2.2 Serial Bus Interface

When using the Serial Bus Interface a write will always

consist of the LM78 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 be at the

desired Address, simply read the LM78 with the Serial

Bus Interface Address byte, followed by the data byte

read from the LM78.

2.

If the Internal Address Register value is unknown, write

to the LM78 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 LM78.

In all other respects the LM78 functions identically for Serial

Bus communications as it does for ISA communications.

Auto-Increment does not operate. When writing to or reading

from a Register which Auto-Increments with ISA communi-

cations, the Register must be manually incremented for

Serial Bus communications.

The default power on Serial Bus address for the LM78 is:

0101101 binary. This address can be changed by writing any

desired value to the Serial Bus address register, which can

be done either via the ISA or Serial Bus. During and Serial

Bus communication on the BUSY bit (bit 7) in the address

register at x5h will be high, and any ISA activity in that

situation should be limited to reading port x5h only.

All of these communications are depicted in the Serial Bus

Interface Timing Diagrams as shown in

Figure 7.

DS012873-10

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

DS012873-11

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

DS012873-12

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

FIGURE 7. Serial Bus Timing

www.national.com

15