DESIGN OF A CUSTOM ASIC INCORPORATING CAN™ AND 1 ...

Table 2.5 1 – Wire® Search Path Direction.
Search Bit Position vs. Last Discrepancy Path Taken
= Take the ‘1’ path.
< Take the same path as last time (from the last ROM number found).
> Take the ‘0’ path.
The search algorithm also keeps track of the last discrepancy that occurs within the first
eight bits of the algorithm. The first eight bits of the 64 – bit registration number is a family
code. As a result, the devices discovered during the search are grouped into family types. The
last discrepancy within that family code can be used to selectively skip whole groups of 1 –
Wire® devices. The 64 – bit ROM number also contains an 8 – bit cyclic-redundancy-check
(CRC). This CRC value is verified to ensure that only correct ROM numbers are discovered.
2.3.7 1 – Wire® CRC Check
Serial data can be checked for errors in a variety of ways. One common way is to include
an additional bit in each packet being checked that will indicate if an error has occurred. The
error detection scheme most effective at locating errors in a serial data stream with a minimal
amount of hardware is the Cyclic Redundancy Check. The most significant byte in the ROM
code of each 1 – Wire® device contains a CRC value based on the previous seven bytes of data
for that part. When the host system begins communication with a device, the 8 – byte ROM is
read, least significant bit first. If the CRC that is calculated by the host agrees with the CRC
contained in byte 7 of the ROM data actually read, the communication is considered valid. If
this is not the case, an error has occurred and the ROM code should be read again.
The CRC can be most easily understood by considering the function as it would actually
be built in hardware, usually represented as a shift register arrangement with feedback as shown
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