Achronix Speedster22i User Macro Guide Bedienungsanleitung PDF herunterladen (Seite 152)

Memories BRAM80KFIFO

Speedster22i Macro Cell Library

AchronixSemiconductorProprietary PAGE 135

Read Error Flag

TheReadError(read_err)flagisassertedinthefollowingrdclkclockcyclewhentheusertries

to read the FIFO while the Empty Flag is high and the Enable Read When Empty

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

EnableReadWhenEmptyparameterissetto1’

b1.

Flag Latency

The empty, full, almost_empty, and  almost_full flags are calculated based on comparisons

betweentheFIFOwritepointerandtheFIFOreadpointer.Thewritepointerissynchronous

tothewrclkdomainandthereadpointerissynchronoustotherdclkdomain.Forthecaseof

anasynchronousFIFO,thewrclkandrdc

lkclocksresideindifferentclockdomains.

Table 6-35: FIFO Pointers and Status Flag Clock Domain Assignments

Flag Associated Clock Domain

FIFO Write Pointer wrclk

FIFO Read Pointer rdclk

empty flag rdclk

almost_empty flag rdclk

full flag wrclk

almost_full flag wrclk

Beforefl

agcalculationscanbemade,circuitryhastomakesurethatbothpointersareinthe

same clock domain as the flag for which the calculation is done.The Write Pointer

SynchronizerFigure6‐10: WriteP

ointerSynchronizerBlockDiagramandtheReadPointer

Synchronizer Figure 6‐11: Read Po

inter Synchronizer Block Diagram are used to transfer

eachofthepointersint

otheotherclockdomain.Inordertosynchronizeagivenpointertothe

opposite clock domain, a series of registers, whose depth is determined by the

wrptr_sync_stages and rdptr_sync_stages parameters, is used.The transfer of a pointer

through these registers adds additional delay to the flag calculat

ion. The versions of the

pointers used for flag calculations are shown below in Table 6‐36

: Pointers Used for FIFO

FlagCalcul

ations.

Table 6-36: Pointers Used for FIFO Flag Calculations

Flag

Write Pointer Used for

ag Calculation

Read Pointer Used for

Flag Calculation

empty flag Synchronized Write Pointer Read Pointer

almost_empty flag Synchronized Write Pointer Read Pointer

full flag Write Pointer Synchronized Read Pointer

almost_full flag Write Pointer Synchronized Read Pointer

Forexample,the emptyfl

agiscomputedfromtheSynchronizedWritePointerandtheRead

Pointer.The write pointer incurs an additional delay of two to five rdclk cyles (set by the

wrptr_sync_stages parameter) before it is used to calculate the empty flag.Therefore, the

emptyflagwillnottransitionfromtheemptytonon‐emptystateforami

nimumoftwordclk

cyclesafterthefirstwritetotheFIFOoccurs.Asimilardelayoccursforthealmost_emptyflag

aswell.Likewise,forthefullandalmost_fullflags,therearetwotofivewrclkcyclesofdelay

intheactual FIFOstatusduetothesynchronizedreadpo

inter.ForanasyncronousFIFO,the

calculationoftheflagsdoesnotimmediatelyreflectthestateoftheFIFO.Whilethisbehavior

isnormalfor asynchronousFIFOs,itshouldbenoted.AsynchronousFIFOhasonlyasingle

clock,sothereisnoclockdom

aincrossingrequired.AsynchronousFIFOhastheadvantage

that the flag calculation is immediate and precise.Table 6‐37: Empty and Al

most Empty

1 2 ... 147 148 149 150 151 152 153 154 155 156 157 ... 223 224

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