LH540204 Просмотр технического описания (PDF) - Sharp Electronics

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LH540204

CMOS 4096 × 9 Asynchronous FIFO

Sharp Electronics 

LH540204

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 LH540204 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 4096 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

LH540204 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

XI

FL/RT

USAGE USAGE USAGE

H1

H

Cascaded

Slave 2

H1

L

Cascaded

Master 2

XO XI

FL

XO XI

FL

L X Standalone

HF (none) RT

NOTES:

1. A reset operation forces XO HIGH for the nth FIFO, thus forcing XI

HIGH for the (n+1)st FIFO.

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

caded grouping mode.

3. H = HIGH; L = LOW; X = Don’t Care.

CMOS 4096 × 9 Asynchronous FIFO

Table 2. Expansion-Pin Usage According to

Grouping Mode

I/O

PIN

I XI

O XO/HF

I FL/RT

STANDALONE

CASCADED

MASTER

CASCADED

SLAVE

Grounded

From XO From XO

(n-1st

(n-1st

FIFO)

FIFO)

Becomes

HF

To XI

(n+1st

FIFO)

To XI

(n+1st

FIFO)

Becomes

RT

Grounded

(Logic

LOW)

Logic

HIGH

Table 3. Status Flags

NUMBER OF UNREAD DATA

WORDS PRESENT WITHIN

FF

HF

EF

4096 × 9 FIFO

0

H

H

L

1 to 2048

H

H

H

2049 to 4095

H

L

H

4096

L

L

H

4

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