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

the desired Address, simply read the LM81 with the Se-

rial Bus Interface Address byte, followed by the data

byte read from the LM81.

2.

If the Internal Address Register value is unknown, or if it

is not the desired value, write to the LM81 with the Serial

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.

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.

DS100072-8

(a) Serial Bus Write to the Internal Address Register followed by the Data Byte

DS100072-9

(b) Serial Bus Write to the Internal Address Register Only

DS100072-10

(c) Serial Bus Read from a Register with the Internal Address Register Preset to Desired Location

FIGURE 5. Serial Bus Timing

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