Datasheet

DS2408
7 of 39
Figure 2. HIERARCHICAL STRUCTURE FOR 1-Wire PROTOCOL
1-Wire Net
Other
Devices
Bus
Master
Command
Level:
1-Wire ROM Function
Commands
DS2408-Specific
Control Function
Commands
DS2408
Available
Commands:
Read ROM
Match ROM
Search ROM
Skip ROM
Conditional Search
ROM
Overdrive Match
Overdrive Skip
Resume
Read PIO Registers
Channel Access Read
Channel Access Write
Write Conditional
Search Register
Reset Activity Latches
Data Field
Affected:
64-BIT ROM, RC-FLAG
64-BIT ROM, RC-FLAG
64-BIT ROM, RC-FLAG
RC-FLAG
64-BIT ROM, RC-FLAG, Port Status,
Cond. Search Settings,
64-BIT ROM, RC-FLAG, OD-Flag
RC-FLAG, OD-Flag
RC-FLAG
PIO Registers
Port Input Latches
Port Output Latches
Conditional Search Register
Activity Latches
Cmd
.
Codes:
33h
55h
F0h
CCh
ECh
69h
3Ch
A5h
F0h
F5h
5Ah
CCh
C3h
PARASITE POWER
The DS2408 can derive its power entirely from the 1-Wire bus by storing energy on an internal capacitor
during periods of time when the signal line is high. During low times the device continues to operate from
this “parasite” power source until the 1-Wire bus returns high to replenish the parasite (capacitor) supply.
If power is available, the V
CC
pin should be connected to the external voltage supply.
Figure 3. 64-BIT LASERED ROM
MSB
LSB
8-BIT
CRC CODE
48-BIT SERIAL NUMBER
8-BIT FAMILY
CODE (29h)
MSB LSB
MSB LSB
MSB LSB
64-BIT LASERED ROM
Each DS2408 contains a unique ROM code that is 64 bits long. The first 8 bits are a 1-Wire family code.
The next 48 bits are a unique serial number. The last eight bits are a CRC of the first 56 bits. See Figure 3
for details. The 1-Wire CRC is generated using a polynomial generator consisting of a shift register and
XOR gates as shown in Figure 4. The polynomial is X
8
+ X
5
+ X
4
+ 1. Additional information about the
Dallas 1-Wire Cyclic Redundancy Check is available in Application Note 27.