Rainbow Electronics DS2781 - Manual (Page 24)

DS2781: Two-Cell Standalone Fuel Gauge IC
24 of 30
Application Note 27, Understanding and Using Cyclic Redundancy Checks with Dallas Semiconductor Touch
Memory Products. (This application note can be found on the Maxim/Dallas Semiconductor website at
www.maxim-ic.com.)
In the circuit in Figure 19, the shift register bits are initialized to 0. Then, starting with the least significant bit 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
number is entered. After the 48th bit of the serial number has been entered, the shift register contains the CRC
value.
Figure 19. 1-Wire CRC Generation Block Diagram
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 tri-state output drivers. The DS2781 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, 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
W. 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
ms (16ms for overdrive speed), slave devices on the bus begin to interpret the low
period as a reset pulse, effectively terminating the transaction.
The DS2781 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 transaction with a reset
pulse. All 1-Wire devices on a multinode bus must operate at the same communication speed for proper operation.
1-Wire timing for both standard and overdrive speeds are listed in the Electrical Characteristics: 1-Wire Interface
tables.
Figure 20. 1-Wire Bus Interface Circuitry
0.2 mA
(typ)
100 W
MOSFET
TX
RX Rx
Tx
RX = RECEIVE
TX = TRANSMIT
Vpullup
(2.0V to 5.5V)
4.7k W
BUS MASTER DS2781 1-WIRE PORT
MSb
XOR XOR
LSb
XOR
INPUT