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DS18S20
CRC GENERATION
CRC bytes are provided as part of the DS18S20’s 64-bit ROM code and in the 9th byte of the scratchpad
memory. The ROM code CRC is calculated from the first 56 bits of the ROM code and is contained in the
most significant byte of the ROM. The scratchpad CRC is calculated from the data stored in the
scratchpad, and therefore it changes when the data in the scratchpad changes. The CRCs provide the bus
master with a method of data validation when data is read from the DS18S20. To verify that data has been
read correctly, the bus master must re-calculate the CRC from the received data and then compare this
value to either the ROM code CRC (for ROM reads) or to the scratchpad CRC (for scratchpad reads). If
the calculated CRC matches the read CRC, the data has been received error free. The comparison of CRC
values and the decision to continue with an operation are determined entirely by the bus master. There is
no circuitry inside the DS18S20 that prevents a command sequence from proceeding if the DS18S20
CRC (ROM or scratchpad) does not match the value generated by the bus master.
The equivalent polynomial function of the CRC (ROM or scratchpad) is:
CRC = X 8 + X 5 + X 4 + 1
The bus master can re-calculate the CRC and compare it to the CRC values from the DS18S20 using the
polynomial generator shown in Figure 8. This circuit consists of a shift register and XOR gates, and the
shift register bits are initialized to 0. Starting with the least significant bit of the ROM code or the least
significant bit of byte 0 in the scratchpad, one bit at a time should shifted into the shift register. After
shifting in the 56th bit from the ROM or the most significant bit of byte 7 from the scratchpad, the
polynomial generator will contain the re-calculated CRC. Next, the 8-bit ROM code or scratchpad CRC
from the DS18S20 must be shifted into the circuit. At this point, if the re-calculated CRC was correct, the
shift register will contain all 0s. Additional information about the Maxim 1-Wire cyclic redundancy check
is available in Application Note 27: Understanding and Using Cyclic Redundancy Checks with Maxim
iButton Products.
Figure 8. CRC Generator
INPUT
(MSB)
XOR
8 of 23
XOR
(LSB)
XOR
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