Data words are read out from the LH540203’s output

port in precisely the same order that they were written in

at its input port; that is, according to a First-In, First Out

(FIFO) queue discipline. Since the addressing sequence

for a FIFO device’s memory is internally predefined, no

external addressing information is required for the opera-

tion of the LH540203 device.

Drop-in-replacement compatibility is maintained with

both larger sizes and smaller sizes of industry-standard

nine-bit asynchronous FIFOs. The only change is in the

number of internally-stored data words implied by the

states of the Full Flag and the Half-Full Flag.

The Retransmit (RT) control signal causes the internal

FIFO-memory-array read-address pointer to be set back

to zero, to point to the LH540203’s first physical memory

location, without affecting the internal FIFO-memory-

array write-address pointer. Thus, the Retransmit control

signal provides a mechanism whereby a block of data,

delimited by the zero physical address and the current

write-address-pointer value, may be read out

repeatedly

an arbitrary number of times. The only restrictions are that

neither the read-address pointer nor the write-address

pointer may ‘wrap around’ during this entire process, i.e.,

advance past physical location zero after traversing the

entire memory. The retransmit facility is not available

when an LH540203 is operating in a depth-expanded

configuration.

The Reset (RS) control signal returns the LH540203

to an initial state, empty and ready to be filled. An

LH540203 should be reset during every system power-up

sequence. A reset operation causes the internal FIFO-

memory-array write-address pointer, as well as the read-

address pointer, to be set back to zero, to point to the

LH540203’s first physical memory location. Any informa-

tion which previously had been stored within the

LH540203 is not recoverable after a reset operation.

Acascading (depth-expansion) scheme may be imple-

mented by using the Expansion In (XI) input signal and

the Expansion Out (XO/HF) output signal. This allows a

deeper ‘effective FIFO’ to be implemented by using two

or more LH540203 devices, without incurring additional

latency (‘fallthrough’ or ‘bubblethrough’) delays, and with-

out the necessity of storing and retrieving any given data

word more than once. In this cascaded operating mode,

one LH540203 device must be designated as the ‘first-

load’ or ‘master’ device, by grounding its First-Load

(FL/RT) control input; the remaining LH540203 devices

are designated as ‘slaves,’ by tying their FL/RT inputs

HIGH. Because of the need to share control signals on

pins, the Half-Full Flag and the retransmission capability

are not available for either ‘master’ or ‘slave’ LH540203

devices operating in cascaded mode.

FUNCTIONAL DESCRIPTION (cont’d)

DATA OUTPUTS

FLAG

LOGIC

WRITE

POINTER

READ

POINTER

DATA INPUTS

DUAL-PORT

RAM

ARRAY

2048 x 9

EF

FF

. . .

540203-1

INPUT

PORT

CONTROL

R

W

RESET

LOGIC

RS

OUTPUT

PORT

CONTROL

EXPANSION

LOGIC

XO/HF

XI

FL/RT

Figure 3.

LH540203 Block Diagram

LH540203

CMOS 2048

× 9 Asynchronous FIFO

2