LM78 datasheet(12/31 Pages) NSC

12 / 31 page

Functional Description (Continued)

2.0 INTERFACE

The LM78 only decodes the three lowest address bits on the

ISA bus. Referring to the ISA bus timing diagrams in

Figure

1 and Figure 2, the Chip Select Input, CS, should be taken

low by external address decoder circuitry to access the

LM78. The LM78 decodes the following base addresses:

-Port x0h: Power On Self Test codes from ISA bus.

-Port x4h: Power On Self Test codes from ISA bus.

-Port x5h: The LM78s Internal Address Register

-Port x6h: Data Register

IORD is the standard ISA bus signal that indicates to the

LM78 that it may drive data on to the ISA data bus.

IOWR is the standard ISA command to the LM78 that it may

latch data from the ISA bus.

SYSCLK is the standard ISA SYSCLK, typically 8.33 MHz.

This clock is used only for timing of the ISA interface of the

LM78. All other clock functions within LM78 such as the ADC

and fan counters are done with a separate asynchronous

internal clock.

A typical application designed to utilize the POST RAM

would decode the LM78 to the address space starting at

80h, which is where POST codes are output to. Otherwise,

the LM78 can be decoded into a different desired address

space.

To communicate with an LM78 Register, first write the ad-

dress of that Register to Port x5h. Read or write data from or

to that register via Port x6h. A write will take IOWR low, while

a read will take IORD low.

If the Serial Bus Interface and ISA bus interface are used

simultaneously there is the possibility of collision. To prevent

this from occurring in applications where both interfaces are

used, read port x5h and if the Most Significant Bit, D7, is

high, ISA communication is limited to reading port x5h only

until this bit is low. A Serial Bus communication occurring

while ISA is active will not be a problem, since even a single

bit of Serial Bus communication requires 10 microseconds,

in comparison to less than a microsecond for an entire ISA

communication.

LM78 Datasheet(PDF) 12 Page - National Semiconductor (TI)