DS1982

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CRC GENERATION

The DS1982 has an 8-bit CRC stored in the most significant byte of the 64-bit ROM. The bus master can

compute a CRC value from the first 56 bits of the 64-bit ROM and compare it to the value stored within

the DS1982 to determine if the ROM data has been received error-free by the bus master. The equivalent

polynomial function of this CRC is: X

Under certain conditions, the DS1982 also generates an 8-bit CRC value using the same polynomial

function shown above and provides this value to the bus master to validate the transfer of command,

address, and data bytes from the bus master to the DS1982. The Memory Function Flow Chart of Figure

6 indicates that the DS1982 computes an 8-bit CRC for the command, address, and data bytes received

for the Write Memory and the Write Status commands and then outputs this value to the bus master to

confirm proper transfer. Similarly the DS1982 computes an 8–bit CRC for the command and address

bytes received from the bus master for the Read Memory, Read Status, and Read Data/Generate 8-Bit

CRC commands to confirm that these bytes have been received correctly. The CRC generator on the

DS1982 is also used to provide verification of error-free data transfer as each page of data from the 1024-

bit EPROM is sent to the bus master during a Read Data/Generate 8-bit CRC command, and for the 8

bytes of information in the status memory field.

In each case where a CRC is used for data transfer validation, the bus master must calculate a CRC value

using the polynomial function given above and compare the calculated value to either the 8-bit CRC

value stored in the 64-bit ROM portion of the DS1982 (for ROM reads) or the 8-bit CRC value computed

within the DS1982. The comparison of CRC values and decision to continue with an operation are

determined entirely by the bus master. There is no circuitry on the DS1982 that prevents a command

sequence from proceeding if the CRC stored in or calculated by the DS1982 does not match the value

generated by the bus master. Proper use of the CRC as outlined in the flow chart of Figure 6 can result in

a communication channel with a very high level of integrity. For more details on generating CRC values

including example implementations in both hardware and software, see the Book of DS19xx iButton

Standards.

INITIALIZATION PROCEDURE “RESET AND PRESENCE PULSES” Figure 10

480

µs ≤ t

480

µs ≤ t

∞ (includes recovery time)

15

µs ≤ t

µs

60

µs ≤ t

µs

RESISTOR

MASTER

DS1982