Transcript
Application Report SLAA514 – December 2011
USB Keyboard Using MSP430™ Microcontrollers David Racine, Luis Reynoso ............................................................................................ MSP430 Apps ABSTRACT This application report describes a low-cost highly-flexible composite USB keyboard implementation based on MSP430F5xx/MSP430F6xx families. Schematics and software are included allowing for an easy implementation and customization. The document explains basic necessary concepts but familiarity with the MSP430™ USB Developers Package (MSP430USBDEVPACK) and USB HID specification is assumed. Source code and additional information described in this application report can be downloaded from http://software-dl.ti.com/msp430/msp430_public_sw/mcu/msp430/USBKBD_430/latest/index_FDS.html.
1 2 3 4 5 6 7
Contents Introduction .................................................................................................................. 2 Implementation .............................................................................................................. 2 Software ...................................................................................................................... 6 Hardware and Peripheral Usage ........................................................................................ 12 Using the USB Keyboard ................................................................................................. 12 Schematics ................................................................................................................. 15 References ................................................................................................................. 16 List of Figures
1
Key Matrix .................................................................................................................... 3
2
Keyboard Schematic........................................................................................................ 3
3
Detection of a Key Using Column-Interrupt Method .................................................................... 4
4
Detection of a Key Using Polling Method ................................................................................ 4
5
"Ghost" Key Detection ...................................................................................................... 5
6
USB Keyboard Software Modules ........................................................................................ 6
7
USB Keyboard Flow Diagram ............................................................................................. 7
8
Digital Keyscan Flow Diagram ............................................................................................ 9
9
USB Keyboard in Windows Device Manager .......................................................................... 13
10
Testing the HID Custom Interface ....................................................................................... 14
11
Schematics ................................................................................................................. 15 List of Tables
1
VID/PID Used by the Device .............................................................................................. 5
2
HID Keyboard Report Format ............................................................................................. 8
3
Communication Protocol Report Descriptor ............................................................................. 8
4
Implemented Protocol ...................................................................................................... 8
5
Configuration Constant Table ............................................................................................ 11
6
ScanCodes ................................................................................................................. 11
7
MSP430F550x/5510 Peripheral Usage ................................................................................. 12
8
MSP430F550x/5510 Pinout Usage ..................................................................................... 12
MSP430, Code Composer Studio are trademarks of Texas Instruments. IAR Embedded Workbench is a trademark of IAR Systems. SLAA514 – December 2011 Submit Documentation Feedback
USB Keyboard Using MSP430™ Microcontrollers Copyright © 2011, Texas Instruments Incorporated
1
Introduction
1
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Introduction This application report describes the implementation of a USB keyboard with the following characteristics: • 101 keys, 2 LEDs: standard HID keyboard and LED usage • 16x8 matrix: allows for easy customization of different keyboard layouts • Composite USB device: In addition to the keyboard interface, it includes an HID-datapipe back-channel which can be used to transmit any custom data • HID boot protocol support, allowing keyboard to be used to interface with a PC's BIOS • "Ghost" key handling in software, to prevent errors from multiple key presses • Uses MSP430F550x/5510 low-cost USB family The Texas Instruments MSP430F550x/5510 devices are ultra-low power microcontrollers featuring a powerful 16-bit RISC CPU, 16-bit registers, and constant generators that contribute to maximum code efficiency. In addition, this MSP430 family includes an integrated USB and PHY supporting USB 2.0 fullspeed communication, four 16-bit timers, a high-performance 10-bit analog-to-digital converter (ADC), two universal serial communication interfaces (USCI), hardware multiplier, DMA, real-time clock module with alarm capabilities, and 31 or 47 I/O pins.
2
Implementation
2.1
Key Matrix The USB keyboard presented in this application report implements a key matrix of rows and columns similar to smaller keypads like the one shown in the application report Implementing An Ultralow-Power Keypad Interface with MSP430 (SLAA139). This implementation uses a 16 rows x 8 columns matrix, which allows up to 128 keys, but it actually uses only 101 keys in total. The key matrix is shown in Figure 1.
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Row0 P2.0
Col0 P1.0
Col1 P1.1
Col2 P1.2
Col3 P1.3
Col4 P1.4
Col5 P1.5
Col6 P1.6
Col7 P1.7
Keypad Enter
Keypad -
Keypad +
Keypad 2
Keypad 3
Keypad .
Keypad 1
Keypad /
Keypad 9
Win
Keypad 7
Home
PageUp
Keypad NumLock
Tab
~ `
1
Q
A
Row1 P2.1 Row2 P2.2
Keypad 0
Row3 P2.3
Right Alt
Left Alt
Row4 P2.4
C
Space bar
F3
F4
CapsLock
3
E
D
Row5 P2.5
X
Z
F2
F1
Esc
2
W
S
Row6 P2.6
V
B
G
T
5
4
R
F
Row7 P2.7
M
N
H
Y
6
7
U
J
Row8 P3.0
> .
ê
| \
F11
F10
9
O
L
Row9 P3.1
Right Shift
Left Shift
Row10 P3.2
< ,
Keypad *
F7
F6
F5
8
I
K
Keypad 8
F9
Row11 P3.3
ç
Row12 P3.4
Right Ctrl
Left Ctrl
Row13 P5.0
? /
é
_ -
F12
0
P
{ [
: ;
Row14 P5.1
“ ‘
Enter
PrtScr
End
+ =
Back space
} ]
Page Down
è
F8
Pause
Scroll Lock
Keypad 4
Keypad 5
Keypad 6
Row15 P2.0
Figure 1. Key Matrix
...
Coln
Col1
Col0
Each key works like a switch, and pulldowns are implemented on each column, keeping the idle state low (see Figure 2).
Row0 Row1 ... Rown-1 Rown
Figure 2. Keyboard Schematic There are multiple ways to scan a key matrix, but this application report uses two methods, referred in this application report as: column-interrupt and polling.
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Implementation
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In the column-interrupt approach, all rows are actively driven at the same time and columns are configured to interrupt the processor when any single key is pressed. This method is useful in low-power modes, because any key can wake up the microcontroller; however, it is important to remark that the key press is only used for that purpose, because it does not provide the exact key being pressed. Figure 3 shows the key matrix behavior when the Enter key is pressed in column-interrupt mode. Actively driven rows and columns are shown in red. Notice that the Col1 pin would detect a change when the Enter key is pressed, but the effect would be the same for any other pin pressed in the same column.
Row0 P2.0
Col0 P1.0
Col1 P1.1
Col2 P1.2
Keypad Enter
Keypad -
Keypad +
Row1 P2.1 . . . Row14 P5.1
...
Keypad 9
“ ‘
Row15 P2.0
Enter
PrtScr
è
F8
Figure 3. Detection of a Key Using Column-Interrupt Method After the system is awake due to a key press using the column-interrupt approach, the polling method can be used to determine which key(s) is(are) being pressed (see Figure 4). In the polling method, each row is scanned separately driving one row at a time in sequential order. The columns are then read giving the exact keys being pressed.
Row0 P2.0
Col0 P1.0
Col1 P1.1
Col2 P1.2
Keypad Enter
Keypad -
Keypad +
Row1 P2.1 . . . Row14 P5.1 Row15 P2.0
...
Keypad 9
“ ‘
Enter
PrtScr
è
F8
Figure 4. Detection of a Key Using Polling Method One of the caveats when using this method is that particular patterns can cause unwanted connections, known as "ghost" keys. This behavior is caused when three or more keys sharing rows and columns are pressed at the same time (see Figure 5).
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Row0 P2.0
Col0 P1.0
Col1 P1.1
Col2 P1.2
Keypad Enter
Keypad -
Keypad +
Row0 P2.0
Keypad 9
Row1 P2.1
Row1 P2.1 . . . Row14 P5.1
“ ‘
Row15 P2.0
Enter
PrtScr
è
F8
...
. . . Row14 P5.1 Row15 P2.0
1. Enter, PtrScr and è keys are pressed
Col0 P1.0
Col1 P1.1
Col2 P1.2
Keypad Enter
Keypad -
Keypad +
Row0 P2.0
Keypad 9
Row1 P2.1
“ ‘
Enter
PrtScr
è
F8
...
. . . Row14 P5.1 Row15 P2.0
Col0 P1.0
Col1 P1.1
Col2 P1.2
Keypad Enter
Keypad -
Keypad +
...
Keypad 9
“ ‘
Enter
PrtScr
è
F8
3. Driving Row15 detects è but it incorrectly detects F8
2. Driving Row14 detects Enter and PtrScr
Figure 5. "Ghost" Key Detection The software included in this application report detects potential "ghost" keys and does not report them to the host.
2.2
USB HID This application report uses the MSP430 application programming interface (API) stack found in the MSP430 USB Developers Package (MSP430USBDEVPACK). The stack is configured to work as a composite HID-HID interface with the first interface being a standard Keyboard and the second interface used as a DataPipe. One of the advantages of using this implementation, which using only HID interfaces, is that no drivers are required. Although the relevant code for the keyboard implementation uses the standard keyboard interface, the DataPipe interface was added to provide users with more flexibility and to facilitate customization. This interface can be used to send or receive any type of data to/from the host, so that the MSP430 microcontroller not only performs the job of a digital keyboard, but it can also be used to perform other jobs taking advantage of the same USB interface and the rest of the peripherals. Some examples include reading sensors using ADC and reporting to PC, controlling actuators using timer PWMs, etc. It should be noted that while the host OS interprets and uses the data from the standard keyboard interface without additional applications or drivers, in the case of the Datapipe interface, a host application is required. Texas Instruments provides an HID API which enables communication between a PC and a MSP430 microcontroller running the HID API stack. This HID API is available in executable format and source code in the MSP430 USB Developers Package (MSP430USBDEVPACK). The keyboard interface supports Boot protocol, which allows it to work with HID-limited hosts (such as some BIOS). VID and PID can be modified according to the particular application but the default code used for this example uses the values shown in Table 1. Table 1. VID/PID Used by the Device
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VID
0x2047
PID
0x0401
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Software
3
Software
3.1
Tools
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The software included in this application report was built and tested using: • IAR Embedded Workbench™ for MSP430 5.30.4 IDE • Code Composer Studio™ (CCS) 5.1.0 IDE
3.2
Software Implementation Figure 6 shows the software layers for the USB keyboard. Application USB Keyboard Comm Protocol
Keyboard Report
USB HID
DKS (Digital Keyscan)
USB API MSP430 Driverlib
UCS
PMM
ticktimer
Timer
GPIOs
DMA
USB
Hardware
Figure 6. USB Keyboard Software Modules Software is designed in a modular way, re-using existing TI libraries such as driverlib and the USB API and adding new modules from low-level drivers to application level. These modules include: • USB Keyboard Description Main application initializing the microcontroller, peripherals, and executing a loop checking and servicing the rest of the modules. Files Src\TI_USBKBD_main.c Flow Diagram
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Initialize: PMM, UCS (clocks), GPIOs, Timers, USB, Timer,
USB Keyboard
Initialization
USB Active?
Y
USB Suspended?
N Disable DKS
Y Sleep
Y
Process RX data from HID0/HID1
N
Data received? N
Initialize DKS and KBD_Report modules
Y
First loop?
N
USB or Keyboard activity?
N
Y Wake MCU
Attend DKS module
Attend KBD_Report module
Y
Force Remote Wakeup
Pending tasks? N Sleep
N
USB, Timer or Keyboard activity?
Y
Figure 7. USB Keyboard Flow Diagram
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Software
•
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Keyboard Report Description Handles the HID Keyboard report, adding and removing keys from the report depending on press/release events and sends the report to the USB Host. Files Src\TI_USBKBD_HIDKBD_report.c Src\Include\ TI_USBKBD_HIDKBD_report_public.h HID Keyboard Report Format Table 2. HID Keyboard Report Format Byte0
•
Bit7
Bit6
Bit5
Bit4
Bit3
Bit2
Bit1
Bit0
Right GUI
Right Alt
Right Shift
Right Ctrl
Left GUI
Left Alt
Left Shift
Left Ctrl
Byte1
Reserved
Byte2
Key_array[0]
Byte3
Key_array[1]
Byte4
Key_array[2]
Byte5
Key_array[3]
Byte6
Key_array[4]
Byte7
Key_array[5]
Communication Protocol Description Handles the HID custom interface, which is used to transfer data to/from an USB host. The current implementation shows a template that can be used for custom development. This module uses the HID-Datapipe as defined in the USB API included in MSP430 USB Developers Package (MSP430USBDEVPACK). Files Src\TI_USBKBD_comm_protocol.c Src\Include\ TI_USBKBD_comm_protocol_public.h HID Custom Interface Report Descriptor Table 3. Communication Protocol Report Descriptor Field
Size
Description
1 byte
The report ID of the chosen report (automatically assigned to 0x3F by the HID-Datapipe calls)
Size
1 byte
The number of valid bytes in the data field
Data
62 bytes
IN Report Report ID
Data payload OUT Report
1 byte
The report ID of the chosen report (must be assigned to 0x3F by the host)
Size
1 byte
The number of valid bytes in the data field
Data
62 bytes
Report ID
Data payload
Data Payload Protocol Table 4. Implemented Protocol
8
Field
Size
CMD
1 byte
Data
61 bytes
Description 1 = Toggle CAPS LED 2 = Toggle NUM LED Unused
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•
DKS (Digital KeyScan) Description Handles the digital keyboard scanning, detecting key press/release events, and reporting them to the keyboard report module. Files Src\TI_USBKBD_DKS.c Src\Include\TI_USBKBD_DKS_public.h Flow Diagram DKS is initialized in “interrupt-column” mode by default if no key is pressed.
Digital KeyScan
Interrupt-column
polling Mode?
N
Key press detected?
Scan key matrix
Y Change to Polling mode
Release
None
Inactive timer N expired?
Key event?
Y
Press
Remove key from report
Change to Interrupt-Column mode
Y “Ghost” key? N
N
Key debounced?
Increment debounce counter
Y
Add key to report
Return
Figure 8. Digital Keyscan Flow Diagram
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Software
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•
USB API / USB HID Description The MSP430 USB API stack is a software solution provided by Texas Instruments that includes support for: – Communications Device Class (CDC) – Human Interface Device class (HID) – Mass Storage Class (MSC) – Personal HealthCare Device Class (PHDC) This software solution, including detailed documentation, is available in the MSP430 USB Developers Package (MSP430USBDEVPACK). Files Src\USB_API\*.* Src\USB_config\*.* Src\USB_App\*.* Ticktimer Description Handles a general purpose interrupt timer that is used as a timebase, to wake-up the processor, and to trigger a new keyboard scan, among other functions. The ticktimer is implemented using TA0.0 with a default time base of 2 ms. Files Src\TI_USBKBD_ticktimer.c Src\Include\TI_USBKBD_ticktimer_public.h MSP430 Driverlib Description The Texas Instruments MSP430 Peripheral Driver Library (Driverlib) is a set of drivers that provide an easy mechanism to use the MSP430 peripherals. This software uses Driverlib to initialize the PMM and UCS modules. Source code and detailed documentation are available in MSP430Ware (www.ti.com/msp430ware). For simplicity purposes, this project includes only the pre-compiled libraries for IAR and CCS using a small memory model and header files. Files Src\ driverlib\*.h Src\ driverlib\driverlib_small_CCS.lib Src\ driverlib\driverlib_small_IAR.r43
•
•
3.3
Configuration and ScanCode Tables For modularity purposes and to allow for an easier optimization or upgrade, the USB keyboard software reserves some Flash sectors for constant tables that define some of the functionality of the application and define the ScanCode table. • Configuration Constant Table Description Contains the USB keyboard version and configuration constants defining the KeyScan functionality, such as debounce counter, ticktimer period, etc. Files Src\TI_USBKBD_SharedTables.c (declaration) Src\Include\TI_USBKBD_public.h (typedef) Declaration const USBKBD_config_const_t USBKBD_configconst_s
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Location USBKBD_CONFIGCONST_SEGMENT (0xFC00-0xFDFF) Contents Table 5. Configuration Constant Table Field
•
Size
Description
MagicKey
4 bytes
Indicates the start of the table. 0xDEADC0DE is used by default.
Version
2 bytes
USB keyboard version in BCD format: 0x0101 - 1.0.1
ticktimer_div
2 bytes
TickTimer divider (based on ACLK): 66 represents a period of 66 / 32768 = ~2 ms
debounce_cycles
2 bytes
Number of debounce cycles in tick counts: 2 represents a debounce of 4 ms with Ticktimer = 2 ms
inactive_timeout
2 bytes
Number of tick counts before going to interrupt_column mode if no key is detected: 8 represents 16 ms with Ticktimer = 2 ms
ScanCode Table Description Contains the USB Keyboard scancode table, mapping each row and column to the corresponding value based on HID Usage Tables. Files Src\TI_USBKBD_SharedTables.c(declaration) Src\Include\TI_USBKBD_public.h(typedef) Declaration const USBKBD_scancodest_t USBKBD_scancodes_s
Location USBKBD_SCANCODES_SEGMENT (0xFA00-0xFBFF) Contents Table 6. ScanCodes Field
Size
MagicKey
4 bytes
keycode
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Description Indicates the start of the table. 0xDEADC0DE is used by default.
128 bytes
Keycodes for each key in the following order: Row0,Col0 Row0,Col1 … Row0,Col7 Row1,Col0 Row1,Col1 … Row15,Col6 Row15,Col7
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Hardware and Peripheral Usage
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Hardware and Peripheral Usage In addition to system modules (UCS, PMM), this keyboard implementation uses the peripherals shown in Table 7. Table 7. MSP430F550x/5510 Peripheral Usage Peripheral
Usage
USB
Communication with host (Composite HID-HID)
Timer_A0 (TA0.0)
TimerTick used as a time base to perform periodic polling, debounce, etc.
In addition to the circuitry required for USB and common functionality (reset, VCC, VSS, crystal, etc.), the USB keyboard uses the pins shown in Table 8. Table 8. MSP430F550x/5510 Pinout Usage
Columns
KSO0 KSO1 KSO2 KSO3 KSO4 KSO5 KSO6 KSO7 KSO8 KSO9 KSO10 KSO11 KSO12 KSO13 KSO14 KSO15
P2.0 P2.1 P2.2 P2.3 P2.4 P2.5 P2.6 P2.7 P3.0 P3.1 P3.2 P3.3 P3.4 P5.0 P5.1 P5.4
Rows
LEDs
KSI0 KSI1 KSI2 KSI3 KSI4 KSI5 KSI6 KSI7 LED0 (CAPS) LED1 (NUM)
P1.0 P1.1 P1.2 P1.3 P1.4 P1.5 P1.6 P1.7 P4.7 P4.6
Schematics showing the implementation on the USB keyboard are found in Section 6.
5
Using the USB Keyboard When connected to a PC, the USB keyboard should be detected by the operating system and enumerated without drivers. Windows shows three devices in the Device Manager (see Figure 9). • Human Interface Devices – USB Human Interface Device: Standard keyboard in intf0 (MI_00) – USB Human Interface Device: Custom interface in intf1 (MI_01) • Keyboards – HID Keyboard Device: Standard keyboard in intf0 (MI_00)
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Using the USB Keyboard
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Figure 9. USB Keyboard in Windows Device Manager The keyboard can now be tested and used as a standard keyboard. In addition to the regular key functionality, the custom interface can be tested using the MSP430 HID USB Application following these steps (see Figure 10): 1. Select the VID and PID (default: VID = 0x2047, PID = 0x0401). 2. Click Set VID PID. 3. Click Connect. 4. The LED should turn green. 5. Write one of the commands in the Send & Receive field. 6. Observe the response from the USB keyboard.
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Using the USB Keyboard
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3
2
4
1 5
6
Figure 10. Testing the HID Custom Interface The MSP430 HID USB Application is available in the MSP430 USB Developers Package (MSP430USBDEVPACK).
5.1
Performance The usual response time for keyboards is approximately 5 to 50 ms. While this depends on different factors such as the mechanical implementation of the keyboard, USB bus load, etc., by using this software, developers have more flexibility to customize the application according to their needs. Whether response time, price, or power consumption is the most important requirement, parameters such as debounce time, polling scan rate, USB polling interval, and microcontroller internal frequency can be adjusted to meet particular requirements. One important factor affecting the response time is the polling rate, which defines the time required to scan all keys. While a key press is detected in a few cycles in column-interrupt mode, the algorithm to recognize the particular pressed key, debounce it, discard "ghost" keys, etc. can take more cycles. During bench tests, this implementation was measured to take ~1870 cycles (which is equivalent to ~233 µs at 8 MHz) for the first pressed key and ~520 cycles (~65 µs at 8 MHz) for each additional pressed key.
5.2
Memory Footprint The following memory footprint was obtained using IAR Embedded Workbench 5.30.4 using the maximum optimization level: Code: 7626 Bytes Constants: 1096 Bytes Data: 679 Bytes
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Schematics
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6
Schematics
R8
R11 1M 10p C35 10p C36
KSI5
LED1
LED0
R14
KSI6
4.7M R15
NC0
R16
IC7
KSI7
4.7M
4.7M
GND
IO1 VCC
6
2
IO2
IO4
5
3
GND IO3
4
TPD4E004
GND GND
4.7u C39
VBUS
D-
USB1
D- 2
1
D+ 3
GND
D+
4
SHIELD
SBW/Power
SBW_VCC
JP3
14 12 10 8 6 4 2
JTAG1
JP1
int
2
LL103A D3
1 2 3
ext
1 2
DVCC1
13 11 9 7 5 3 1
SBWTCK
SBWTDIO
VBUS
48 47 46 45 44 43 42 41 40 39 38 37 36 35 34 33
GND IN OUT 1
GND NR/FB 3
EN
C20
Q1G$1
3
2
Q1G$2 4
C3
47pF
4MHz
XT2
1
C4 47pF
GND
220n
VBUS_LDO
4.7uF
XT2OUT
XT2IN
C21
10nF
GND
220n C38
8
4
C11 100nF
GND
V18 VUSB VBUS PU.1/DM PUR PU.0/DP VSSU C33
5
LED0 LED1
DVCC1 KSO12 KSO11 KSO10 KSO9 KSO8 KSO7
Digital Keyboard
1uF
GND
R42
JP6 C19
330R
SBWTCK XT2OUT XT2IN AVSS V18 VUSB VBUS PU.1/DM PUR PU.0/DP GND
1 VUSB_VCC
VCC1 SBW_VCC
470nF
VBUS_LDO3
SBWTDIO
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
GND
VCC1
R5 47K
GND
GND
SHIELD1
C8
KSO13 KSO14
KSO15 GP
C9
GND
KSO0 KSO1 KSO2 KSO3 KSO4 KSO5 KSO6
KSI0 KSI1 KSI2 KSI3 KSI4 KSI5 KSI6 KSI7
TP 64 63 62 61 60 59 58 57 56 55 54 53 52 51 50 49
17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
5
6
C13 100nF
33k
DNP
LL103A D2
R36
GND
+ 10uF/6,3V
VBUS
0.1u C40
GND
C5 C7
100nF
AVCC GND
1
GND
AVCC
DVCC1
C10
GND
10uF/6,3V +
R35 27R
R13 4.7M
2
R34 27R
KSI4
4.7M
GND
R19 330R
R18 330R
KSI3
4.7M
R10
1
GND GND GND
KSI2
R7
100R
S3
USB Interface
R6 4.7M
R33 1.4k PUR PU.0/DP PU.1/DM
KSI1
4.7M
VUSB
R3
GND
Digital Keys
KSI0
LED1
GP KSO7 KSO0 KSI1 KSI7 KSO9 KSI6 KSI5 KSO3 KSI4 KSI2 KSO1 KSI3 KSI0 KSO13 KSO5 KSO2 KSO4 KSO8 KSO6 KSO11 KSO10 KSO12 KSO14 KSO15
Digital Connector
LEDs
GND LED0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
P$1 P$2 P$3 P$4 P$5 P$6 P$7 P$8 P$9 P$10 P$11 P$12 P$13 P$14 P$15 P$16 P$17 P$18 P$19 P$20 P$21 P$22 P$23 P$24 P$25 P$26 P$27 P$28 P$29 P$30
A
GND
VUSB_VCC
GND
D4 LL103A
Figure 11. Schematics
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References
7
References 1. 2. 3. 4. 5. 6.
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USB HID specification (www.usb.org/developers/devclass_docs/HID1_11.pdf) MSP430x5xx/MSP430x6xx Family User's Guide (SLAU208) MSP430F550x/MSP430F5510 Mixed Signal Microcontroller data sheet (SLAS645) MSP430 USB Developers Package (MSP430USBDEVPACK) (www.ti.com/tool/msp430usbdevpack). MSP430Ware (www.ti.com/msp430ware) Implementing an Ultralow-Power Keypad Interface With the MSP430 (SLAA139)
USB Keyboard Using MSP430™ Microcontrollers Copyright © 2011, Texas Instruments Incorporated
SLAA514 – December 2011 Submit Documentation Feedback
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