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

of the bit, then write the desired value of that bit. After one complete pass, the bus
master knows the contents of the ROM in one device. The remaining number of devices
and their ROM codes can be identified by additional passes.
MATCH ROM [55h] – This command followed by a 64 – bit ROM sequence, allows the
bus master to address a specific slave device on a bus topology having one or more
slaves. Only the device that exactly matches the 64 – bit ROM sequence will respond to
the following memory function command. All slaves that do not match the 64 – bit ROM
sequence wait for a reset pulse.
SKIP ROM [CCh] – This command can save time in a single slave network by allowing
the bus master to access the memory functions without providing the 64 – bit ROM code.
If more than one slave is present on the bus and, for example, a read command is issued
following the SKIP ROM command, data collision will occur on the bus as multiple
slaves transmit simultaneously (open-drain pulldowns produce a wired – AND result).
All iButtons and 1 – Wire® devices support at least some basic set of ROM function
commands such as those previously mentioned. The number of supported ROM functions
depends on the device itself but most importantly, these ROM functions (there are a total of 9)
have the same command number regardless of the iButton or 1 – Wire® device being used.
After a ROM function command has been successfully executed, the bus master may then
provide any one of the memory function commands specific to the device being addressed. (See
device documentation for specific number and type of memory functions as these can be
different for each device and also not all devices support these commands.) This allows further
data and other transactions to take place between the bus master and slave devices.
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