Running CRC

RunningCRC

Purpose:	Calculate the next CRC WORD given the prior CRC WORD and the next BYTE in the sequence.
Assumes:	The CRC generator polynomial is x¹⁵ + x¹³ + x⁰ (which is the value 0xA001 or 1010000000000001.)
Passed:	R25:R24	WORD	wPriorCRC	the previous 16-bit CRC value (i.e., its accrued value so far)
	R22	BYTE	bNewByte	the next data BYTE to include in the CRC
Returns:	R25:R24	WORD	wCRC	the new 16-bit CRC value, with bNewByte included
Alters:	No Registers (i.e., only the R25:R24 value returned and the FLAGS)


Example:	.ORG 0x1000 CRCOverAnyType: ADIW R26, 0 ; If the buffer pointer is NULL, there is BREQ ACRCOverAny_Ret ; nothing to do, so just exit immediately MOVW R24, R26 ; Only need to point R25:R24 at the buffer CALL AParseValue ; The ULONG result is in R25:R24:R23:R22 SBRS R21, BIT_PV_VALID ; Done if a value could not be parsed from it RJMP ACRCOverAny_Ret ; (could read param #2 here as the BYTE count) LDI R27, -1 ; Maintain the CRC in R27:R26; initialize it LDI R26, -1 ; to 0xFFFF MOVW R30, R22 ; Save the target address: the R23:R22 portion LDI R19, '=' ; For PrintDescAndHexBYTE & PrintDescAndHexWORD MOV R21, R24 ; BYTE 3 selects the data source (1 is EEPROM, CPI R21, 3 ; 2 = FLASH, any other SRAM), so if it is 3 or BRLO ACRCOverAny_Loop ; above, set it to zero, thereby limiting the LDI R21, 0 ; values to only 0, 1, or 2 CRCOverAny_Loop: LD R22, Z ; Assuming the value is in SRAM, read it CPI R21, 0 ; If it is, in fact in SRAM, we've got the new BREQ ACRCOverAny_Check ; data byte MOVW R24, R30 ; Move the 'Z' pointer to the common parameter SBRS R21, 1 ; which both ReadEEPROM... and ReadPGM... CALL AReadEEPROM_BYTE ; share, then call one or the other of them, SBRC R21, 1 ; based on whether R21 is B00000010 or not CALL AReadPGM_BYTE MOV R22, R24 ; Move the EEPROM or FLASH data BYTE into R22 CRCOverAny_Check: CPI R22, 0xFF ; If the new data BYTE is 0xFF, we're done BREQ ACRCOverAny_Ret MOVW R24, R26 ; Place the prior accrued CRC in R25:R24 CALL ARunningCRC ; Call to calculate the CRC; R22 is retained MOVW R26, R24 ; Save the new CRC back into R27:R26 LDI R25, ACSZ_New >> 8 LDI R24, ACSZ_New & 0xFF ; "New BYTE = 0x--" CALL APrintDescAndHexBYTE LDI R25, ACSZ_CRC >> 8 LDI R24, ACSZ_CRC & 0xFF ; "; new CRC = 0x----" MOVW R22, R26 CALL APrintDescAndHexWORD ; Print the string and the value CALL APrintNewLine ; Print a Carriage Return and Line Feed ADIW R30, 1 ; Advance the address to the next BYTE RJMP ACRCOverAny_Loop ; Loop back to read the next BYTE CRCOverAny_Ret: RET Leading0s: .LONG 0 CSZ_New: .STRING "New BYTE" CSZ_CRC: .STRING "; new CRC"
Test it:	m>DP+ 0x1000 0x80 FLASH (program memory) contents: 1000: 10 96 59 F1 CD 01 0E 94-6D 30 50 FF 26 C0 BF EF ..Y.....m0P.&... 1010: AF EF FB 01 3D E3 58 2F-53 30 08 F0 50 E0 60 81 ....=.X/S0..P.`. 1020: 50 30 41 F0 CF 01 51 FF-0E 94 21 30 51 FD 0E 94 P0A...Q...!0Q... 1030: 2C 30 68 2F 6F 3F 89 F0-CD 01 0E 94 1A 30 DC 01 ,0h/o?.......0.. 1040: 90 E1 80 E6 0E 94 62 30-90 E1 89 E6 BD 01 0E 94 ......b0........ 1050: 63 30 0E 94 5B 30 31 96-E2 CF 08 95 00 00 00 00 c0..[01......... 1060: 4E 65 77 20 42 59 54 45-00 3B 20 6E 65 77 20 43 New BYTE.; new C 1070: 52 43 00 FF FF FF FF FF-FF FF FF FF FF FF FF FF RC.............. m>@ 0x1000 0x2106F ; The '2' selects FLASH and address = 0x106F New BYTE = 0x43; new CRC = 0xB1FE New BYTE = 0x52; new CRC = 0x7DB1 New BYTE = 0x43; new CRC = 0x85FC New BYTE = 0x00; new CRC = 0x4185 m>DES EEPROM contents: 0000: 10 3C 00 9F 01 00 84 FF-04 00 FF FF 28 3C 50 FF .<..........(.P. 0010: C0 00 3F FF 40 08 03 67-FF 25 74 FF FF FF FF FF ..?.@..g.%t..... 0020: 24 28 05 00 FF FF 2A FF-FF 14 30 20 00 30 FF FF $(....*...0 .0.. 0030: 80 16 FF FF FF FF FF FF-60 00 B0 00 90 01 FF FF ........`....... 0040: FF FF FF FF FF FF 00 2F-FF FF FF FF FF FF FF FF ......./........ m>@ 0x1000 0x10000 ; The '1' selects EEPROM and address = 0 New BYTE = 0x10; new CRC = 0x8CBE New BYTE = 0x3C; new CRC = 0x610C New BYTE = 0x00; new CRC = 0x0561 New BYTE = 0x9F; new CRC = 0x8084 New BYTE = 0x01; new CRC = 0xA341 New BYTE = 0x00; new CRC = 0x3063 New BYTE = 0x84; new CRC = 0x4A70 m>
Notes:	When a CRC WORD is initialized to 0xFFFF, changes in blocks that begin with runs of zeros can be detected. When the initial CRC is 0x0000, they cannot be detected.
	Refer to the CRC System Function for more general information about CRCs. More more information is available online.
	The CRC System Function calculates a 16-bit CRC over a block of SRAM
Dropin: (setup code)	; Setup for the CRC() call: ; Passed: R25:R24 WORD wPriorCRC the existing CRC value accrued so far ; R22 BYTE bNewByte the next data BYTE to combine into the CRC ; Returns: R25:R24 WORD wCRC the new 16-bit CRC value, including the new byte ; Alters: R24, R25 and the FLAGS (R25:R24 has the return value)
Also see:	The CRC System Function.