LM81CIMT-31 National Semiconductor, LM81CIMT-31 Datasheet - Page 12

• Manufacturer Part Number: LM81CIMT-31
• Description: Serial Interface ACPI-Compatible Microprocessor System
• Manufacturer: National Semiconductor

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Functional Description
2.2 Serial Bus Interface
The Serial Bus control lines consist of the SMBData (serial
data), SMBCLK (serial clock) and A0-A1 (address) pins. The
LM81 can operate only as a slave. The SMBCLK line only
controls the serial interface, all other clock functions within
LM81 such as the ADC and fan counters are done with a
separate asynchronous internal clock.
When using the Serial Bus Interface a write will always con-
sist of the LM81 Serial Bus Interface Address byte, followed
by the Internal Address Register byte, then the data byte.
There are two cases for a read:
1. If the Internal Address Register is known to already be at
2. If the Internal Address Register value is unknown, or if it
the desired Address, simply read the LM81 with the Se-
rial Bus Interface Address byte, followed by the data
byte read from the LM81.
is not the desired value, write to the LM81 with the Serial
(c) Serial Bus Read from a Register with the Internal Address Register Preset to Desired Location
(a) Serial Bus Write to the Internal Address Register followed by the Data Byte
(b) Serial Bus Write to the Internal Address Register Only
(Continued)
FIGURE 5. Serial Bus Timing
12
The default Serial Bus address of the LM81 is set to 010
11(A1)(A0). All bits, except for A0 and A1, can be changed by
writing to the Serial Bus address register. A0 and A1 will al-
ways reflect the state of the A0 and A1 input pins.
All of these communications are depicted in the Serial Bus
Interface Timing Diagrams as shown in Figure 5 .
Serial Bus Timeout can be initiated by holding the SMBCLK
and/or SMBData lines low for greater than t
max). Serial Bus Timeout resets the serial bus interface cir-
cuitry to the idle state and readies the LM81 for a new serial
bus communication.
Bus Interface Address byte, followed by the Internal Ad-
dress Register byte. Then restart the Serial Communica-
tion with a Read consisting of the Serial Bus Interface
Address byte, followed by the data byte read from the
LM81.
DS100072-8
DS100072-10
DS100072-9
TIMEOUT
(35 ms