DS1985

16 of 25

The bus master requires a pullup resistor at the master end of the bus, with the bus master circuit

equivalent to the one shown in Figures 7a and 7b. The value of the pullup resistor should be

approximately 5 k

Ω for short line lengths.

A multidrop bus consists of a 1-Wire bus with multiple slaves attached. The 1-Wire bus has a maximum

data rate of 16.3 kbits per second. If the bus master is also required to perform programming of the

EPROM portions of the DS1985, a programming supply capable of delivering up to 10 milliamps at 12

volts for 480

µs is required. 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

µs, one or more of the devices on the bus may be

reset.

Transaction Sequence

The sequence for accessing the DS1985 via the 1-Wire port is as follows:

§

Initialization

§ ROM Function Command

§ Memory Function Command

§ Read/Write Memory/Status

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 a Presence Pulse(s) transmitted by the

slave(s).

The Presence Pulse lets the bus master know that the DS1985 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, it can issue one of the four ROM function commands. All

ROM function commands are 8 bits long. A list of these commands follows (refer to flowchart in Figure

8):

Read ROM [33H]

This command allows the bus master to read the DS1985’s 8-bit family code, unique 48-bit serial

number, and 8-bit CRC. This command can be used only if there is a single DS1985 on the bus. If more

than one slave is present on the bus, a data collision will occur when all slaves try to transmit at the same

time (open drain will produce a wired-AND result).