DS2423

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Read-Data Time Slot

RESISTOR

MASTER

DS2423

0V

Master

Sampling

Window

CRC GENERATION

With the DS2423 there are two different types of CRCs (Cyclic Redundancy Checks). One CRC is an 8-

bit type and is 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 DS2423 to

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

when reading the ROM of the DS2423. It is computed at the factory and lasered into the ROM.

+ Counter command 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 DS2423 chip (Figure 12) will calculate a new

16-bit CRC as shown in the command flow chart of Figure 7. The bus master compares the CRC value

read from the device to the one it calculates from the data and decides whether to continue with an

operation or re-read the portion of the data with the CRC error.

With the initial pass through the Read Memory + Counter flow chart the 16-bit CRC value is the result of

shifting the command byte into the cleared CRC generator, followed by the two address bytes, data bytes,

value of the counter associated with the page and zero bits. Subsequent passes through the Read Memory

+ Counter flow chart will generate a 16-bit CRC that is the result of clearing the CRC generator and then

shifting in the data bytes, the value of the counter and the zero bits.

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 DS2423

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.

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

software, see the Book of DS19xx iButton Standards.