Rainbow Electronics DS1920 - Manual (Page 9)

DS1920
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HARDWARE CONFIGURATION
The 1-Wire bus has only a single line by definition; it is important that each device on the bus be able to
drive it at the appropriate time. To facilitate this, each device attached to the 1-Wire bus must have open
drain or 3-state outputs. The 1-Wire port of the DS1920 (data contact) is open drain with an internal
circuit equivalent to that shown in Figure 8. A multidrop bus consists of a 1-Wire bus with multiple
slaves attached. The 1-Wire bus requires a pull up resistor of approximately 5 k
Ω
. The idle state for the 1-
Wire bus is high. If for any reason a transaction needs to be suspended, the bus MUST be left in the idle
state if the transaction is to resume. If this does not occur and the bus is left low for more than 120 ms,
one or more of the devices on the bus will be reset.
HARDWARE CONFIGURATION Figure 8
TRANSACTION SEQUENCE
The protocol for accessing the DS1920 via the 1-Wire port is as follows:
§ Initialization
§ ROM Function Command
§ Memory/Control Function Command
§ Transaction/Data
INITIALIZATION
All transactions on the 1-Wire bus begin with an initialization sequence. The initialization sequence
consists of a reset pulse transmitted by the bus master followed by presence pulse(s) transmitted by the
slave(s).
The presence pulse lets the bus master know that the DS1920 is on the bus and is ready to operate. For
more details, see the “1-Wire Signaling” section.
ROM FUNCTION COMMANDS
Once the bus master has detected a presence pulse, it can issue one of the five ROM function commands.
All ROM function commands are eight bits long. A list of these commands follows (refer to flowchart in
Figure 5):