Rainbow Electronics DS2770 - Manual (Page 17)

DS2770
17 of 27
1-WIRE BUS INTERFACE CIRCUITRY Figure 16
TRANSACTION SEQUENCE
The protocol for accessing the DS2770 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 DS2770
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 five Net
Address 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 17 presents a transaction
flowchart of the five Net Address commands.
Read Net Address [33h or 39h]. This command allows the bus master to read the DS2770’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, followed by a 64-bit net address, allows the bus master to
specifically address one DS2770 on the 1-Wire bus. Only the DS2770 that exactly matches the 64-bit
address responds to the subsequent Function command. All other slaves ignore the Function command
and wait for a reset pulse. This command can be used with one or more slave devices on the bus.
A
Typ.
T
X
R
X
R
X
T
X
R
X
= Receive
T
X
= Transmit
BUS MASTER DS2770 1-WIRE PORT
I
PD
2.0V < V
PULLUP
< 5.5V