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COMMERCIALANDINDUSTRIAL
TEMPERATURERANGES
IDT72V51436/72V51446/72V51456 3.3V, MULTI-QUEUE FLOW-CONTROL DEVICES
(16 QUEUES) 36 BIT WIDE CONFIGURATION 589,824, 1,179,648 and 2,359,296 bits
As mentioned, every queue within a multi-queue device has its own almost
fullstatus,whenaqueueisselectedonthewriteport,thisstatusisoutputviathe
PAFflag.ThePAFflagvalueforeachqueueisprogrammedduringmulti-queue
device programming (along with the number of queues, queue depths and
almost empty values). The
PAFoffsetvalue,m,forarespectivequeuecanbe
programmedtobeanywherebetween‘0’and‘D’,where‘D’isthetotalmemory
depthforthatqueue.The
PAFvalueofdifferentqueueswithinthesamedevice
can be different values.
When queue switches are being made on the write port, the
PAFflagoutput
will switch to the new queue and provide the user with the new queue status,
on the second cycle after a new queue selection is made, on the same WCLK
cycle that data can actually be written to the new queue. That is, a new queue
can be selected on the write port via the WRADD bus, WADEN enable and a
rising edge of WCLK. On the second rising edge of WCLK following a queue
selection,the
PAFflagoutputwillshowthefullstatusofthenewlyselectedqueue.
The
PAF is flag output is double register buffered, so when a write operation
occursatthealmostfullboundarycausingtheselectedqueuestatustogoalmost
full the
PAFwillgoLOW2WCLKcyclesafterthewrite.Thesameistruewhen
a read occurs, there will be a 2 WCLK cycle delay after the read operation.
So the
PAF flag delays are:
from a write operation to
PAF flag LOW is 2 WCLK + tWAF
The delay from a read operation to
PAFflagHIGHistSKEW2+WCLK+tWAF
Note, if tSKEW is violated there will be one added WCLK cycle delay.
The
PAFflagissynchronoustotheWCLKandalltransitionsofthePAFflag
occur based on a rising edge of WCLK. Internally the multi-queue device
monitorsandkeepsarecordofthealmostfullstatusforallqueues.Itispossible
that the status of a
PAFflagmaybechanginginternallyeventhoughthatflagis
not the active queue flag (selected on the write port). A queue selected on the
read port may experience a change of its internal almost full flag status based
on read operations. The multi-queue flow-control device also provides a
duplicateofthe
PAFflagonthePAF[7:0]flagbus,thiswillbediscussedindetail
in a later section of the data sheet.
See Figures 22 and 23 for Almost Full flag timing and queue switching.
ALMOST EMPTY FLAG
As previously mentioned the multi-queue flow-control device provides a
single Programmable Almost Empty flag output,
PAE. The PAE flag output
provides a status of the almost empty condition for the active queue currently
selected on the read port for read operations. Internally the multi-queue flow-
controldevicemonitorsandmaintainsastatusofthealmostemptyconditionof
all queues within it, however only the queue that is selected for read operations
has its empty status output to the
PAEflag.Thisdedicatedflagisoftenreferred
toasthe“activequeuealmostemptyflag”.Thepositionofthe
PAEflagboundary
within a queue can be at any point within that queues depth. This location can
be user programmed via the serial port or one of the default values (8 or 128)
can be selected if the user has performed default programming.
As mentioned, every queue within a multi-queue device has its own almost
emptystatus,whenaqueueisselectedonthereadport,thisstatusisoutputvia
the
PAEflag.ThePAEflagvalueforeachqueueisprogrammedduringmulti-
queue device programming (along with the number of queues, queue depths
and almost full values). The
PAEoffsetvalue,n,forarespectivequeuecanbe
programmedtobeanywherebetween‘0’and‘D’,where‘D’isthetotalmemory
depthforthatqueue.The
PAEvalueofdifferentqueueswithinthesamedevice
can be different values.
When queue switches are being made on the read port, the
PAEflagoutput
will switch to the new queue and provide the user with the new queue status,
on the second cycle after a new queue selection is made, on the same RCLK
cycle that data actually falls through to the output register from the new queue.
That is, a new queue can be selected on the read port via the RDADD bus,
RADENenableandarisingedgeofRCLK.OnthesecondrisingedgeofRCLK
followingaqueueselection,thedatawordfromthenewqueuewillbeavailable
at the output register and the
PAEflagoutputwillshowtheemptystatusofthe
newly selected queue. The
PAE is flag output is double register buffered, so
when a read operation occurs at the almost empty boundary causing the
selected queue status to go almost empty the
PAEwillgoLOW2RCLKcycles
after the read. The same is true when a write occurs, there will be a 2 RCLK
cycle delay after the write operation.
So the
PAE flag delays are:
from a read operation to
PAE flag LOW is 2 RCLK + tRAE
The delay from a write operation to
PAEflagHIGHistSKEW2+RCLK+tRAE
Note, if tSKEW is violated there will be one added RCLK cycle delay.
The
PAEflagissynchronoustotheRCLKandalltransitionsofthePAEflag
occur based on a rising edge of RCLK. Internally the multi-queue device
monitorsandkeepsarecordofthealmostemptystatusforallqueues.Itispossible
that the status of a
PAEflagmaybechanginginternallyeventhoughthatflagis
not the active queue flag (selected on the read port). A queue selected on the
writeportmayexperienceachangeofitsinternalalmostemptyflagstatusbased
on write operations. The multi-queue flow-control device also provides a
duplicateofthe
PAEflagonthePAE[7:0]flagbus,thiswillbediscussedindetail
in a later section of the data sheet.
See Figures 24 and 25 for Almost Empty flag timing and queue switching.