Rainbow Electronics DS2890 - Manual (Page 15)

DS2890
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FIGURE 18. BUS MASTER WITH DS2480B DRIVER
VDD
POL
RXD
TXD
VPP
1-W
GND
NC
DS2480B
+5V
to 1-WIRE connection
of DS2890
serial in
serial out
HOST CPU
serial
port
BUS MASTER
TRANSACTION SEQUENCE
The protocol for accessing the DS2890 via the 1-Wire port is as follows:
· Initialization
· ROM Function Command
· Potentiometer 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 DS2890 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 eight ROM function commands that
the DS2890 supports. All ROM function commands are 8 bits long. A list of these commands follows
(refer to Figure 23 and Figure 24 flowcharts):
READ ROM [33H]
This command allows the bus master to read the DS2890’s 8-bit family code, unique 48-bit serial
number, and 8-bit CRC. This command should 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 48-bit serial number
read by the master will be invalid.
MATCH ROM [55H]
The match ROM command, followed by a 64-bit ROM sequence, allows the bus master to address a
specific DS2890 on a multi-drop bus. Only the DS2890 that exactly matches the 64-bit ROM sequence
will respond to the following memory function command. All slaves that do not match the 64-bit ROM
sequence will wait for a reset pulse. This command can be used with a single or multiple devices on the
bus.