Rainbow Electronics DS2788 - Manual (Page 23)

1-Wire Bus System
The 1-Wire bus is a system that has a single bus mas-
ter and one or more slaves. A multidrop bus is a 1-Wire
bus with multiple slaves. A single-drop bus has only
one slave device. In all instances, the DS2788 is a
slave device. The bus master is typically a micro-
processor in the host system. The discussion of this
bus system consists of four topics: 64-bit net address,
hardware configuration, transaction sequence, and
1-Wire signaling.
64-Bit Net Address
Each DS2788 has a unique, factory-programmed
1-Wire net address that is 64 bits in length. The first
eight bits are the 1-Wire family code (32h for DS2788).
The next 48 bits are a unique serial number. The last
eight bits are a cyclic redundancy check (CRC) of the
first 56 bits (see Figure 18). The 64-bit net address and
the 1-Wire I/O circuitry built into the device enable the
DS2788 to communicate through the 1-Wire protocol
detailed in the 1-Wire Bus System section.
CRC Generation
The DS2788 has an 8-bit CRC stored in the MSB of its
1-Wire net address. To ensure error-free transmission
of the address, the host system can compute a CRC
value from the first 56 bits of the address and compare
it to the CRC from the DS2788. The host system is
responsible for verifying the CRC value and taking
action as a result. The DS2788 does not compare CRC
values and does not prevent a command sequence
from proceeding as a result of a CRC mismatch. Proper
use of the CRC can result in a communication channel
with a very high level of integrity.
The CRC can be generated by the host using a circuit
consisting of a shift register and XOR gates as shown
in Figure 19, or it can be generated in software.
Additional information about the Dallas 1-Wire CRC is
available in Application Note 27: Understanding and
U s i n g C y c l i c R e d u n d a n c y C h e c k s w i t h D a l l a s
S e m i c o n d u c t o r i B u t t o n P r o d u c t s ( www.maxim-
ic.com/appnoteindex ).
In the circuit in Figure 19, the shift register bits are ini-
tialized to 0. Then, starting with the LSb of the family
code, one bit at a time is shifted in. After the 8th bit of
the family code has been entered, then the serial num-
ber is entered. After the 48th bit of the serial number
has been entered, the shift register contains the CRC
value.
Hardware Configuration
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
DS2788 uses an open-drain output driver as part of the
bidirectional interface circuitry shown in Figure 20. If a
bidirectional pin is not available on the bus master,
D
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Stand-Alone Fuel-Gauge IC with
LED Display Drivers
______________________________________________________________________________________ 23
8-BIT CRC 48-BIT SERIAL NUMBER
8-BIT FAMILY
CODE (32h)
MSb LSb
Figure 18. 1-Wire Net Address Format
MSb XOR XOR
XOR
INPUT
LSb
Figure 19. 1-Wire CRC Generation Block Diagram
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