DC fifo behaviour at underflow/overflow

kaz · 2012-12-15T12:38:01+00:00

We all know that a fifo should operate without getting empty or full. Does anybody have experience of what sort of output disorder can one expectwhen operating in the wrong state (underflow or overflow). I am asking that because naturally one thinks of some data samples gettinglost when a fifo is in this wrong state but I am facing another output patternat final system output and trying to find a cause. The pattern I get is an odd/even offset by some 8 samples in one case or every 8th sampleduplicated in another case. For case1 if I realign that stream it gets correct so I amnot actually losing samples. The system is too large and remotely tested andthere is not much room to do any test at the time being. I have suspicion of a dc fifo in the path that may enter wrong state(underflow/overflow). It is altera dc fifo in stratix iv writing on ~368MHz clock and reading on ~245MHz, 32bits wide and 8 words deep. Any thoughts appreciated Kaz --------------------------------------- Posted through

Dec 15, 2012 21 Replies

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Michael S 13 years ago

I disabled built-in protections and forcefully overwrote protections in user logic.

rdreq Thanks

C_VECTOR (31 DOWNTO 0);

;

OGIC_VECTOR (31 DOWNTO 0);

WNTO 0);

URAL;

STRING;

ATURAL;

;

STRING;

;

OGIC_VECTOR (31 DOWNTO 0);

;

C_VECTOR (31 DOWNTO 0)

There is a known bug in Quartus 11.1 that applies to use_eab => "ON". According to the knowledge base, it was fixed in 11.1SP1.

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There is another known bug that is not fixed in 11.1SP1/11.1SP2, but it only applies to use_eab => "OFF" so you shouldn't care about it.

Looking at your parameters, rdsync_delaypipe => 5 and wrsync_delaypipe => 5 sound like an overkill. It's unlikely that value of 5 really improve anything over value of 4. And if overflow happens nevertheless (don't ask me how, I don't know) then longer sync pipelines certainly make self-recovery more difficult.

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Michael S 13 years ago

No, I did a functional simulation in Quartus 9.1 internal simulator. Quartus 9.1 ISIM can't simulate Stratix-IV, so I told him that it is Cyclone-III. For a functional simulation it should make no difference.

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