Achronix Speedster22i User Macro Guide Manuale Utente Scaricare il pdf (Pagina 151)

Memories BRAM80KFIFO

Speedster22i Macro Cell Library

AchronixSemiconductorProprietary PAGE 134

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 depend upon the en_rd_when_empty

parameter.

Iftheen_rd_when_emptyparameterissetto1’b0,attemptstoreadtheFIFOwhen

the Empty flag is asserted will be blocked, the Read Error (read_err) flag will be set in the

following rdclk clock cycle, and the Read Pointer will remain unchanged. If the

en_rd_when_emptyparameterissetto

1’b1,attemptstoreadtheFIFOwhentheEmptyflagis

asserted will result in a valid read operation from the FIFO with the Read Pointer

incrementing by one word value.The user should disregard  the Read Error flag when the

en_rd_when_empty parameter is set to 1’b1.The use may keep

 t r ack of the Read Pointer

valuebymonitoringtherdcountoutputpins.

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6‐34: ConditiontoAssertalmost_emptyFlagbased

on afull_offset  ParameterAssignment).Thealmost_empty flag may be used to determine

thenumberofblindreadsfromtheFIFOthatcanbeperfromedwithoutmonitoringtheempty

flag.Forexample,iftheaempty_offsetparameterissetto17’h0004andthealmost_emptyflag

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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 depend upon the en_wr_when_full param eter.If the en_wr_when_full

parameter is set to

 1’b0, attempts to write the FIFO when the Full flag is asserted will be

blocked,theWriteError(writ e_err)flagwillbesetinthefollowingwrclkclockcycle,andthe

Write Pointer will remain unchanged. If the en_wr_when_full parameter is set to 1’b1,

attemptstowritetheFIFOwhen

theFullflagisassertedwillresultinavalidwriteoperation

to the FIFO with the Write Pointer incrementing by one word value.The user should

disregard theWriteErrorflag when the en_wr_when_full parameter issetto1’b1.Theuse

maykeeptrackoftheWritePointervalueby

monitoringthewrcountoutputpins.

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

ofblindwritestotheFIFOthatcanbemadewithoutmonitoringthefullflag.Forexample,if

theafull_offsetparameteris

setto17’h00004andthealmost_fullflagisdeasserted,theuseris

guaranteedthatthereareatleastfiveemptylocationsintheFIFO.Theusermaywriteallfive

wordswithoutmonitoringthefullflagandbeguaranteedthatthesewordswillbewrittento

theFIFOandthe

write_errflagwillnotbeasserted.TheAlmostFullflagissynchronouswith 

thewrclkclockinpu t.

Write Error Flag

TheWriteError(write_err) flag isassertedinthe following wrclkclock  cyclewhenthe user

tries to write the FIFO while the Full Flag is high and  the Enable Write When Full

(en_wr_when_full) parameter is set to 1’b0.The Write Error flag is undefined when the

EnableWriteWhenFull

parameterissetto1’b1.

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