DS2720

13 of 21

Figure 10. 1-WIRE BUS INTERFACE CIRCUITRY

TRANSACTION SEQUENCE

The protocol for accessing the DS2720 through the 1-Wire port is as follows:

§ Initialization

§ Net Address Command

§ Function Command

§ Transaction/Data

The sections that follow describe each of these steps in detail.

All transactions of the 1-Wire bus begin with an initialization sequence consisting of a reset pulse

transmitted by the bus master followed by a presence pulse simultaneously transmitted by the DS2720

and any other slaves on the bus. The presence pulse tells the bus master that one or more devices are on

the bus and ready to operate. For more details, see the 1-Wire Signaling section.

NET ADDRESS COMMANDS

Once the bus master has detected the presence of one or more slaves, it can issue one of the ROM

commands described in the following paragraphs. The name of each net address command is followed by

the 8-bit opcode for that command in square brackets. Figure 11 presents a transaction flowchart of the

ROM commands.

Read Net Address [33h or 39h].

This command allows the bus master to read the DS2720’s 1-Wire net

address. This command can only be used if there is a single slave on the bus. If more than one slave is

present, a data collision occurs when all slaves try to transmit at the same time (open drain produces a

wired-AND result). The RNAOP bit in the status register selects the opcode for this command, with

RNAOP = 0 indicating 33h and RNAOP = 1 indicating 39h.

Match Net Address [55h].

This command allows the bus master to specifically address one DS2720 on

the 1-Wire bus. Only the addressed DS2720 responds to any subsequent function command. All other

slave devices ignore the function command and wait for a reset pulse. This command can be used with

one or more slave devices on the bus.

0.5

mA

(typ)

100

W

MOSFET

Tx

Rx

Rx

Tx

Rx = RECEIVE

Tx = TRANSMIT

(2.0V TO 5.5V)

4.7k

W

BUS MASTER

DS2720 1-WIRE PORT