OPERATIONAL DESCRIPTION (cont’d)

Retransmit

The FIFO can be made to reread previously-read data

by means of the Retransmit function. A retransmit opera-

tion is initiated by pulsing the RT input LOW. Both R and

W must be deasserted (HIGH) for the duration of the

retransmit pulse. The FIFO’s internal read-address

pointer is reset to point to location zero, the first physical

memory location, while the internal write-address

pointer remains unchanged.

After a retransmit operation, those data words in the

region in between the read-address pointer and the

write-address pointer may be reaccessed by subsequent

read operations. A retransmit operation may affect the

state of the status flags FF, HF, and EF, depending on

the relocation of the read-address pointer. There is no

restriction on the number of times that a block of data

within an LH540203 may be read out, by repeating the

retransmit operation and the subsequent read operations.

The maximum length of a data block which may be

retransmitted is 2048 words. Note that if the write-address

pointer ever ‘wraps around’ (i.e., passes location zero

more than once) during a sequence of retransmit opera-

tions, some data words will be lost.

The Retransmit function is not available when the

LH540203 is operating in depth-cascaded mode,

because the FL/RT control pin must be used for first-load

selection rather than for retransmission control.

Table 1. Grouping-Mode Determination

During a Reset Operation

XI

FL/

RT

MODE

XO/HF

USAGE

XI

USAGE

FL/RT

USAGE

H

Cascaded

Slave

2

XO

XI

FL

L

Cascaded

Master

2

XO

XI

FL

L

X

Standalone

HF

(none)

RT

NOTES:

2. The terms ‘master’ and ‘slave’ refer to operation in depth-cas-

caded grouping mode.

3. H = HIGH; L = LOW; X = Don’t Care.

Table 2. Expansion-Pin Usage According to

Grouping Mode

I/O

PIN

STANDALONE

CASCADED

MASTER

CASCADED

SLAVE

I

XI

Grounded

From XO

(n-1st

FIFO)

From XO

(n-1st

FIFO)

O

XO/HF

Becomes

HF

To XI

(n+1st

FIFO)

To XI

(n+1st

FIFO)

I

FL/RT

Becomes

RT

Grounded

(Logic

LOW)

Logic

HIGH

Table 3. Status Flags

NUMBER OF UNREAD DATA

WORDS PRESENT WITHIN

2048

× 9 FIFO

FF

HF

EF

0H

H

L

1 to 1024

H

H

H

1025 to 2047

H

L

H

2048

L

L

H

LH540203

CMOS 2048

× 9 Asynchronous FIFO

4