This document describes the design and implementation of a capacitive sensing matrix keyboard using PSoC CapSense. Key points: - The keyboard uses an 8x8 matrix of capacitive sensors connected to a PSoC microcontroller to detect key presses. Additional keys are directly connected. - Firmware scans the matrix and direct keys, detects presses using row-column intersections, and sends output over USB HID. - A two-sensor design for each key improves detection accuracy. The PCB, schematic, and firmware implementation are described. - The keyboard presents as a standard USB HID keyboard to the PC. A demo project shows typing on the capacitive matrix keyboard.

1. 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 Characteristic Value
compact devices. Keyboard sensors can be connected to
Total Keys Quantity 69 (64 matrix + 5 separated)
the microcontroller pin individually. However, such a
connection scheme limits the number of sensors to the Matrix Structure 8x8
number of available sensing pins. For applications with
only a small number of sensors, this is not an issue. For Sensor Dimension 11x11 mm
keyboards with many sensors, a new connection scheme 2
Communication Interface I C™
must be devised to maximize the number of sensors while
maintaining a small pin count. Creating a matrix of sensors Overlay Thickness 1-6 mm
can drastically raise the number of sensors in an
2
PC Interface USB, via I C-USB bridge
application without increasing the pin overhead needed to
control them.
Power Supply Voltage From USB 5 ± 0.25V
Figure 1. CapSense Matrix Keyboard Usage Standard HID keyboard, no
PC Implementation
additional GUI required
Keys Matrix Structure
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
This Application Note presents an example scheme for a matrix. The 64 keys in this matrix represent alphanumeric
matrixed capacitive sensing keyboard. A system block keys as well as special keys such as WIN and EXC.
diagram is shown in Figure 1.
Shift, Ctrl, Alt and some other keys can be activated at the
Technical specifications for the matrixed keyboard same time as other keys. Scanning these keys separately
presented here are listed in Table 1. allows correct detection of simultaneous sensor activation
of these and keys in the general matrix.
February 16, 2007 Document No. 001-41442 Rev. ** 1
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2. AN2407
The sensor design implemented on the demonstration Figure 3. Key Press/Sensor Activation Examples
board increases the reliability of sensor activation
Sensors Keys
detection. Each key consists of four segments. Segments
opposite each other are connected to each other, creating COL1 COL2
two interwoven yet electrically distinct sensors for each
key; there is a row sensor and column sensor for each ROW1 Key1 Key2
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.
ROW2
Key3 Key4
Figure 2. Matrix Keys Structure
a)
COL1 COL2
ROW1 Key1 Key2
ROW2
Key3 Key4
b)
COL1 COL2
ROW1 Key1 Key2
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 ROW2
Key3 Key4
activations.
Figures 3a shows the activation scheme for a single
c)
pressed key. Figure 3b shows the activation scheme for a
simultaneous-key press that can be detected by the COL1 COL2
controller. Figures 3c and 3d show different simultaneous-
ROW1 Key1 Key2
key 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.
ROW2
To avoid this phenomenon, keyboard design logic allows Key3 Key4
only one key to register as pressed by selecting only one
row and column sensor at a time. The selected sensor is
d)
determined by using a sorting algorithm for signals.
Legend:
Activated Row Sensor Detected as Not
Pressed
Activated Column Sensor
Detected as
Non-activated Row Sensor
Pressed
Non-activated Column Sensor
Touched Sensor
February 16, 2007 Document No. 001-41442 Rev. ** 2
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3. AN2407
Keyboard Schematic and PCB Keyboard Firmware
The proposed keyboard uses minimal external The PSoC Designer project for this Application Note uses
components and no mechanical keys. All the keys are the CapSense Sigma Delta (CSD) User Module (UM) for
capacitive sensors implemented on the PCB as copper capacitive sensing and the EzI2C UM to transmit the
results of the capacitive scans to the host though an I2C
pads. The proposed keyboard consists of a PSoC and
several resistors and capacitors. The keyboard schematic interface. The firmware scans the individual and matrix
is shown in Appendix A. Series resistors are placed on sensors and sends the key codes to the I2C-USB bridge.
each sensing trace and close to the PSoC. These Keys activation detection logic and sound generation is
resistors decrease the influence of the RF noise caused implemented in the firmware of the PSoC, not in the host
by cell phones, microwave ovens, etc. For more or GUI. The CSD UM parameters are shown in Figure 4.
information on this, see AN2292, “Layout Guidelines for
Figure 4. CSD UM Parameters
PSoC CapSense” and AN3218, “EMC Considerations for
PSoC CapSense.”
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-to-
sensor 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 A sorting algorithm is used to detect which buttons are
(PWM). This signal is amplified through VT1 and played really pressed when multiple keys are active. When a
via speaker LS1. finger is near a key, non-zero signals exist on several
neighboring key row and column elements at the same
Connector J3 is used for ISSP and for I2C data transfer.
time. The activation may be on a particular key (row and
The following requirements should be applied to board column), but other rows and columns exhibit some signal
layout for optimal matrix keyboard performance. as well. The most upper element is used to detect which
element is really being pressed by the sensor when
• Reduce spacing between row and column sensors for
multiple sensors show possible activation.
each button (8-12 mill/0.2-0.3 mm).
• Keep the size and spacing of sensors consistent After the active row and column are determined, the
when possible. internal-use key code is derived using the following
• Use the physical key structure to determine the formula
connection matrix structure for simplified layout.
KeyCode=(RowMaxSenNum<<3)+ColMaxSenNum[15]-8;
The Enter and Space keys are not special function keys
For Shift, Ctrl, Alt, Space, and Return key activation
as are Ctrl, Shift and Alt. But they are very large by the
detection, CSD high-level APIs are used. After, the
mechanical design. Therefore, these keys have individual
scanning key state is read from the
PSoC pins and are scanned with individual threshold
CSD_baSnsOnMask[2] array.
parameters.
February 16, 2007 Document No. 001-41442 Rev. ** 3
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4. AN2407
When finger is between two keys, false activation is Figure 7. HID Report Description
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.
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.
The sample text in Figure 8 was typed using the USB
Figure 5. USB HID Keyboard Endpoint Descriptors 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.
February 16, 2007 Document No. 001-41442 Rev. ** 4
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5. AN2407
Appendix A. Keyboard Schematic
C_1 C_2 C_3 C_4 C_5 C_6 C_7 C_8
R16 R17 R18 R19 R20 R21 R23 R22
200 200 200 200 200 200 200 200
R8 200
R_1 3
4
3
4
3
4
3
4
3
4
3
4
3
4
3
4
1 1 1 1 1 1 1 1
Esc ! @ # $ % ^ &
2 1 2 2 2 3 2 4 2 5 2 6 2 7 2
R10 200 K1 K2 K3 K4 K5 K6 K7 K8
R_2
3
4
3
4
3
4
3
4
3
4
3
4
3
4
3
4
1 1 1 1 1 1 1 1
~ Q W E R T Y U
2 2 2 2 2 2 2 2
R9 200 K17 K10 K11 K12 K13 K14 K15 K16
R_3
3
4
3
4
3
4
3
4
3
4
3
4
3
4
3
4
1 1 1 1 1 1 1 1
TAB A S D F G H J
2 2 2 2 2 2 2 2
R12 200 K9 K18 K19 K20 K21 K22 K23 K24
R_4
3
4
3
4
3
4
3
4
3
4
3
4
3
4
3
4
1 1 1 1 1 1 1 1
BACK Z X C V B N M
2 2 2 2 2 2 2 2
R14 200 K41 K26 K27 K28 K29 K30 K31 K32
R_5
3
4
3
4
3
4
3
4
3
4
3
4
3
4
3
4
1 1 1 1 1 1 1 1
* ( ) __ + PRINT Scroll Lock PAUSE
8 2 9 2 0 2 _ 2 = 2 2 2 2
R11 200 K36 K37 K38 K39 K40 K70 K71 K72
R_6
3
4
3
4
3
4
3
4
3
4
3
4
3
4
3
4
1 1 1 1 1 1 1 1
I O P { } Insert Home PUp
2 2 2 [ 2 ] 2 2 2 2
R13 200 K44 K45 K46 K47 K48 K64 K65 K67
R_7
3
4
3
4
3
4
3
4
3
4
3
4
3
4
3
4
1 1 1 1 1 1 1 1
K L : quot; | DELETE End PDown
2 2 ; 2 ' 2 2 2 2 2
R24 200 K52 K53 K54 K55 K56 K42 K66 K68
R_8
3
4
3
4
3
4
3
4
3
4
3
4
3
4
3
4
1 1 1 1 1 1 1 1
<
< > ? < >
>
, 2 . 2 / 2 2 2 2 2 2
K59 K60 K61 K73 K43 K49 K50 K51
VCC
ENTER
SPACE
SHIFT
R5 200
ALT1
C3 C4
+
ALT
ALT2 4.7uF
0.1uF
R6 200
SHIFT
SHIFT1
SHIFT2
R7 200
SPACE SPACE
R15 200
ENTER ENTER
32
31
30
29
28
27
26
25
R26 200
CTRL1 CTRL
Vdd
CTRL2
Vss
P0[3]
P0[5]
P0[7]
P0[6]
P0[4]
P0[2]
R_1
R_2
R_3
1 24 R_4
P0[1] P0[0]
2 23 R_5
C_7 P2[7] P2[6]
LS1 3 22 R_6
C_8 P2[5] P2[4]
4 U1 21 R_7
C_6 P2[3] P2[2]
5 CY 8C21434 20 R_8
C_5 P2[1] P2[0]
6 19 CTRL
C_4 P3[3] P3[2]
7 18 ALT
SHILD P3[1] P3[0]
8 17
CEM-1206S C_1
SPK P1[7] XRES
C2 4.7uF C_2
R25
P1[5]
P1[3]
P1[1]
P1[0]
P1[2]
P1[4]
P1[6]
C_3
VT1
Vss
SPK
BC807-40 T/R
R1 1k
9
10
11
12
13
14
15
16
5K1 R3 2K2
VD1
VCC
STPS340U
R27 6K2
C1
J3
10nF
1 VCC
2
3
4
5
ISSP/DATA
February 16, 2007 Document No. 001-41442 Rev. ** 5
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6. 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%
February 16, 2007 Document No. 001-41442 Rev. ** 6
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7. AN2407
Appendix C. Device Photo
Figure 12. CapSense Matrix Keyboard with I2C-USB Bridge Photo
February 16, 2007 Document No. 001-41442 Rev. ** 7
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8. AN2407
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.
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of Cypress Semiconductor Corp. All other trademarks or registered trademarks referenced herein are the property of their respective owners.
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February 16, 2007 Document No. 001-41442 Rev. ** 8
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