Achronix Speedster22i User Macro Guide Uživatelský manuál stáhnout pdf (Strana 182)

Memories LRAMFIFO

Speedster22i Macro Cell Library

AchronixSemiconductorProprietary PAGE 165

Table 6-56: Condition to Assert almost_full Flag based on afull_offset Parameter Assignment

Condition when

almost_full flag is asserted

Condition when almost_full flag is

deasserted

afull_offset or fewer empty loca-

tions remain in the FIFO.

There are at least (afull_offset +1) empty

locations remaining in the FIFO.

aempty_offset

The aempty_offset  parameter defines the word depth at which the FIFO almost_empty

changes.Thealmost_emptyflagmaybeusedtodeterminethenumberofblindreadsfromthe

FIFO that can be  performed without monitoring the empty flag.For example, if the

aempty_offset parameter is set to 7’h04 and the almost_empty fl

ag is deasseted, the user is

guaranteed that there are at leastfive words in the FIFO. The user may read all five words

withoutmonitoringtheemptyflagandbeguaranteedthatthesewordswillbereadfromthe

FIFO and the read_err flag will not be asserted. The default va

lue of the aempty_offset

parameteris7’h04,correspondingtofourorfewerwordsremainingintheFIFO.

Table 6-57: Condition to Assert almost_empty Flag based on aem

pty_offset Parameter Assignment

Condition when

almost_empty flag is

asserted

Condition when almost_empty flag is

deasse

rted

aempty_offset or fewer

words remain in the FIFO.

There are at least (aempty_offset +1) words in

the FIFO.

Status Flags

Empty Flag

TheEmpty(empty)flagisassertedaftertheFIFOisresetorwhenallofthedatahasbeenread

fromtheFIFO.TheEmptyflagissynchronoustotherdclkclockdomain.Furtherattemptsto

readtheFIFOwhentheEmptyflagisassertedwillbeblocked,theReadError(re

ad_err)flag

willbesetinthefollowingrdclkclockcycle,andtheReadPointerwillremainunchanged.

Almost Empty Flag

The Almost Empty (almost_empty) flag is asserted when there are aempty_offset or fewer

wordsremainingintheFIFO(SeeTable6‐57

: ConditiontoAssertalmost_emptyFlagbased

onaempty_offsetParameterAssignment).Thealmost_emptyfl

agmaybeusedtodetermine

thenumberofblindreadsfr

omtheFIFOthatcanbeperformedwithoutmonitoringtheempty

flag.Forexample,iftheaempty_offsetparameterissetto7’h04andthe almost_emptyflagis

deasserted,theuserisguaranteedthatthereareatleastfivewordsintheFIFO.Theusermay

read all fi

vewords without monitoring the empty flag and be  guaranteed that these words

willbereadfromtheFIFOandtheread_errflagwillnotbeasserted.TheAlmostEmptyflag

issynchronouswiththerdclkclockinput.

Full Flag

TheFull(full)flagisassertedwhenalloftheavailablelocationsoftheFIFOhavebeenwritten.

Itissynchronoustothewrclkclockdomain.FurtherattemptstowritetheFIFOwhentheFull

flag is asserted will be blocked, the Write Error (write_err) flag will be set in the f

ollowing

wrclkclockcycle,andtheWritePointerwillremainunchanged.

Almost Full Flag

The Almost Full (almost_full) flag is asserted when there are afull_offset or feweravailable

locationsremainingintheFIFO.Thealmost_fullflagmaybeusedtodeterminethenumber

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