ECE 477
Digital Systems Senior Design Project
Spring 2009
Homework 9: Software Design Considerations
Due: Friday, March 27, at NOON
Team Code Name: _The Magic Wand____________________________ Group No. __5___
Team Member Completing This Homework: _Nina Vintila___________________________
E-mail Address of Team Member: _____mvintila _________ @ purdue.edu
NOTE: This is the last in a series of four “design component” homework assignments, each of
which is to be completed by one team member. The body of the report should be 3-5 pages,
not including this cover sheet, references, attachments or appendices.
Evaluation:
SCORE
DESCRIPTION
Excellent – among the best papers submitted for this assignment. Very few
corrections needed for version submitted in Final Report.
Very good – all requirements aptly met. Minor additions/corrections needed for
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version submitted in Final Report.
Good – all requirements considered and addressed. Several noteworthy
8
additions/corrections needed for version submitted in Final Report.
Average – all requirements basically met, but some revisions in content should
7
be made for the version submitted in the Final Report.
Marginal – all requirements met at a nominal level. Significant revisions in
6
content should be made for the version submitted in the Final Report.
Below the passing threshold – major revisions required to meet report
*
requirements at a nominal level. Revise and resubmit.
* Resubmissions are due within one week of the date of return, and will be awarded a score of
“6” provided all report requirements have been met at a nominal level.
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Comments:
ECE 477
Digital Systems Senior Design Project
Spring 2009
1.0 Introduction
The Magic Wand is a hand writing recognition device that uses changes in acceleration to
determine character writing patterns and displays interpreted characters to a display. The
product consists of two separate parts: the Magic Wand, a pen shaped device, and a base station
that performs the motion analysis to determine characters. The portable device will be able to
collect motion and battery related information and wirelessly send it to the base station. It will
sample accelerometer data around 120 Hz and transmit the data along with battery status to the
base station. The base station will buffer incoming acceleration data and wait for an ‘end of
character’ signal to begin analyzing the buffered data and look for patterns that correspond to
specific characters.
2.0 Software Design Considerations
The Magic Wand product requires two different software design styles due to the fact that
the Wand itself differs in functionality from the base station. While the portable device merely
collects data from accelerometers, the base station processes the data and provides feedback tot
the user.
The Wand is primarily a data acquisition device and transmitter. The highest priority
process on the pen is the acquisition and conversion of the analog data from the accelerometer.
This process is done with a frequency of 120 Hz. The pen also uses an external interrupt from
the LTC4150 fuel gauge to monitor battery use. The aggregate information (accelerometer and
battery information) is formed into a structured packet and transmitted to the base station.
The base station’s main purpose is to buffer the data received from the pen, provide nonvolatile storage for the battery information, and analyze the buffered data from the pen to look
for character patterns. The MRF24J40MA transceiver [1] triggers an interrupt when data is
received, communicating the data to the dsPIC33 via SPI. The dsPIC33 parses the transmitted
data and stores the accelerometer data to RAM and stores the battery information from the pen to
non-volatile storage. A periodic interrupt triggers an LCD update routine that looks at the
current battery counts and analyzed characters in the display buffer and displays them on the
LCD.
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Digital Systems Senior Design Project
Spring 2009
2.1 Memory Mapping
The PIC18F2320 [2] microcontroller was chosen for use on the portable device due to its
convenient compatibility with the transceiver module. The memory map for the PIC18F2320 is
displayed in Figure 2.1.1. Program memory will begin at 18h and continue until 1FFFh. This
allows for 8KB of memory to hold the main program along with variables. The MiWi protocol
requires 3336 Bytes of program memory and 150 Bytes of RAM. Thus, special care must be
given during the development stage that the program memory is used efficiently due to its
limited amount of space. The return address stack allows for up to 31 program calls and is stored
before the reset vector at location 0000h. From location 2000h on the memory is unimplemented
in the PIC18F2320.
Figure 2.1.1 PIC18 Memory Map [2]
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Digital Systems Senior Design Project
Spring 2009
The dsPIC33FJ128GP204 microcontroller [3] was chosen for the base station due to its
compatibility with the WiMi communication protocol, while providing more memory to work
with for processing the motion information. The dsPIC33FJ128GP204 has 128KB of user
memory with 85.5KB of available program memory. This will provide ample space for the main
program, along with space to store the buffered character data. A total of 5KB of flash memory
will be reserved to store the data received from the pen. This reserved space provides plenty of
space for a single character. If the buffer begins to be full an error message will be displayed on
the LCD screen and the buffer will be reset.
Figure 2.1.2 dsPIC33 Memory Map
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Digital Systems Senior Design Project
Spring 2009
2.2 Mapping of external interfaces
The pen microcontroller has three ports, named PORTA, PORTB and PORTC. PORTA
is used for input of analog accelerometer signals. PORTB contains the external interrupt pins
that will be connected to the fuel gauge and transceiver respectively, as well as the programming
pins and the connection to the pushbutton at the tip of the pen. PORTC interfaces the SPI
communication between the transceiver and the microcontroller. A detailed table of these
mappings is included in table 1 in Appendix C of this document.
The base station microcontroller also has three ports named in an identical fashion.
PORTA is only connected to the polarity signal of the fuel gauge, which indicates whether the
battery is charging or discharging. PORTB is connected to all of the signals to and from the
transceiver, as well as the programming pins. Lastly, all of the pins of PORTC will be used as
digital outputs to LCD screen. Table 2 in Appendix C contains a detailed listing of the pin usage.
2.3 Utilization of integrated peripherals
2.3.1 PIC18F2320
The Analog to Digital convertor will be used to take input data from the accelerometers
and store it into a buffer for transmission to the base station. This module uses 5 registers:
ADRESH, ADRESL, ADCON0, ADCON1 and ADCON2. The first two registers hold the result
of the conversion, the high and low byte respectively. The program will trigger a 10 bit, left
justified conversion, and thus only the high byte will be used as a result, discarding the two least
significant bits stored in the low byte. ADCON0 is used to select the channels for the conversion
(0, 1 and 2 in our case), to enable the ATD module and to indicate status of the conversion.
ADCON1 sets the types of inputs that the ATD pins serve as (analog or digital) to allow the
unused channels to function as general purpose input/output. ADCON2 sets right/left justified
option, the acquisition time and conversion clock.
The timer module will be used in interrupt mode to sample the pushbutton. The registers
pertinent to its functionality are the INTCON and T0CON. The 5th bit of INTCON enable TIM
module interrupts, while T0CON sets the timer period after which and interrupt will be
generated.
The final integrated peripheral used by the pen microcontroller is the SPI module in
master mode, which will be used for communication to the WiMi transceiver. It will be set up
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Digital Systems Senior Design Project
Spring 2009
using the SSPCON1 control register which contains clock polarity, master mode and clock
frequency information, and SSPSTAT status register which includes a buffer full flag and a
clock edge selection. The serial receive/transmit buffer is stored in register SSPBUFF, and the
shift register is called SSPSR.
2.3.2 dsPIC33FJ128GP204
The dsPIC33 will be using two integrated peripherals the SPI and timer. The SPI module
consists of a 16-bit shift register (SPI1SR), a buffer register (SPI1BUF), a control register
(SPI1CON), along with a status register (SPI1STAT). The SPI on the dsPIC will operate in
master mode.
The timer on the dsPIC33 will be used in the updating and refreshing of the LCD. The
T2CON register is used to set up the timer and control its functionality. The dsPIC33 provides
the capability to mush 2 timers together and create a 32 bit counter, but for our purposes we will
not need to do that. Address x00001A will be used in the interrupt vector table to specify how
the interrupt will be handled.
2.4 Overall Organization of Application code
Both the pen and base station will implement a ‘hybrid’ style of operation. This works
best for our operation as the pen and base station use external interrupts along with timers and
some program driven controls. A flowchart of the main program for the pen and base station and
included in appendix A, figures A.1 and A.2 respectively.
The pen will use a continuous loop to check and see if the timers or external interrupts
have set any flags. The timer’s expiration (every 8 ms) will trigger an interrupt that will check
the status of the pressure sensor pen tip (a simple push button will accomplish this function) and
set a corresponding flag. The status of the pushbutton determines wither or not a 3 channel ATD
conversion will be run on the next iteration through the main loop and the information will be
placed in a transmission buffer. The external interrupt from the coulomb counter [4] will trigger
the current count to be incremented in the designated memory location. When the buffer is
filled, the accelerometer information is sent to the base station along with the battery status. If a
sufficiently long period of time has passed without any characters being drawn, a packet will be
transmitted containing only and update on battery discharge status.
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Digital Systems Senior Design Project
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The base station will also use a continuous loop to check flags and determine if timers or
external interrupts have transpired. An external interrupt will be triggered from the transceiver
to indicate that data is ready to be read. The coulomb counter will also use an external interrupt
to indicate to the microcontroller that 0.307 Coulombs (0.085 mAh) of charge has been drained
or, in the case of the base with a rechargeable battery, charged to the battery. A timer will be
used to trigger an LCD update. When the main loop sees the LCD update flag set it will call a
routine to display the current battery life of each devices, display if the two devices are
connected, and display the current buffer of characters that have been recognized.
The main debugging tool that we will be using throughout the development and testing
stages will be the use of the LCD that will already be connected and set up on the board as a way
to observe the changes in internal variables during operation of the device.
3.0 Software Design Narrative
3.1 Pen code hierarchy
This section includes a detailed narrative of each module for the portable device software.
The general hierarchical arrangement of the program is included in figure B.1 in appendix B.
Except for the ATD conversion routine (which has been tested on the development board) and
the MiWi protocol routines (provided by Microchip), the other modules are in development, with
the pseudo-code finalized.
The main module (main.c) contains the continuous polling loop illustrated in the functional
flowchart (figure A.1). This loop continuously checks if the push button or data ready flags have
been set since the previous iteration. If one of them is set, it clears it before calling the
appropriate functions to service that instance of a signaled flag.
Before starting the infinite loop, however, the main function calls a series of initialization
functions. IOinit( ) sets the data direction registers for ports A, B and C according to the intended
use, as indicated in table 1 of appendix C. Var_init( ) initializes any variables as needed for the
operation of the program. Periph_init( ) initializes the control registers for the SPI and ATD
module, while timer_init( ) fixes the value of the timer period and enables timer interrupts.
If the push button flag is set, it means that at the last probing of the push button value tip it
was found to be depressed, and thus the pb_handle( ) function is called. If the push button flag is
cleared, but the previous push button state was pressed, then an end-of-character condition is
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Digital Systems Senior Design Project
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generated and added to the transmission buffer. If the data ready flag is set, then it is first cleared
before the ATD conversion results are added to the transmission buffer. If the buffer is full, the
transmit_axes( ) function is called. Lastly, if the data ready flag is not set, but the coulomb cont
value is higher than a fixed threshold, then the transmit_battery( ) function is called.
The pb_handle( ) function first clears the push button flag, before initializing a three channel
ATD conversion on channels 0, 1, and 2, and stores each of these results in memory. Finally, it
sets a data ready flag to be used by the main loop.
The transmit_axes( ) function forms and sends a MiWi transmission packet using the
modules provided by Microchip [5] to the base station. In this packet will include all the data in
the ATD buffer and additionally the current value of the coulomb counter variable. This variable
will then be cleared to avoid overflow, as the base station will be keeping track of the overall
state of the pen battery.
The transmit_battery( ) function simply forms a packet with only the coulomb counter value
and transmits it to the base station, clearing the variable. This function is anticipated for the
situation when the pen is on for a longer period of time without any characters being written, and
this can cause and overflow in the counter variable.
The timer interrupt subroutine will be used to sample to push button value and set the flag if
the button is found to be pressed. The int0( ) will be generated as part of the handshaking
protocol with the transceiver and will be handled by the corresponding modules, while int1( )
will simply increment the local coulomb counter value.
3.2 Base code hierarchy
This section includes a detailed narrative of each module for the base station software. The
general hierarchical arrangement of the program is included in figure B.2 in appendix B. While
the routines for the MiWi protocol have been provided by Microchip, the rest of the code is
currently under development from pseudo-code phase into c modules.
The main module (main.c) contains the continuous polling loop illustrated in the functional
flowchart (figure A.2). This loop continuously checks if the receiver or end-of-character flag has
been set since the previous iteration. If one of them is set, it clears it before calling the
appropriate functions to service that instance of a signaled flag.
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Digital Systems Senior Design Project
Spring 2009
Before starting the infinite loop, however, the main function calls a series of initialization
functions. IOinit( ) sets the data direction registers for ports A, B and C according to the intended
use, as indicated in table 1 of appendix C. Var_init( ) initializes any variables as needed for the
operation of the program. Periph_init( ) initializes the control registers for the SPI module, while
timer_init( ) fixes the value of the timer period and enables timer interrupts.
If the receiver flag is set, then the pen is attempting to send information to the base, and the
Receive_MiWi( ) function is called after the flag is cleared. If the end-of-character flag is set,
then it is cleared and then the char_analysis( ) function is called.
The Receive_MiWi( ) function will use the provided MiWi modules to read the data from
the transceiver through the SPI lines, and parse this data. The local pen battery coulomb counter
will be incremented with the value of the corresponding variable included in the transmission
stack by the pen, and the end-of-character flag will be set if this condition is indicated in the
transmission. Finally, if any ATD data is included in the stack, then it is stored into a circular
buffer. If the buffer is full, and no end of character condition has been transmitted, then an error
message is stored into the LCD buffer, the ATD data buffer is cleared and an error flag is raised.
This flag will only be cleared when an end of character condition is received from the pen. This
provision will prevent the data buffer to be overfilled by random “doodling” by the user, which
provides much more accelerometer data that any character can generate.
The char_analysis( ) function will use the data stored in the buffer to attempt a character
match. This algorithm has not yet been finalized, and will be developed in further detail after the
data from the PCB mounted accelerometer can be observed on an oscilloscope to get a better
understanding of the nature of the signals received. If a match has been found, then this character
is stored in the LCD buffer. On the contrary, if the data yields no clear results for a match, a ‘?’
will be displayed to indicate no match was found.
The timer interrupt subroutine will call for an update on the LCD display. The first line out
of the four available will contain battery life information. The values for the coulomb counters
for the two batteries will be proportional to the number of “bars” on each of the batteries. The
last three lines will be reserved for characters and/or error messages. If the buffer is filled almost
completely, the first line will be deleted and the following two will be scrolled up to allow for an
easy-to-follow display.
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Digital Systems Senior Design Project
Spring 2009
The int0( ) subroutine will be associated to the transceiver connection, and will thus set the
receiver flag, while the int1( ) subroutine will simply increment or decrement the local coulomb
counter flag depending on whether the battery is charging or discharging as indicated by the
polarity signal connected to port A.
4.0 Summary
In this report, a detailed overview of software design considerations was presented, followed
by an in-depth description of the code functionality and structure. Both microcontrollers will run
in a continuous polling loop that will be checking for flags set in interrupt subroutines.
Additional features then the ones already described, that enhance user experience and reliability
of the product, may be implemented if time permits.
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Spring 2009
List of References
[1] Microchip MRF24J40MA Data Sheet, [Online Document], 2008 Nov, [cited 2009 Mar 27],
Avaliable HTTP: http://ww1.microchip.com/downloads/en/DeviceDoc/70329b.pdf
[2] Microchip PIC18F2220/2320/4220/4320 Data Sheet, [Online Document], 2007 Dec, [cited
2008 March 27], Available HTTP:
http://ww1.microchip.com/downloads/en/DeviceDoc/39599g.pdf
[3] Microchip dsPIC33FJ32GP302/304, dsPIC33FJ64GPX02/X04, and
dsPIC33FJ128GPX02/X04 Data Sheet, [Online Document], 2008 Mar, [cited 2008 March
27], Available HTTP: http://ww1.microchip.com/downloads/en/DeviceDoc/70292B.pdf
[4] Linear Technology LTC4150 Coulomb Counter/ Battery Gas Gauge, [Online Document],
2003, [cited 2008 march 27],
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1037,C1134,P235
4,D1556
[5]
MiWi™ P2P for PIC18, PIC24, dsPIC, PIC32, [Online Document], 2008 Nov, [cited 2008
March 27], Available HTTP:
http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2680&dDocN
ame=en538363, [Individual module files: MSPI.c; P2P.c; SymbolTime.c; MRF24J40.h;
MSPI.h; P2P.h; SymbolTime.h; GenericTypeDefs.h]
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Appendix A: Flowchart/Pseudo-code for Main Program
Figure A.1 Functional Flowchart for Pen Device
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Digital Systems Senior Design Project
Figure A.2 Functional Flowchart for Base Station
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Appendix B: Hierarchical Block Diagram of Code Organization
ISR
main.c
timer( )
Initialization
function
pb_handle( )
transmit_axes( )
IOinit( )
var_init( )
periph_init( )
timer_init( )
int1( )
int0( )
transmit_battery( )
MiWi modules [5]
Figure B.1 : Pen Hierarchical Design
ISR
main.c
timer( )
Initialization
function
IOinit( )
var_init( )
periph_init( )
timer_init( )
Char_analysis( )
Receive_MiWi( )
MiWi modules [5]
Figure B.2: Base Hierarchical Design
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int1( )
UpdateLCD( )
int0( )
ECE 477
Digital Systems Senior Design Project
Appendix C: Port Pins and Use
PEN PORT USAGE
Register
PORTA
PORTB
PORTC
PIN
AN0
AN1
AN2
RA3
RA4
RA5
RA6
RA7
INT0
INT1
RB2
RB3
RB4
RB5
RB6
RB7
RC0
RC1
RC2
SCK
SDI
RDO
RC6
RC7
Description
Accelerometer Z Axis
Accelerometer Y Axis
Accelerometer X Axis
Not Used
Not Used
Not Used
Not Used
Not Used
MiWi Transceiver Interrupt
Fuel Gauge Interrupt
Not Used
Pen Tip Push Button
Not Used
PGM
PGC
PGD
GPIO Debug Pin
Not Used
MiWi Wake
SPI Clock
SPI Data In
SPI Data Out
Chip Select
Transceiver Reset
Table 1 – Port Mapping on PIC18
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Digital Systems Senior Design Project
Table 2 – Port Mapping on dsPIC33
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