DS1963S

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CRC GENERATION

With the DS1963S there are two different types of CRCs (Cyclic Redundancy Checks). One CRC is an

8-bit type. It is computed at the factory and lasered into the most significant byte of the 64-bit ROM. The

equivalent polynomial function of this CRC is X

8

+ X

5

+ X

4

+ 1. To determine whether the ROM data has

been read without error the bus master can compute the CRC value from the first 56 bits of the 64-bit

ROM and compare it to the value read from the DS1963S. This 8-bit CRC is received in the true form

(non-inverted) when reading the ROM.

The other CRC is a 16-bit type, generated according to the standardized CRC16-polynomial function X

16

+ X

15

+ X

2

+ 1. This CRC is used for error detection with the Read Authenticated Page command,

Compute SHA, when reading the scratchpad and for fast verification of a data transfer when writing to

the scratchpad. It is the same type of CRC as is used with NV RAM based iButtons for error detection

within the iButton Extended File Structure. In contrast to the 8-bit CRC, the 16-bit CRC is always

returned or sent in the complemented (inverted) form. A CRC-generator inside the DS1963S chip (Figure

13) will calculate a new 16-bit CRC as shown in the command flow chart of Figure 7. The bus master

may compare the CRC value read from the device to the one it calculates from the data and decides

whether to continue with an operation or to re-read the portion of the data with the CRC error.

With the Write Scratchpad command the CRC is generated by first clearing the CRC generator and then

shifting in the command code, the Target Addresses TA1 and TA2 and all the data bytes. The DS1963S

will transmit this CRC only if the data bytes written to the scratchpad include scratchpad ending offset

11111b. The data may start at any location within the scratchpad. This algorithm applies regardless of the

state of the HIDE-flag. However, if the HIDE-flag is set the data bytes that follow the target address are

used for the CRC-calculation only. They are not received in the scratchpad.

With the Read Scratchpad command the CRC is generated by first clearing the CRC generator and then

shifting in the command code, the Target Addresses TA1 and TA2, the E/S byte, and the scratchpad data

starting at the scratchpad offset. The DS1963S will transmit this CRC only if the reading continues

through the end of the scratchpad, regardless of the actual ending offset. If the HIDE-flag is set the CRC-

calculation uses FFh-bytes instead of the scratchpad data, which remains hidden.

With the Read Authenticated Page command the 16-bit CRC value is the result of shifting the command

byte into the cleared CRC generator, followed by the two address bytes, the data bytes, and the values of

the write-cycle counters of the addressed memory page and its associated secret. The write cycle counters

are shifted in with their least significant byte first. With the Compute SHA command the CRC results

from shifting the command byte into the cleared CRC generator, followed by the Target Addresses TA1

and TA2 and the SHA Control byte.

For more details on generating CRC values including example implementations in both hardware and

software, see the Book of DS19xx iButton Standards.