LM87

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SNAS034J – APRIL 2000 – REVISED MARCH 2013

Serial Bus Interface

Figure 6. (a) Serial Bus Write to the Internal Address Register followed by the Data Byte

Figure 7. (b) Serial Bus Write to the Internal Address Register Only

Figure 8. (c) Serial Bus Read from a Register with the Internal Address Register Preset to Desired

Location

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,

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 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

Communication with a Read consisting of the Serial Bus Interface 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 power up. If ADD is tied to ground the

value for XY is 10. If ADD is tied to Vcc XY will be set to 01. If ADD is not connected, XY will be 00. XY = 11 is

not a possible combination.

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