10

256 x 36, 1,024 x 36

COMMERCIAL TEMPERATURERANGE

A HIGH on the BE/FWFT input when the Reset (RS1) input goes

from LOW to HIGH will select a Big-Endian arrangement. In this case, the

most significant byte (word) of the long word written to Port A will be read

from Port B first; the least significant byte (word) of the long word written

to Port A will be read from Port B last.

ALOWontheBE/FWFTinputwhentheReset(RS1)inputgoesfromLOW

to HIGH will select a Little-Endian arrangement. In this case, the least

significant byte (word) of the long word written to Port A will be read from

Port B first; the most significant byte (word) of the long word written to Port

A will be read from Port B last. Refer to Figure 2 for an illustration of the

BE function. See Figure 3 (Reset) for an Endian select timing diagram.

— TIMING MODE SELECTION

AfterReset,theFWFTselectfunctionisactive,permittingachoicebetween

two possible timing modes: IDT Standard mode or First Word Fall Through

(FWFT) mode. Once the Reset (RS1) input is HIGH, a HIGH on the BE/

FWFT

input during the next LOW-to-HIGH transition of CLKA and CLKB

will select IDT Standard mode. This mode uses the Empty Flag function

(EF) to indicate whether or not there are any words present in the FIFO

memory. It uses the Full Flag function (FF) to indicate whether or not the

FIFO memory has any free space for writing. In IDT Standard mode,

every word read from the FIFO, including the first, must be requested

using a formal read operation.

OncetheReset(RS1)inputisHIGH,aLOWontheBE/FWFTinputduring

the next LOW-to-HIGH transition of CLKA and CLKB will select FWFT

mode. This mode uses the Output Ready function (OR) to indicate

whether or not there is valid data at the data outputs (B0-B35). It also uses

the Input Ready function (IR) to indicate whether or not the FIFO memory

has any free space for writing. In the FWFT mode, the first word written

to an empty FIFO goes directly to data outputs, no read request

necessary. Subsequent words must be accessed by performing a formal

read operation.

Following Reset, the level applied to the BE/FWFT input to choose

the desired timing mode must remain static throughout FIFO operation.

Refer to Figure 3 (Reset) for a First Word Fall Through select timing

diagram.

PROGRAMMING THE ALMOST-EMPTY AND ALMOST-FULL FLAGS

Two registers in the IDT72V3623/72V3643 are used to hold the

offset values for the Almost-Empty and Almost-Full flags. The Almost-

Empty flag (AE) Offset register is labeled X and Almost-Full flag (AF)

Offset register is labeled Y. The offset registers can be loaded with preset

values during the reset of the FIFO, programmed in parallel using the

FIFO’s Port A data inputs, or programmed in serial using the Serial Data

(SD) input (see Table 1). SPM, FS0/SD, and FS1/SEN function the same

way in both IDT Standard and FWFT modes.

— PRESET VALUES

ToloadaFIFO’sAlmost-EmptyflagandAlmost-FullflagOffsetregisters

withoneofthethreepresetvalueslistedinTable1,theSerialProgramMode

(SPM)andatleastoneoftheflag-selectinputsmustbeHIGHduringtheLOW-

to-HIGH transition of the Reset input (RS1). For example, to load the

preset value of 64 into X and Y, SPM, FS0 and FS1 must be HIGH when

RS1

returns HIGH. For the relevant preset value loading timing diagram,

see Figure 3.

SIGNAL DESCRIPTION

RESET (RS1, RS2)

Afterpowerup,aResetoperationmustbeperformedbyprovidingaLOW

pulse to RS1 and RS2 simultaneously. Afterwards, the FIFO memory of

the IDT72V3623/72V3643 undergoes a complete reset by taking its

Reset(RS1andRS2)inputLOWforatleastfourPortAclock(CLKA)andfour

Port B clock (CLKB) LOW-to-HIGH transitions. The Reset inputs can switch

asynchronously to the clocks. A Reset initializes the internal read and

write pointers and forces the Full/Input Ready flag (FF/IR) LOW, the

Empty/Output Ready flag (EF/OR) LOW, the Almost-Empty flag (AE)

LOW, and the Almost-Full flag (AF) HIGH. A Reset (RS1) also forces the

Mailbox flag (MBF1) of the parallel mailbox register HIGH, and at the

same time the RS2 and MBF2 operate likewise. After a Reset, the FIFO’s

Full/Input Ready flag is set HIGH after two write clock cycles to begin

normal operation.

ALOW-to-HIGHtransitionontheFlFOReset(RS1)inputlatchesthevalue

of the Big-Endian (BE) input for determining the order by which bytes are

transferredthroughPortB.

ALOW-to-HIGHtransitionontheFlFOReset(RS1)inputalsolatchesthe

values of the Flag Select (FS0, FS1) and Serial Programming Mode (SPM)

inputs for choosing the Almost-Full and Almost-Empty offset programming

method ( for details see Table 1, Flag Programming, and Almost-Empty and

Almost-Full flag offset programming section). The relevant Reset timing

diagram can be found in Figure 3.

PARTIAL RESET (PRS)

The FIFO memory of the IDT72V3623/72V3643 undergoes a

limitedresetbytakingitsPartialReset(PRS)inputLOWforatleastfourPortA

clock (CLKA) and four Port B clock (CLKB) LOW-to-HIGH transitions. The

Partial Reset input can switch asynchronously to the clocks. A Partial

Reset initializes the internal read and write pointers and forces the Full/

Input Ready flag (FF/IR) LOW, the Empty/Output Ready flag (EF/OR)

LOW, the Almost-Empty flag (AE) LOW, and the Almost-Full flag (AF)

HIGH. A Partial Reset also forces the Mailbox flag (MBF1, MBF2) of the

parallel mailbox register HIGH. After a Partial Reset, the FIFO’s Full/Input

Ready flag is set HIGH after two Write Clock cycles to begin normal

operation. See Figure 4, Partial Reset (IDT Standard and FWFT Modes)

for the relevant timing diagram.

Whateverflagoffsets,programmingmethod(parallelorserial),andtiming

mode(FWFTorIDTStandardmode)arecurrentlyselectedatthetimeaPartial

Resetisinitiated,thosesettingswillberemainunchangeduponcompletionof

the reset operation. A Partial Reset may be useful in the case where

reprogramming a FIFO following a Reset would be inconvenient.

BIG-ENDIAN/FIRST WORD FALL THROUGH (BE/FWFT)

— ENDIAN SELECTION

This is a dual purpose pin. At the time of Reset, the BE select function

is active, permitting a choice of Big- or Little-Endian byte arrangement for

data read from Port B. This selection determines the order by which bytes

(or words) of data are transferred through this port. For the following

illustrations, assume that a byte (or word) bus size has been selected for

Port B. (Note that when Port B is configured for a long word size, the Big-

NOTE:

1. Either a HIGH or LOW can be applied to a “don’t care” input with no change to the logical operation of the FIFO. Nevertheless, inputs that are temporarily “don’t care” (along with unused

inputs) must not be left open, rather they must be either HIGH or LOW.