Rainbow Electronics DS2788 - Manual (Page 24)

D
S
2
7
8
8 separate output and input pins can be connected
together.
The 1-Wire bus must have a pullup resistor at the bus-
master end of the bus. For short line lengths, the value
of this resistor should be approximately 5k Ω. The idle
state for the 1-Wire bus is high. If, for any reason, a bus
transaction must be suspended, the bus must be left in
the idle state to properly resume the transaction later. If
the bus is left low for more than 120 µs (16µs for over-
drive speed), slave devices on the bus begin to inter-
p r e t t h e l o w p e r i o d a s a r e s e t p u l s e , e f f e c t i v e l y
terminating the transaction.
The DS2788 can operate in two communication speed
modes, standard and overdrive. The speed mode is
determined by the input logic level of the OVD pin with
a logic 0 selecting standard speed and a logic 1
selecting overdrive speed. The OVD pin must be at a
stable logic level of 0 or 1 before initializing a transac-
tion with a reset pulse. All 1-Wire devices on a multin-
ode bus must operate at the same communication
speed for proper operation. 1-Wire timing for both stan-
dard and overdrive speeds are listed in the Electrical
Characteristics: 1-Wire Interface tables.
Transaction Sequence
The protocol for accessing the DS2788 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 initial-
ization sequence consisting of a reset pulse transmitted
by the bus master, followed by a presence pulse simul-
taneously transmitted by the DS2788 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 net address com-
mands described in the following paragraphs. The name
of each ROM command is followed by the 8-bit op code
for that command in square brackets. Figure 21 presents
a transaction flowchart of the net address commands.
Read Net Address [33h or 39h]. This command allows
the bus master to read the DS2788’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 pre-
sent, a data collision occurs when all slaves try to trans-
m i t a t t h e s a m e t i m e ( o p e n d r a i n p r o d u c e s a
wired-AND result). The RNAOP bit in the Status register
selects the op code for this command, with RNAOP = 0
indicating 33h and RNAOP = 1 indicating 39h.
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
24 ______________________________________________________________________________________
Rx
BUS MASTER
Rx = RECEIVE
Tx = TRANSMIT
4.7k Ω
0.2 µA
(TYP)
V PULLUP
(2.0V TO 5.5V)
Tx
Rx
Tx
100 Ω MOSFET
DS2788 1-Wire PORT
Figure 20. 1-Wire Bus Interface Circuitry