How to Calculate a CRC (AVR ASM)
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;**** A P P L I C A T I O N N O T E A V R 236 ************************ ;* ;* Title: CRC check of program memory ;* Version: 1.3 ;* Last updated: 11.11.2004 ;* Target: AT90Sxxx, ATtinyxxx, ATmegaxxx ;* (All AVR Devices with LPM instr, not AT90S1200) ;* ;* Support E-mail: avr@atmel.com ;* ;* NOTE: Always check out Atmels web site, www.atmel.com for the latest and updated ;* version of the software. ;* ;* DESCRIPTION ;* This application note describes how to perform CRC computation ;* of code memory contents using a simple algoritm. ;* To generate CRC checksum load the register "status" with 00 and call the routine ;* "crc_gen". The resulting checksum is placed in the registers ;* byte_2(low byte) and byte_3(high byte). ;* ;* To check the CRC checksum load the register "status" with FF and call the routine ;* "crc_gen". The resulting checksum is placed in the registers ;* byte_2(low byte) and byte_3(high byte). If the checksum is 00 the program code is ;* correct, if the checksum is non-zero an error has been introduced in the program code ;************************************************************************** .include "8515def.inc" ;***** Constants .equ LAST_PROG_ADDR = 0x1FFF; Last program memory address(byte address) .equ CR = 0x8005; CRC divisor value ;************************************************************************** ;* ;* PROGRAM START - EXECUTION STARTS HERE ;* ;************************************************************************** ; .cseg .org $0000 rjmp RESET ;Reset handle ;*************************************************************************** ;* ;* "crc_gen" - Generation and checking of CRC checksum ;* ;* This subroutine generates the checksum for the program code. ;* 32 bits are loaded into 4 register, the upper 16 bits are XORed ;* with the divisor value each time a 1 is shifted into the carry flag ;* from the MSB. ;* ;* If the status byte is 0x00,the routine will generate new checksum: ;* After the computing the code 16 zeros are ;* appended to the code and the checksum is calculated. ;* ;* If the status byte is different from 0X00, the routine will check if the current checksum is valid. ;* After the computing the code the original checksum are ;* appended to the code and calculated. The result is zero if no errors occurs ;* The result is placed in registers byte_2 and byte_3 ;* ;* Number of words :44 + return ;* Number of cycles :program memory size(word) * 175(depending on memory contens) ;* Low registers used :6 (byte_0,byte_1,byte_2,byte_3) ;* High registers used :7 (sizel,sizeh,crdivl,crdivh,count,status,zl,zh) ;* ;*************************************************************************** ;***** Subroutine Register Variables .def byte_0 = r0 ; Lower byte of lower word .def byte_1 = r1 ; Upper byte of lower word .def byte_2 = r2 ; Lower byte of upper word .def byte_3 = r3 ; Upper byte of upper word .def crc = r4 ; CRC checksum low byte .def crch = r5 ; CRC checksum high byte .def sizel = r17; Program code size register .def sizeh = r18 .def crdivl = r19; CRC divisor register .def crdivh = r20 .def count = r21; Bit counter .def status = r22; Status byte: generate(0) or check(1) crc_gen: ldi sizel,low(LAST_PROG_ADDR) ;Load last program memory address ldi sizeh,high(LAST_PROG_ADDR) clr zl ;Clear Z pointer clr zh ldi crdivh,high(CR);Load divisor value ldi crdivl,low(CR) lpm ;Load first memory location mov byte_3,byte_0;Move to highest byte adiw zl,0x01 ;Increment Z pointer lpm ;Load second memory location mov byte_2,byte_0 next_byte: cp zl,sizel ;Loop starts here cpc zh,sizeh ;Check for end of code brge end ;Jump if end of code adiw zl,0x01 lpm ;Load high byte mov byte_1,byte_0;Move to upper byte adiw zl,0x01 ;Increment Z pointer lpm ;Load program memory location rcall rot_word;Call the rotate routine rjmp next_byte end: ;ret;uncomment this line if checksum is stored in last flash memory address. ldi count,0x11 cpi status,0x00 brne check clr byte_0 ;Append 16 bits(0x0000) to clr byte_1 ;the end of the code for CRC generation rjmp gen check: mov byte_0,crc ;Append the original checksum to mov byte_1,crch ;the end of the code for CRC checking gen: rcall rot_word;Call the rotate routine mov crc,byte_2 mov crch,byte_3 ret ;Return to main prog rot_word: ldi count,0x11 rot_loop: dec count ;Decrement bit counter breq stop ;Break if bit counter = 0 lsl byte_0 ;Shift zero into lowest bit rol byte_1 ;Shift in carry from previous byte rol byte_2 ;Preceede shift rol byte_3 brcc rot_loop ;Loop if MSB = 0 eor byte_2,crdivl eor byte_3,crdivh;XOR high word if MSB = 1 rjmp rot_loop stop: ret ;*************************************************************************** ;* ;* EERead_seq ;* ;* This routine reads the ;* EEPROM into the global register variable "temp". ;* ;* Number of words :4+ return ;* Number of cycles :8 + return ;* High Registers used :4 (temp,eeadr,eeadrh,eedata) ;* ;*************************************************************************** .def temp = r16 .def eeadr = r23 .def eeadrh = r24 .def eedata = r25 ;***** Code eeread: out EEARH,eeadrh;output address high byte out EEARL,eeadr;output address low byte sbi EECR,EERE ;set EEPROM Read strobe in eedata,EEDR;get data ret ;*************************************************************************** ;* ;* EEWrite ;* ;* This subroutine waits until the EEPROM is ready to be programmed, then ;* programs the EEPROM with register variable "EEdwr" at address "EEawr" ;* ;* Number of words :7 + return ;* Number of cycles :13 + return (if EEPROM is ready) ;* Low Registers used :None ;* High Registers used:;4 (temp,eeadr,eeadr,eedata) ;* ;*************************************************************************** .def temp = r16 .def eeadr = r23 .def eeadrh = r24 .def eedata = r25 eewrite: sbic EECR,EEWE;If EEWE not clear rjmp EEWrite ; Wait more out EEARH,eeadrh;Output address high byte out EEARL,eeadr;Output address low byte out EEDR,eedata;Output data sbi EECR,EEMWE sbi EECR,EEWE ;Set EEPROM Write strobe ret ;************************************************************************ ;* ;* Start Of Main Program ;* .cseg .def crc = r4 ;Low byte of checksum to be returned .def crch = r5 ;High byte of checksum to be returned .def temp = r16 .def status = r22;Status byte: generate(0) or check(1) .def eeadr = r23 .def eeadrh = r24 .def eedata = r25 RESET: ldi r16,high(RAMEND);Initialize stack pointer out SPH,r16 ;High byte only required if ldi r16,low(RAMEND);RAM is bigger than 256 Bytes out SPL,r16 ldi temp,0xff out DDRB,temp ;Set PORTB as output out PORTB,temp;Write 0xFF to PORTB clr status ;Clear status register, prepare for CRC generation rcall crc_gen ldi eeadr,0x01;Set address low byte for EEPROM write ldi eeadrh,0x00;Set address high byte for EEPROM write mov eedata,crc;Set CRC low byte in EEPROM data rcall eewrite ;Write EEPROM ldi eeadr,0x02;Set address low byte for EEPROM write ldi eeadrh,0x00;Set address high byte for EEPROM write mov eedata,crch;Set CRC high byte in EEPROM data rcall eewrite ;Write EEPROM out PORTB,crc;Output CRC low value to PORTB mainloop: sbic EECR,EEWE;If EEWE not clear rjmp mainloop; Wait more ;********** Insert program code here ************* ldi eeadr,0x01;Set address low byte for EEPROM read ldi eeadrh,0x00;Set address high byte for EEPROM read rcall eeread ;Read EEPROM mov crc,eedata;Read CRC low byte from EEPROM ldi eeadr,0x02;Set address low byte for EEPROM read ldi eeadrh,0x00;Set address high byte for EEPROM read rcall eeread ;Read EEPROM mov crch,eedata;Read CRC low byte from EEPROM ser status ;Set status register, prepare for CRC checking rcall crc_gen loop: out PORTB,crc;Output CRC low value to PORTB rjmp loop .exit |