User Macro Guide

Memories BRAM80KFIFO
Speedster22i Macro Cell Library
AchronixSemiconductorProprietary PAGE 137
FIFO Operational Modes
TheFIFOmacrosupportsbothsingleclocksynchronous(sameclockconnectedtowrclkand
rdclkinputswithout anyphaseoffsetbetweenthetwoclocks) anddualclock asynchronous
(twounrelatedclocksortworelatedclocks)modesofoperation.Forsynchronousoperation,
both the wrclk and rdclk  inputs must be connected  to
 the same clock net.Phase offsets
between the two clocks in synchronous mode is not allowed.For asynchronous mode, the
usermayconnectthewrclkandrdclkinputstotwodifferentclocks.TheFIFOwilltreatthe 
twoclocksasiftheyareunrelated.
Asynchronous FIFO Operation
When the FIFO is configured as an asynchronous FIFO (sync_mode = 1’b0), there are two
modesofoperationavailable.ThetwomodesoftheFIFOiscontrolledbytheFirstWordFall
Through(fwft)parameter.ThisparametercontrolswhatisontheoutputoftheFIFOafterthe
firstwordhas
beenwri tten toaFIFOthatwaspreviouslyinanemptystate.Forasynchronous
operation,theusermustsettheen_out_regparameterto1’b1.
Asychronous FIFO Standard Mode (sync_mode = 1’b0, fwft = 1’b0)
Afteraresetoperation,orafterthelastwordhasbeenreadfromtheFIFO,theFIFOwillbein
an empty state as indicated by a high level on the empty flag.When the FIFO in set to
standardmode(fwft= 1’b0),theoutputoftheFIFOremainsunchanged
afterthefirstwriteto
a FIFO in the empty state.After the first writeoperation the empty flag will be deasserted
indicating that there isdatain the FIFOthatmaybe read.The usermustread theFIFOby
settingtherdenhighatwhichtimethefirst
wordwrittenintotheFIFOwillbeavailableatthe
FIFO outputs at the next rising edge of the rdclk input.Each subsequent read operation
updatestheFIFOoutputswiththe nextstoreddatawordifitisavailable(emptyflag=false).
Asychronous FIFO First Word Fall Through Mode (sync_mode = 1’b0, fwft = 1’b1)
Afteraresetoperation,orafterthelastwordhasbeenreadfromtheFIFO,theFIFOwillbein
anemptystateasindicatedbyahighlevelontheemptyflag.WhentheFIFOinsettoFirst
WordFallThroughmode(fwft=1’b1),theoutputofthe
FIFOwillbeupdatedafterthefirst
write to a FIFO in the empty state.After the first write operation the empty flag will be
deasserted indicating that there is data in the FIFO that may be read.In First Word Fall
Throughmode,thefirstwordwrittenintothe
FIFOisplacedonthe FIFOoutputsbeforethe
firstreadoperation occurs.Eachsubsequent read operationupdatesthe FIFOoutputswith
thenextstoreddatawordifitisavailable(emptyflag=1’b0).NotethatwhenselectingFirst
Word Fall Through mode, the FIFO must be configured for asynchronous
 operation
(sync_mode = 1’b0).First Word Fall Through mode is not supported when the FIFO is
configuredforsynchronousFIFOoperation(sync_mode=1’b1).
Synchronous FIFO Operation
The synchronous FIFO mode supports the standard mode.The First Word Fall Through
modeisnotsupportedwhensynchronous FIFOoperationisselected.Theusermustnotset
thefwftparameterto1’b1whensynchronousFIFOoperationisselected.Theusermayeither
allowtheFIFOtopickupthedefault
valueofthefwftparameter(1’b0)orsetitto1’b0.
Synchronous FIFO Standard Mode (sync_mode = 1’b1)
Thesynchronous FIFO standardmodehas the advantagethatthereisnolatencyin theflag
calculations, so the  flags represent the exact state of the FIFO.In sycnchronous mode, the
Reset
Output
“regce” 1 1

reg_srval
Operation regce_priority rstreg outregce rdclk dout