[3]
P1[1]
Vss
P1[0]
P1[2]
P1[4]
P1[6]
CEM-1206S
1
2
3
4
5
6
7
8
R3 2K2
VD1
STPS340U
R27 6K2
C1
10nF
J3
1
2
3
4
5
VCC
ISSP/DATA
October 14, 2011
Document No. 001-41442 Rev. *A
5
AN2407
Appendix B. Keyboard Layout
Figure 9. PCB Top. Scale 50%
Figure 10. PCB Bottom. Scale 50%
Figure 10. PCB Silk Screen Top. Scale 50%
October 14, 2011
Document No. 001-41442 Rev. *A
6
AN2407
Appendix C. Device Photo
Figure 12. CapSense Matrix Keyboard with I2C-USB Bridge Photo
October 14, 2011
Document No. 001-41442 Rev. *A
7
AN2407
Document History
Document Title: Capacitance Sensing - PC-Compatible USB CapSense Matrix Keyboard – AN2407
Document Number: 001-41442
Revision
ECN
Orig. of
Change
Description of Change
Submission
Date
**
1541809
ANBA
10/04/2007
New spec.
*A
3407058
ZINE
10/14/2011
Obsolete spec.
In March of 2007, Cypress recataloged all of its Application Notes using a new documentation number and revision code. This new documentation
number and revision code (001-xxxxx, beginning with rev. **), located in the footer of the document, will be used in all subsequent revisions.
PSoC is a registered trademark of Cypress Semiconductor Corp. "Programmable System-on-Chip," PSoC Designer are trademarks of Cypress
Semiconductor Corp. All other trademarks or registered trademarks referenced herein are the property of their respective owners.
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October 14, 2011
Document No. 001-41442 Rev. *A
8