AN1752 Data Structures 8-Bit Microcontrollers Freescale Semi

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AN1752 Data Structures 8-Bit Microcontrollers Freescale Semi

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AN1752
Data Structures 8-Bit Microcontrollers
Freescale Semiconductor, Inc.
Brad Bierschenk Consumer Systems Group Applications Engineering Austin, Texas
Introduction
data structure describes information organized stored computer system. Although data structures usually presented context large computers, same principles applied embedded 8-bit processors. efficient appropriate data structures improve both dynamic (time-based) static (storage-based) performance microcontroller software. This application note presents data structures which useful development microcontroller software. applications presented here means absolute. find infinite variety ways apply these basic data structures microcontroller application.
Strings
string sequence elements accessed sequential order. string data structure usually refers sequence characters. example, message which output display stored string ASCII character bytes memory.
Motorola, Inc., 1998, 2001
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Application Note
Storing Strings string elements must identified starting ending address. starting address string defined using absolute address label using base address group strings identifying particular strings with offset into group. There several methods terminating string information. common terminating string using special character mark string. terminating character value $04, ASCII (end-of-transmission) byte.
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Figure shows example string data.
DATA MESSAGE POINTER
ADDRESS
Figure String Data Structure Another method terminating string identify length. length then used counter value, eliminating need extra byte storage string. string ASCII characters terminated without using extra byte storage using sign (most significant bit) indicator last byte string. Because ASCII character data only seven bits long, last byte string indicated most significant location. When using this method, programmer must careful strip sign before using ASCII character value.
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Application Note Strings
Accessing Strings
efficient accessing string with indexed addressing mode INCX DECX instruction. Listing String Storage Example Listing String Access Example illustrate this string storage scheme
Listing String Storage Example
Absolute string addresses specifying string data *-Message1 'This string' Message2 'This another string' Indexed string addressing Another specifying string data *-Msgs Message3 *-Msgs 'This string' Message4 *-Msgs 'This another string'
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Listing String Access Example
String display code generic method displaying entire string. *-LoadMsg #Message1 ;Offset into Loop Messages,X ;Load character #$04 ;Check StringDone ;End string ShowByte ;Show character INCX ;Point next Loop String storage code *-Messages Message1 *-Messages 'This string'
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Application Note
String Applications Practical applications strings include storing predefined "canned" messages. This useful applications which require output text displays, giving users information prompting users input. Strings also effective storing initialization strings hardware such modems. Strings also store predefined command data sequences communicate with other devices.
Stacks
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stack series data elements which accessed only end. analogy this data structure stack dinner plates; first plate placed stack last plate taken from stack. this reason, stack considered LIFO (last first out) structure. stack useful when latest data desired. stack will typically have predefined maximum size.
shows representation stack.
DATA EMPTY EMPTY STACK POINTER EMPTY DATA BYTE DATA BYTE DATA BYTE DATA BYTE DATA BYTE
ADDRESS STACK BOTTOM STACK GROWS THIS DIRECTION STACK (MAXIMUM)
Figure Stack Data Structure
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Application Note Stacks
Just like physical stack items, software stack bottom top. Software should keep track location stack. This address either point first piece valid data point next available location. following examples will pointing next available location.
Stack Reading Writing
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read operation stack called "pulling" "popping"), write operation stack called "pushing." When pulls data from stack, data removed from stack stack pointer adjusted. When data pushed onto stack, stack pointer adjusted data added stack. implementation Figure push operation would first decrement stack pointer then store data address pointed (stack pointer) pull operation would retrieve data (stack pointer) then increment stack pointer. error conditions intrinsic this data structure; underflow overflow. stack underflow occurs when user attempts pull information empty stack. stack overflow occurs when user attempts push information onto stack which full. When using this data structure, these conditions should attended underflow condition should return error. overflow, either reject data return error, stack "wrap" around bottom, destroying data bottom stack.
Hardware Stack
Motorola MCUs utilize stack structure saving program context before transferring program control. This interaction result jump interrupt. result interrupt, stack used push values (condition code register) registers, well 16-bit (program counter) value. When jump instruction encountered, value pushed stack. returning from interrupt (RTI instruction) program context (registers pulled from stack. When returning from jump (RTS instruction) pulled from stack.
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Application Note
HC05 Stack HC05 Family MCUs have limited stack access. only operation that performed with MCU's stack pointer reset instruction will reset stack pointer $FF. HC05 stack pointer also limited size bytes. When stack pointer grows beyond address $C0, stack pointer wraps around $FF, destroying existing data that address.
HC08 Stack
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HC08 Family MCUs more flexible stack structure. stack pointer address. HC08 MCUs also have added addressing mode which indexed stack pointer. this way, user pass parameters subroutines using hardware stack, accessing parameters using stack pointer indexed addressing. Other HC08 Family instructions allow data pushed pulled stack. stack pointer also transferred index register vice-versa. With addition these instructions addressing modes, user good control over stack HC08 MCU.
Stack Applications
stack useful dynamically allocating memory passing parameters from subroutines. Typically, variables statically allocated assembly time. example:
Statically allocated variables RAMSPACE Another method statically allocate variable RAMSPACE+4 RAMSPACE+5
MyVar1 MyVar2 MyVar3 MyVar4 MyVar5
This appropriate global variables, which need available throughout program flow. However, local variables which only used specific subroutines, this method most efficient. space these variables dynamically allocated using software stack stack, freeing memory. same
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Application Note Stacks
method applied subroutine input output parameters, passing them stack instead register. Listing Software Stack shows software implementation stack, which would appropriate HC05 Family microcontrollers.
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Application Note
Listing Software Stack
STACK.ASM simple software stack implementation Simply shows PUSH PULL operations stack; intended complete application. StackPtr points next (empty) available location Written MC68HC705P6A *-*-* Memory equates *-RAMSPACE ROMSPACE $100 RESETVEC $1FFE Stack equates *-STACKSIZE STACKBOT ;Bottom software stack STACKMAX {STACKBOT-STACKSIZE+1} ;Maximum address stack variables *-ORG RAMSPACE ;First address StackPtr ;Pointer next stack byte Start program code *-ORG ROMSPACE ;Start Init #STACKBOT ;Initialize stack pointer StackPtr Some simple read write operations illustration only *-LDA #$01 PushA ;Write stack FullErr PushA ;Write stack FullErr PushA ;Write stack FullErr PushA ;Write stack FullErr PushA ;Write stack FullErr PushA ;Write stack FullErr PushA ;Write stack FullErr PushA ;Write stack AN1752 MOTOROLA
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Application Note Stacks
Loop
FullErr PushA FullErr PushA FullErr PullA EmptyErr PullA EmptyErr PullA EmptyErr
;Write FULL stack ;Write FULL stack ;Read from stack ;Read from stack ;Read from stack
;Your code here ;Your code here ;Your code here
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EmptyErr FullErr
Subroutines code access data structure *-*-* PUSH subroutine Push contents accumulator onto stack indicate full error *-PushA StackPtr ;Get stack pointer #STACKMAX ;Check full stack Full DECX ;Decrement stack pointer ;Store data StackPtr ;Record stack pointer ;Clear carry ;Return Full ;Set carry error ;Return PULL subroutine PULL byte stack into accumulator indicate empty stack error *-PullA StackPtr ;Get stack pointer #STACKBOT ;Check empty stack Empty ;Get data INCX ;Increment stack pointer StackPtr ;Record stack pointer ;Clear carry ;Return Empty ;Set carry ;Return Vector definitions *-ORG RESETVEC Init AN1752 MOTOROLA
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Application Note
Using software stack, subroutine allocate variables pushing (allocating) bytes stack, accessing them with indexed addressing (relative stack pointer variable) pulling them (deallocating) before returning. this way, same space used multiple subroutines. Parameters passed from subroutines well. input parameter pushed stack. When subroutine entered, access input parameter relative stack pointer. same token, subroutine push output parameter onto stack passed back calling routine. hardware stack stack pointer also used these purposes. Because expanded instruction set, stack easily exploited HC08 Family microcontrollers. Listing Using HC08 Stack Operationsshows example using HC08 stack pass parameters allocate local variables.
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Listing Using HC08 Stack Operations Using stack pass parameters allocate variables optimizes memory usage.
Code segment example using HC08 stack pass parameters allocate local variables. intended complete application. *-LDA PSHA PSHA PULA PULA Loop #$AA ;Load some data passed ;Push parameter subroutine ;Push parameter subroutine ;Call subroutine ;Parameter passed back ;Parameter passed back Result1 ;Your code here
Result2
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Application Note Stacks
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Subroutine which uses stack variable access SP->Empty LOCAL2 LOCAL1 PARAM2 PARAM1 -*-PARAM1 ;Parameters passed PARAM2 LOCAL1 ;Local variables LOCAL2 *-Sub PSHA ;Allocate local variable PSHA ;Allocate local variable ROLA ROLA PULA PULA PARAM1,SP LOCAL1,SP PARAM2,SP LOCAL2,SP LOCAL1,SP PARAM1,SP LOCAL2,SP PARAM2,SP ;Load parameter passed something ;Store local variable ;Load parameter passed ;Store local variable ;Store value passed back ;Store value passed back ;Deallocate local variable memory ;Deallocate local variable memory ;Return
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Application Note Queues
queue series elements which accepts data from extracts data from other end. analogy this data structure would checkout line supermarket. first people first people out. this reason, considered FIFO (first first out) structure. This useful when accessing data order received. queue will usually have predefined maximum size.
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Figure illustrates queue.
DEQUEUE FROM 'GET' POINTER DATA DATA DATA DATA ENQUEUE 'PUT' POINTER DATA EMPTY EMPTY EMPTY QUEUE BOTTOM QUEUE GROWS THIS DIRECTION QUEUE
DATA
ADDRESS
Figure Queue Reading Writing read operation queue called "dequeue," write operation "enqueue." pointers necessary queue, head line, tail. enqueue operation, after checking size queue, data stored location pointed "put" pointer, pointer adjusted. dequeue operation, data read from "get" pointer location, pointer adjusted. Queues usually have fixed size, important keep track number items queue. This done with variable containing size queue with pointer arithmetic.
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Application Note Queues
Queue Errors
with stack structure, queue subject underflow overflow errors. write, "enqueue" operation, should nondestructive should error queue full. read, "dequeue" operation, should destructive (remove data element) should error queue empty.
Queue Applications
practical application FIFO queue data buffer. Queues used buffers transmitted received data with printers serial communication devices. Listing Queue Example shows example queue software. good application this would store data received from SIOP (serial input/output port) processing later.
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Listing Queue Example
Illustrates implementation queue 705P6A Register definitions Memory definitions RAMSPACE ROMSPACE RESETVEC
$100 $1FFE
Queue data structure definitions These three equates defines data structure change queue, change data structure, code. QMAX ;Maximum size QTOP ;Top array QBOT QTOP+QMAX-1 ;Bottom array
variables TempA TempX RAMSPACE
GetPtr PutPtr QCount
;8-bit pointer ;8-bit pointer ;Counter size
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Application Note
Start program code *-ORG ROMSPACE Start InitQ #QTOP ;Initialize pointers variables GetPtr PutPtr QCount Write read from good application this place bytes received from into queue, retrieve them later *-JSR #$FF #$55 Loop
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Subroutines *-*-* enqueues data byte passed accumulator Checks full returns carry full. Otherwise returns cleared carry successful enqueue. *-EnQ TempX Save register contents QCount ;Check full CMPX #QMAX QFull full error PutPtr ;Store data CMPX #QBOT ;Check wrap WrapPut ;Wrap pointer INCX ;Adjust pointer EnQDone WrapPut #QTOP ;Successful enqueue
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Application Note Queues
;Unsuccessful enqueue QFull
EnQDone
PutPtr TempX QCount
;Store pointer ;Restore register ;Increment count variable ;Clear carry ;Return ;Restore register ;Set carry ;Return
TempX
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Dequeue byte from queue, return byte accumulator queue empty, return carry indicate error. Otherwise, return clear carry data *-DeQ TempX ;Save register contents QCount ;Check empty CMPX #$00 QEmpty GetPtr CMPX #QBOT ;Check wrap condition WrapGet INCX DeQDone WrapGet #QTOP ;Successful dequeue DeQDone GetPtr ;Record pointer TempX ;Restore register QCount ;Decrement counter ;Clear carry ;Return ;Unsuccessful dequeue QEmpty TempX ;Restore register ;Set carry ;Return Vector definitions *-ORG RESETVEC Start
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Application Note MACQ (Multiple Access Circular Queue)
multiple access circular queue circular buffer) modified version queue data structure. fixed-length, order-preserving data structure, always contains most recent entries. useful data flow problems, when only latest data interest. Once initialized always full, write operation always discards oldest data.
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Figure depicts MACQ.
Reading Writing
After being initially filled, write operation will place data MACQ, shift existing data downward. last byte will discarded, result latest data existing buffer. read operation non-destructive return number data bytes desired from MACQ.
LATEST DATA HERE DATA DATA DATA DATA DATA DATA DATA DATA DATA DATA DATA DATA DATA DATA DATA DATA DATA ADDRESS
OLDEST DATA DISCARDED
Figure Result MACQ Write
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Application Note MACQ (Multiple Access Circular Queue)
Applications
MACQ useful data streams which require latest data afford have destructive write operation. example, predict weather forecaster might temperature readings from last five days predict next day's temperature. Daily temperature readings recorded MACQ, latest data available. MACQs also useful digital filters. example, they used calculate second derivative, running average, etc.
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Example
Listing MACQ illustrates implementation MACQ circular buffer. This could effectively used storing converter readings. this way, latest conversion results would accessible through circular buffer.
Listing MACQ
Illustrates implementation multiple-access circular queue. (MACQ) MACQ fixed-length, order-preserving, indexable data structure. Once initialized, MACQ always full. write MACQ destructive, discarding oldest data. read from MACQ non-destructive. 705P6A Register definitions Memory definitions RAMSPACE ROMSPACE RESETVEC
$100 $1FFE
MACQueue data structure definitions These three equates defines data structure change queue, change data structure,and code. QSIZE ;Maximum size QTOP ;Top array QBOT QTOP+QSIZE-1 ;Bottom array
variables TempA TempX TempData QPtr RAMSPACE
;8-bit pointer
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Application Note
Start program code *-ORG ROMSPACE Start InitQ #QBOT ;Initialize pointer QPtr Write read from MACQ useful application this would store converter readings, latest readings available. *-LDA #$55 WriteQ #$56 WriteQ #$57 WriteQ #$58 WriteQ #$AA WriteQ #$AB WriteQ #$AC WriteQ #$AD WriteQ WriteQ ReadQ ReadQ ReadQ Loop
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Subroutines *-*-* WriteQ, contains data written write destructive full once initialized always full. *-WriteQ TempX ;Store register value QPtr ;Load pointer CMPX #QTOP-1 ;See full QFull ;Store data AN1752 MOTOROLA
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Application Note Tables
QPtr WQDone
;Decrement pointer ;Store pointer
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Once MACQ initialized, it's always full QFull TempData QBOT-1 SwapLoop DECX CMPX #QTOP SwapLoop #QTOP TempData WQDone TempX
;Start shifting data down
ReadQ contains queue index location read returns value *-ReadQ TempX #QTOP (index) QTOP pointer address desired value Vector definitions *-ORG RESETVEC Start
Tables
table viewed vector identically structured lists. table common storing "lookup" data, such display data vector bytes. Figure shows example table.
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Application Note
TOP-OF-TABLE POINTER
DATA $0100 $0500 $0800 $0090 $1200 $2200
ADDRESS
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$0100 $0100
Figure Table Representation table commonly used look information. Table entries accessed with offset from base address table. Therefore, read from table typically done computing offset desired data accessing using indexed addressing mode.
Table Applications
table data structure common applications. using tables perform character conversions. example, table used convert binary numbers equivalents. (liquid crystal display) displays, ASCII character byte need converted segment bitmaps display. table could used this purpose. Another application table "jump" table. This table vector values which addresses loaded vectored Some program parameter converted offset into jump table, appropriate vector fetched certain input. example, their memory maps Motorola MCUs have built-in vector table, used interrupt exception processing. These vector tables allow preprogrammed addresses defined certain exceptions. When exception occurs, program counter value fetched from appropriate table entry.
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Application Note Tables
Another utilizing table data structure store predefined values lookup. example this storing interpolation data table, perform mathematical functions. This table documented application note, M68HC08 Integer Math Routines, Motorola document order number AN1219. Another example involves using table sinusoidal values produce sine wave output application note Arithmetic Waveform Synthesis with HC05/08 MCUs, Motorola document order number AN1222. equation calculate data CPU-intensive approximated with discrete values, these values precalculated stored table. this way, value quickly fetched, saving time.
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Table Example
Listing Table example tables convert ASCII data segment values.
Listing Table
Code segment example using table store segment values Could used when data registers define segment values display position. Takes ASCII character, converts offset into table segment values, uses offset access segment bitmap values. *-Loop Character ;Load ASCII character Convert ;Convert character ;Offset into table ;Load first byte LCD1 ;Store data register ;Load second byte LCD2 ;Store data register Loop ;Repeat Convert ASCII character byte offset value into table segment values. Valid ASCII values (decimal): 32-47, 48-57, 65-90 *-Convert #!48 ;Check "special" character Special #!65 ;Check numeric character Numeric Alpha #!90 ;Check invalid value ConvError
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Application Note
CLRA ROLA #!39 ConvDone #!32 ConvError #!32 ConvDone #!57 ConvError #!32 ConvDone ;Convert table offset ;Check invalid value ;Convert table offset ;Check invalid value ;Convert table offset ;Invalid value shows blank ;Multiply offset bytes data position
Special
Numeric
ConvError ConvDone
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Lookup table segment values ASCII character values Some characters displayed 15-segment LCD, they marked invalid, will displayed blank space. *-Table $0000 $0000 ;'!' INVALID $0201 ;'"' $0000 ;'#' INVALID $A5A5 ;'$' $0000 ;'%' INVALID $0000 ;'&' INVALID $0001 ;''' $000A ;'(' $5000 ;')' $F00F ;'*' $A005 ;'+' $0000 ;',' INVALID $2004 ;'-' $0800 ;'.' $4002 ;'/' $47E2 ;'0' $0602 ;'1' $23C4 ;'2' $2784 ;'3' $2624 ;'4' $21A8 ;'5' $25E4 ;'6' $0700 ;'7' $27E4 ;'8' $27A4 ;'9' $2764 ;'A' $8785 ;'B' $01E0 ;'C' $8781 ;'D' $21E4 ;'E' $2164 ;'F' AN1752 MOTOROLA
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Application Note Linked Lists
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EndTable
$05E4 $2664 $8181 $06C0 $206A $00E0 $1662 $1668 $07E0 $2364 $07E8 $236C $25A4 $8101 $06E0 $4062 $4668 $500A $9002 $4182 *-Table
;'G' ;'H' ;'I' ;'J' ;'K' ;'L' ;'M' ;'N' ;'O' ;'P' ;'Q' ;'R' ;'S' ;'T' ;'U' ;'V' ;'W' ;'X' ;'Y' ;'Z' ;End table label
Linked Lists
list data structure whose elements vary precision. example, record containing person's name, address, phone number could considered list. linked list group lists, each which contains pointer another list. Figure represents linked list.
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Application Note
NEXTPTRA
NEXTPTRB
NEXTPTRC
DATA1A DATA2A DATA3A DATA4A
DATA1B DATA2B DATA3B DATA4B LISTB
DATA1C DATA2C DATA3C DATA4C LISTC
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LISTA
Figure Linked List Each list structure contains same type information, including link next item list. link might absolute address offset from some base address. doubly linked list, pointers kept both next previous item list. linked list traversed easily simply following pointers from list next.
Linked List Applications
linked list used traditionally define dynamically allocated database, which elements ordered resorted adjusting links. However, small microcontroller, there more appropriate applications linked lists. linked list used structure command interpreter. Each command could contain string characters, address subroutine call that command, link next command linked list. this way, command string could input, searched linked list, appropriate action taken when string found.
State Machines
Another useful application linked list define state machine. state machine represented discrete number states, each which output pointers next state(s). Figure
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Application Note Linked Lists
STATE STATE
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STATE INPUT/OUTPUT
STATE
Figure State Machine state machine considered Mealy Moore machine. Mealy machine's output function both inputs current state. Moore machine output dependent only current state. This state machine model useful controlling sequential devices such vending machines, stepper motors, robotics. These machines have current internal state, receive input, produce output, advance next state. first model process sequential machine, then convert this behavior linked-list structure write interpreter important goal able make modifications state machine changing data structure (linked list) code.
State Machine Example
example, consider traffic light controller which determines light patterns intersection. light patterns needed, north/south directions east/west directions. Consider that bulk traffic travels north/south road, sensors placed east/west road intersection determine when traffic needs cross. Figure
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Application Note
INPUT
INPUT
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Figure Traffic Light Controller Example This example modeled Moore state machine, with output function current state. next state function current state state input. Figure shows state graph this example. initial state will green light north/south direction light east/west direction. controller remains this state, until input seen east/west direction. flow continues shown diagram. output shown diagram pattern light array activate lights state.
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Application Note Linked Lists
INPUT OUTPUT 11011110 DELAY INPUT OUTPUT 11011101 DELAY
INPUT INPUT
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OUTPUT 11110101 DELAY INPUT
OUTPUT 11110011 DELAY
Figure Traffic-Light Controller State Graph Simulation This example simulated using LEDs 68HC705P6A MCU. pushbutton switch used simulate input sensor. Figure illustrates simulation circuit. Using bits output port, pattern generated display appropriate north/south east/west lights (LEDs). Table shows bitmap this application.
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Application Note
OUTPUTS 74LS04 705P6A
OUTPUTS 74LS04
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INPUT
Figure Circuit Simulation Traffic-Light Controller Table Traffic Light Bitmap Port
Position Meaning
used
North/South Signal
East/West Signal
With hardware place, that left define state machine software. This done implementing linked-list data structure code access interpret machine. this particular example, each list data structure defines current state traffic light. Each list contains: byte which bitmap lights. delay value; time controller remains state next state pointer input next state pointer input
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Application Note Linked Lists
main loop program should execute program flow charted Figure software this simulated traffic light controller documented Listing Traffic Controller State Machine.
LOAD INITIAL STATE
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OUTPUT LIGHT PATTERN LOAD NEXT STATE POINTER (OFFSET) DELAY GIVEN VALUE LOAD NEXT STATE POINTER (OFFSET)
INPUT INPUT
INPUT
Figure State Machine Program Flow Listing Traffic Controller State Machine
Traffic light controller example Illustrates linked list implementation state machine 705P6A Register definitions PORTA PORTD DDRA DDRD Memory definitions RAMSPACE ROMSPACE RESETVEC variables
$100 $1FFE RAMSPACE
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Application Note
TempA TempX
Start program code *-ORG ROMSPACE Start #$00 PORTA ;Predefine output levels #$FF DDRA ;Make Port outputs BCLR 7,PORTD ;Make Port input
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Loop
BRCLR
#INITST STATES+LIGHTS,X PORTA STATES+DELAY,X SecDelay 7,PORTD,In0 STATES+NEXT0,X Loop STATES+NEXT1,X Loop
;Index initial state ;Get light pattern ;Output light pattern ;Get delay seconds ;Cause delay ;Check input ;Get next state offset ;(input ;Get next state offset ;(input
DATA STRUCTURE STATE MACHINE LINKED LIST (05) Offsets base address scheme adequate small table (<255 bytes) *-LIGHTS ;Offset light pattern DELAY ;Offset time delay NEXT0 ;Offset pointer NEXT1 ;Offset pointer STATES ;Base address states INITST *-STATES ;Initial state offset North/South green light, East/West light *-STATES ;Offset into STATES %11011110 ;Output state ;Delay state ;Next state input ;Next state input yellow light, light *-STATES %11101110 light, green light *-STATES %11110011 ;Delay state AN1752 MOTOROLA
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Application Note Linked Lists
yellow light, light *-STATES %11110101
;Delay state
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Delay subroutines Cause delay second Accumulator value) 1MHz *-SecDelay #$00 SecDone Delay0 Delay0 DECA SecDelay SecDone Cause delay ~1/2 second *-Delay0 TempX #$B2 DLoop0 CMPX #$00 DDone0 Delay1 DECX DLoop0 DDone0 TempX Cause about 2.8msec delay 1MHz *-Delay1 TempA #$FF DLoop1 #$00 DDone1 DECA DLoop1 DDone1 TempA Vector definitions *-ORG RESETVEC Start
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Application Note Conclusion
data structures necessarily limited large, complicated computers. Although data structure powerful concept such context, same principles applied smaller processors such 8-bit microcontrollers. code implement these data structures does necessarily have complex confusing. goal programming should modularize commonly used functions, that they reused other applications with minimal amount modification. appropriate data structure concepts improve static dynamic performance application, without affecting portability legibility.
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AN1752 Data Structures 8-Bit Microcontrollers Freescale Semi

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