2.2 Serial Bus Interface

The Serial Bus control lines consists of the SDA (serial data),

SCL (serial clock) and A0-A1 (address) pins. The LM80 can

only operate as a slave. The SCL line only controls the serial

interface, all other clock functions within LM80 such as the

ADC and fan counters are done with a separate asynchro-

nous internal clock.

When using the Serial Bus Interface a write will always con-

sist of the LM80 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 be at the de-

sired Address, simply read the LM80 with the Serial Bus

Interface Address byte, followed by the data byte read

from the LM80.

2.

If the Internal Address Register value is unknown, write

to the LM80 with the Serial Bus Interface Address byte,

followed by the Internal Address Register byte. Then re-

start the Serial Communication with a Read consisting of

the Serial Bus Interface Address byte, followed by the

data byte read from the LM80.

The default power on Serial Bus address for the LM80 is:

0101(A2)(A1)(A0) binary, where A0-A2 reflect the state of

the pins defined by the same names.

All of these communications are depicted in the Serial Bus

Interface Timing Diagrams as shown in

Figure 5.

DS100040-8

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

DS100040-9

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

DS100040-10

(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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