DS1921 datasheet(25/44 Pages) DALLAS

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DS1921H/Z

25 of 44

HARDWARE CONFIGURATION Figure 12

Open Drain

Port Pin

RX = RECEIVE

TX = TRANSMIT

100

Ω

MOSFET

VPUP

DATA

5µA

Typ.

BUS MASTER

DS1921 1-Wire PORT

RPUP

TRANSACTION SEQUENCE

The protocol for accessing the DS1921H/Z 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 DS1921H/Z 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 seven ROM function commands. All

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

Figure 13).

Read ROM [33h]

This command allows the bus master to read the DS1921H/Z’s 8-bit family code, temperature range code,

plus unique 36-bit serial number and 8-bit CRC. This command can only be used if there is a single slave

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). The resultant family code and

temperature range code plus 36-bit serial number will result in a mismatch of the CRC.

Match ROM [55h]

The Match ROM command, followed by a 64-bit ROM sequence, allows the bus master to address a

specific DS1921H/Z on a multidrop bus. Only the DS1921H/Z that exactly matches the 64-bit ROM

sequence will respond to the following memory function command. All other slaves will wait for a reset

pulse. This command can be used with a single or multiple devices on the bus.

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