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DS2436Z

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DS2436
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1-WIRE CRC CODE Figure 6
1-WIRE BUS SYSTEM
The 1-Wire bus is a system which has a single bus master and one or more slaves. The DS2436 behaves
as a slave. All data is communicated LSB first. The discussion of this 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 only a single line by definition; 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 have open
drain or three-state outputs. The 1-Wire port of the DS2436 (DQ pin) is open drain with an internal circuit
equivalent to that shown in Figure 7. A multidrop bus consists of a 1-Wire bus with multiple slaves
attached. The 1-Wire bus requires a pullup resistor of approximately 5 kΩ.
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 for the transaction 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 this does not occur
and the bus is left low for more than 480 μs, all components on the bus will be reset.
TRANSACTION SEQUENCE
The protocol for accessing the DS2436 via the 1-Wire port is as follows:
Initialization
ROM Function Command
Memory Function Command
Transaction/Data
INITIALIZATION
All transactions on the 1-Wire bus begin with an initialization sequence. The initialization sequence
consists of a reset pulse transmitted by the bus master followed by presence pulse(s) transmitted by the
slave(s).
The presence pulse lets the bus master know that the DS2436 is on the bus and is ready to operate. For
more details, see the “1-Wire Signaling” section.
ROM FUNCTION COMMANDS
Once the bus master has detected a presence, it can issue one of the four ROM function commands. All
ROM function commands are 8 bits long. A list of these commands follows (refer to flowchart in Figure
5).
DS2436
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HARDWARE CONFIGURATION Figure 7
Read ROM [33h]
This command allows the bus master to read the DS2436’s 8-bit family code, unique 48-bit serial
number, and 8-bit CRC. This command can only be used if there is a single DS2436 on the bus. If more
than one slave is present on the bus, a data collision will occur when all slaves try to transmit at the same
time (open drain will produce a wired-AND result).
Match ROM [55h]
The match ROM command followed by a 64-bit ROM sequence allows the bus master to address a
specific DS2436 on a multidrop bus. Only the DS2436 that exactly matches the 64-bit ROM sequence
will respond to the following memory function command. All slaves that do not match the 64-bit ROM
sequence will wait for a reset pulse. This command can be used with a single or multiple devices on the
bus.
Skip ROM [CCh]
This command can save time in a single drop bus system by allowing the bus master to access the
memory functions without providing the 64-bit ROM code. If more than one slave is present on the bus
and a read command is issued following the Skip ROM command, data collision will occur on the bus as
multiple slaves transmit simultaneously (open drain pulldowns will produce a wired-AND result).
Search ROM [F0h]
When a system is initially brought up, the bus master might not know the number of devices on the 1-
Wire bus or their 64-bit ROM codes. The Search ROM command allows the bus master to use a process
of elimination to identify the 64-bit ROM codes of all slave devices on the bus.
Example of a ROM Search
The ROM search process is the repetition of a simple, three-step routine: read a bit, read the complement
of the bit, then write the desired value of that bit. The bus master performs this simple, three-step routine
on each bit of the ROM. After one complete pass, the bus master knows the contents of the ROM in one
device. The remaining number of devices and their ROM codes may be identified by additional passes.
The following example of the ROM search process assumes four different devices are connected to the
same 1-Wire bus. The ROM data of the four devices is as shown:
ROM1 00110101...
ROM2 10101010...
ROM3 11110101...
ROM4 00010001...
DS2436
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The search process is as follows:
1. The bus master begins the initialization sequence by issuing a reset pulse. The slave devices respond
by issuing simultaneous presence pulses.
2. The bus master will then issue the search ROM command on the 1-Wire bus.
3. The bus master reads a bit from the 1-Wire bus. Each device will respond by placing the value of the
first bit of their respective ROM data onto the 1-Wire bus. ROM1 and ROM4 will place a 0 onto the
1-Wire bus, i.e., pull it low. ROM2 and ROM3 will place a one onto the 1-Wire bus by allowing the
line to stay high. The result is a logical AND of all devices on the line, therefore the bus master sees a
0. The bus master reads another bit. Since the search ROM data command is being executed, all of the
devices on the 1-Wire bus respond to this second read by placing the complement of the first bit of
their respective ROM data onto the 1-Wire bus. ROM1 and ROM4 will place a 1 onto the 1-Wire,
allowing the line to stay high. ROM2 and ROM3 will place a 0 onto the 1-Wire; thus it will be pulled
low. The bus master again observes a 0 for the complement of the first ROM data bit. The bus master
has determined that there are some devices on the 1-Wire bus that have a 0 in the first position and
others that have a 1.
The data obtained from the two reads of the three-step routine have the following interpretations:
00 There are still devices attached which have conflicting bits in this position.
01 All devices still coupled have a 0 bit in this bit position.
10 All devices still coupled have a 1 bit in this bit position.
11 There are no devices attached to the 1-Wire bus.
4. The bus master writes a 0. This deselects ROM2 and ROM3 for the remainder of this search pass,
leaving only ROM1 and ROM4 connected to the 1-Wire bus.
5. The bus master performs two more reads and receives a 0 bit followed by a 1-bit. This indicates that
all devices still coupled to the bus have 0s as their second ROM data bit.
6. The bus master then writes a 0 to keep both ROM1 and ROM4 coupled.
7. The bus master executes two reads and receives two 0-bits. This indicates that both 1-bits and 0-bits
exist as the third bit of the ROM data of the attached devices.
8. The bus master writes a 0 bit. This deselects ROM1 leaving ROM4 as the only device still connected.
9. The bus master reads the remainder of the ROM bits for ROM4 and continues to access the part if
desired. This completes the first pass and uniquely identifies one part on the 1-Wire bus.
10. The bus master starts a new ROM search sequence by repeating steps 1 through 7.
11. The bus master writes a 1 bit. This deselects ROM4, leaving only ROM1 still coupled.
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DS2436Z

Mfr. #:
Buy DS2436Z
Manufacturer:
Maxim Integrated
Description:
Battery Management
Lifecycle:
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