Datasheet

DS1822-PAR
7 of 19
proceeding if the DS1822-PAR CRC (ROM or scratchpad) does not match the value generated by the bus
master.
The equivalent polynomial function of the CRC (ROM or scratchpad) is: CRC = X
8
+ X
5
+ X
4
+ 1
The bus master can recalculate the CRC and compare it to the CRC values from the DS1822-PAR using
the polynomial generator shown in Figure 8. This circuit consists of a shift register and XOR gates, and
the shift register bits are initialized to 0. Starting with the least significant bit of the ROM code or the
least significant bit of byte 0 in the scratchpad, one bit at a time should shifted into the shift register. After
shifting in the 56
th
bit from the ROM or the most significant bit of byte 7 from the scratchpad, the
polynomial generator will contain the re-calculated CRC. Next, the 8-bit ROM code or scratchpad CRC
from the DS1822-PAR must be shifted into the circuit. At this point, if the re-calculated CRC was correct,
the shift register will contain all 0s. Additional information about the Dallas 1-Wire cyclic redundancy
check is available in Application Note 27 entitled Understanding and Using Cyclic Redundancy Checks
with Dallas Semiconductor Touch Memory Products.
CRC GENERATOR Figure 8
1-WIRE BUS SYSTEM
The 1-Wire bus system uses a single bus master to control one or more slave devices. The DS1822-PAR
is always a slave. When there is only one slave on the bus, the system is referred to as a “single-drop”
system; the system is “multidrop” if there are multiple slaves on the bus.
All data and commands are transmitted least significant bit first over the 1-Wire bus.
The following discussion of the 1-Wire bus system is broken down into three topics: hardware
configuration, transaction sequence, and 1-Wire signaling (signal types and timing).
HARDWARE CONFIGURATION
The 1-Wire bus has by definition only a single data line. Each device (master or slave) interfaces to the
data line via an open drain or 3-state port. This allows each device to “release” the data line when the
device is not transmitting data so the bus is available for use by another device. The 1-Wire port of the
DS1822-PAR (the DQ pin) is open drain with an internal circuit equivalent to that shown in Figure 9.
The 1-Wire bus requires an external pullup resistor of approximately 5 kΩ; thus, the idle state for the 1-
Wire bus is high. If for any reason a transaction needs to be suspended, the bus must be left in the idle
state if the transaction is to resume. Infinite recovery time can occur between bits so long as the 1-Wire
bus is in the inactive (high) state during the recovery period. If the bus is held low for more than 480μs,
all components on the bus will be reset. In addition, to assure that the DS1822-PAR has sufficient supply
current during temperature conversions, it is necessary to provide a strong pullup (such as a MOSFET) on
the 1-Wire bus whenever temperature conversions are taking place (as described in the PARASITE
POWER section).
(MSB) (LSB)
XOR XOR
XOR
INPUT