DS1990A

8 of 10

CRC ASSEMBLY LANGUAGE PROCEDURE Table 1

DO_CRC:

CRC_LOOP:

ZERO:

PUSH

ACC

PUSH

B

PUSH

ACC

MOV

B,#8

XRL

A,CRC

RRC

A

MOV

A,CRC

JNC

ZERO

XRL

A,#18H

RRC

A

MOV

CRC,A

POP

ACC

RR

A

PUSH

ACC

DJNZ

B,CRC_LOOP

POP

ACC

POP

B

POP

ACC

RET

; save the accumulator

; save the B register

; save bits to be shifted

set shift=8bits

;

; calculate CRC

; move it to the carry

; get the last CRC value

; skip if data=0

; update the CRC value

;

; position the new CRC

; store the new CRC

; get the remaining bits

; position the next bit

; save the remaining bits

; repeat for eight bits

; clean up the stack

; restore the B register

; restore the accumulator

CRC GENERATION

To validate the data transmitted from the DS1990A, the bus master may generate a CRC value from the

data as it is received. This generated value is compared to the value stored in the last eight bits of the

DS1990A. The bus master computes the CRC over the 8-bit family code and all 48 ID number data bits,

but not over the stored CRC value itself. If the two CRC values match, the transmission is error-free.

An example of how to generate the CRC using assembly language software is shown in Table 1. This

assembly language code is written for the DS5000 Soft microcontroller which is compatible with the

8031/51 Microcontroller family. The procedure DO_CRC calculates the cumulative CRC of all the bytes

passed to it in the accumulator. It should be noted that the variable CRC needs to be initialized to 0 before

the procedure is executed. Each byte of the data is then placed in the accumulator and DO-CRC is called

to update the CRC variable. After all the data has been passed to DO_CRC, the variable CRC will contain

the result. The equivalent polynomial function of this software routine is:

CRC = x

For more details, see the Book of DS19xx iButton Standards.