Generation Checking Program Memory Supports AVR® Controllers with Inst

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Generation Checking Program Memory Supports AVR® Controllers with Inst

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AVR236: Check Program Memory
Generation Checking Program Memory Supports AVR® Controllers with Instruction Compact Code Size, Words (CRC Generation Checking) Requirement Checksum Stored EEPROM Execution Time: (AT90S8515 MHz) Bits Implementation, Easily Modified Bits Supports CRC-16 Standard, Easily Modified CRC-CCITT, CRC-32
8-bit RISC Microcontroller Application Note
Introduction
This application note describes (Cyclic Redundancy Check) theory implementation detect errors program memory Atmel microcontroller. widely used method detecting errors messages transmitted over noisy channels. standards secure microcontroller applications introduced method detecting errors Program memory microcontrollers. preferable implement calculation compact code with requirement data storage memory since frees resources actual application. implementation used this application note optimized minimum code size register usage. Figure Checking Program Memory Using 16-bit Divisor
BYTE ADDRESS CODE 0011011 10011100 10000101 1011 ADDRESS 1FFF
DIVISOR
10000000 00000101
Rev. 1143B-AVR-04/02
Theory Operation
Checksums originally used communication through noisy channels. number (the checksum) computed function transmitted message. Receiver uses same function compute checksum, compares computed value with value received from transmitting side. this application note checksum constructed function code, stored internal EEPROM. microcontroller later same function calculate checksum code compare with appended checksum. Example: Checksum calculated summing numbers code:
Code: Code with checksum:
This checksum simply numbers code. second byte code corrupted from error will detected when original checksum compared with computed checksum.
Original code with checksum: Code with error: Wrong
first byte code corrupted from second byte corrupted from checksum will detect errors.
Original code with checksum: Code with error: 39-> Correct
problem with this checksum that simple. detect errors multiple bytes code detect errors checksum itself. This example shows that addition sufficient detect errors. calculations division instead addition calculate checksum code. principles similar, using division multiple errors burst errors will detected. algorithm treat Program memory enormous binary number, which divided another fixed binary number. remainder this division checksum. microcontroller will later perform same division compare remainder with calculated checksum. Note that division uses polynomial (modulo-2) arithmetic, which similar regular binary arithmetic, except uses carry. addition numbers with polynomial arithmetic simply XOR'ing data. Example: Addition polynomial arithmetic: 1011 0110 1101 0011 0110 0101
addition equal XOR'ing numbers. Lets define some properties polynomial arithmetic: M(x) G(x) R(x) k-bit number (the code checked). (n+1) number (the divisor polynom). n-bit number such that (the remainder checksum). R(x) M(x)*2 Q(x) Where Q(x) quotient G(x) G(x)
AVR236
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AVR236
Q(x) described M(x)*2 R(x) Q(x) M(x) equals adding zeros code G(x)
M(x)*2 replaced last equation G(x)
R(x) R(x) M(x)*2 +R(x) -=Q(x) Q(x)+ -G(x) G(x) G(x) Which equal Q(x) since divisor remainder same number, adding itself same XORing which results zero.
Example Division hexadecimal number which binary number 0110 1010, divided with
divisor 1001 hex). checksum will remainder operation 0110 1010 divided with 1001. First append zeros original message (where width divisor).
011010100000 1001 01100 0000 1101 1001 1000 1001 0011 0000 0110 0000 1100 1001 1010 1001 0110 0000 1100 1001 0101 Remainder Checksum Quotient Ignored
checksum added original code. resulting code will 6A5. When this code checked, code checksum divided divisor. remainder this division zero errors occurred, non-zero otherwise.
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Several standards used today detection. characteristics divisor vary from bits, ability detect errors varies with width divisor used. Some commonly used divisors are:
CRC-16 1000 0000 0000 0101= 8005(hex) CRC-CCITT 0001 0000 0010 0001= 1021 (hex) CRC-32 0000 0100 1100 0001 0001 1101 1011 0111 04C11DB7 (hex)
Observe that bits divisors, actual numbers bits bits divisor number bits always
Software Description
Main Program
main program supplied show operation both generation checking. checksum generated stored internal EEPROM, read back before checking performed. most applications, checksum will generated programmer placed last address Program memory. Figure Flowchart Main Program
START
INITIALIZATION UNIT STACK POINTER PORTS
STATUS
CRC_GEN GENERATE CHECKSUM STORE CHECKSUM EEPROM
STATUS
LOAD CHECKSUM FROM EEPROM
CRC_GEN CHECK CHECKSUM
OUTPUT CHECKSUM
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AVR236
main program call routine CRC_gen with Status Register 0x00 after reset generate checksum code. generated checksum stored EEPROM. Register 0xFF, value different from 0x00.
Checksum Generation
operation based principle rotating entire Program memory bit. shifted into Carry Flag. Carry Flag (one), word XOR'ed with divisor. Note that Program memory which shifted into Carry Flag also XOR'ed with divisor. Since they both result will always zero division ignored. Program memory zeros appended code. checksum resulting value complete operation.
Checksum Checking
same principles applied generation, generated checksum appended code, replacing zeros. result calculation including appended checksum zero errors occurred, non-zero otherwise. checksum included Program code, only checking part computation needs done Program code. same routine used both generation checking. Global Register Status loaded with 0x00 function call perform generation. Status Register loaded with value different from 0x00 function call, function performs checksum checking. flowchart shows flow crc_gen routine which includes both generation checking. flowcharts Figure Figure describes operation crc_gen subroutine.
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Figure CRC_gen Subroutine
START
LOAD DIVISOR VALUE
LOAD INITIAL VALUE
LOAD BYTES
CODE
ROTATE WORD ROUTINE
STATUS
APPEND ZEROS
APPEND CHECKSUM
ROUTINE WORD ROUTINE
RETURN
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AVR236
Figure Rotate Subroutine
START
LOAD COUNTER (HEX)
DECREMENT COUNTER
COUNTER
SHIFT DATA LEFT
CARRY CODE DIVISOR
RETURN
Modifications
code example implements 16-bit checksum CRC-16 computation. code easily modified support 32-bit checksum increasing size code buffer from bits, increasing size divisor from bits. checksum generated programmer placed last memory location, only code checking checksum needs included program. code "end" section routine removed. Please comments code. Some CRC-algorithms requires data register have initial value different from 0x00. other values used, initial values loaded into registers, replacing first instructions. comments code more information. algorithm reflected, which means that bytes shifted first instead MSB, routine support this replacing (Logical Shift Left) (Rotate Left) instructions with (Logical Shift Right) (Rotate Right) instructions. Other implementations computation exists with higher speed, most them lookup table increase speed operation. requirements such application makes them suitable more complex systems.
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Resources
Table Memory Usage
Function main Code Size words Cycles Register Usage R16, R22, R23, R24, R17, R18, R19, R20, R21, R22, R30, R16, R23, R24, Interrupt Description Initialization Example Program Generate Check Checksum Write Checksum EEPROM Read Checksum from EEPROM
CRC_gen
words
700.000 (approx.)
EEwrite
words
cycles
EERread TOTAL
words words
cycles
R16, R23, R24,
Table Peripheral Usage
Peripheral Bytes EEPROM Pins Description Storing Value Output Byte LEDs
References
Fred Halsall "Data Communication, Computer Networks Open Systems" 1992 Addison-Wesley Publishers Ross Williams "The Painless Guide Error Detection Algorithms"
AVR236
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AVR236
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Generation Checking Program Memory Supports AVR® Controllers with Inst

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