DS2778 Maxim Integrated Products, DS2778 Datasheet - Page 36

• Manufacturer Part Number: DS2778
• Description: 2-Cell Stand-Alone Li+ Fuel-Gauge IC
• Manufacturer: Maxim Integrated Products

2-Cell, Stand-Alone, Li+ Fuel-Gauge IC with
Protector and Optional SHA-1 Authentication
also used to provide verification of error-free data trans-
fer as each EEPROM page is sent to the master during
a Read Data/Generate CRC command and for the
8 bytes of information in the status memory field.
In each case where a CRC is used for data transfer val-
idation, the bus master must calculate the CRC value
using the same polynomial function and compare the
calculated value to the CRC either stored in the
DS2775/DS2776 net address or computed by the
DS2775/DS2776. The comparison of CRC values and
decision to continue with an operation are determined
entirely by the bus master. There is no circuitry in the
DS2775/DS2776 that prevents a command sequence
from proceeding if the stored or calculated CRC from
the DS2775/DS2776 and the calculated CRC from the
host do not match.
Because the 1-Wire bus has only a single line, it is
important that each device on the bus be able to drive
it at the appropriate time. To facilitate this, each device
attached to the 1-Wire bus must connect to the bus with
open-drain or three-state output drivers. The DS2775/
DS2776 use an open-drain output driver as part of the
bidirectional interface circuitry shown in Figure 24. If a
bidirectional pin is not available on the bus master,
separate output and input pins can be connected
together.
The 1-Wire bus must have a pullup resistor at the bus
master end. A value of between 2kΩ and 5kΩ is recom-
mended. 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. Note that if the bus is left low for more
than t
Figure 24. 1-Wire Bus Interface Circuitry
36
______________________________________________________________________________________
LOW0
, slave devices on the bus begin to interpret
Hardware Configuration
Rx
Tx
BUS MASTER
Tx = TRANSMIT
(2.0V TO 5.5V)
Rx = RECEIVE
V
PULLUP
4.7kΩ
the low period as a reset pulse, effectively terminating
the transaction.
The protocol for accessing the DS2775/DS2776
through the 1-Wire port is as follows:
• Initialization
• Net Address Commands
• Function Command(s)
• Data Transfer (not all commands have data transfer)
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 DS2775/DS2776 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.
Once the bus master has detected the presence of one
or more slaves, it can issue one of the net address
commands described in the following sections. The
name of each Net Address command (ROM command)
is followed by the 8-bit op code for that command in
square brackets.
This command allows the bus master to read the
DS2775/DS2776’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).
100nA
(TYP)
DS2775/DS2776 1-Wire PORT (DQ)
100Ω MOSFET
Transaction Sequence
Net Address Commands
Tx
Rx
Read Net Address [33h]
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Initialization