2.2 Serial Bus Interface

The Serial Bus control lines consist of the SMBData (serial

data), SMBCLK (serial clock) and ADD (address) pin. The

LM87 can operate only as a slave. The SMBCLK line only

controls the serial interface, all other clock functions within

LM87 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

consist of the LM87 Serial Bus Interface Address byte, fol-

lowed 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 LM87 with the

Serial Bus Interface Address byte, followed by the data

byte read from the LM87.

2.

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

is not the desired value, write to the LM87 with the Serial

Bus Interface Address byte, followed by the Internal

Address Register byte. Then restart the Serial Commu-

nication with a Read consisting of the Serial Bus Inter-

face Address byte, followed by the data byte read from

the LM87.

The Serial Bus address of the LM87 is set to 010 11(X)(Y).

All bits, except for X and Y, are fixed and cannot be changed.

The values for X and Y are set by the state of the ADD pin on

10099508

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

10099509

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

10099510

(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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