[SOLVED] FIFO Depth calculation for an async FIFO

[fragnen]

Data is written as 80 data per 100 clock cycles to a FIFO and being read at 4 data per 10 clock cycles. The write and read clock of the FIFO is running at 100 MHz and 200 MHz respectively and this write and read clock are asynchronous to each other. What will be the depth of this FIFO.

In this case, the depth of 32 should be sufficient if this FIFO was a synchronous FIFO if the write and read clock was synchronous to each other. But since write and read clock are asynchronous to each other, the FIFO will be an asynchronous FIFO and the depth of 32 will not work. Please provide a proper depth of this asynchronous FIFO.

 

[barry]

I'm confused. Your input data rate is 80M-samples/sec, same as your output data rate. Unless you've got some big latency between write and read that you're not telling us about, where's the problem?

 

[ads-ee]

Unless the two clock are derived from the same source clock there is no possible depth that will work with a sustained transfer. Clocks derived from different sources won't ever be exactly the same frequency and that difference will cause an asynchronous FIFO to eventually overflow or underflow as the actual ingress and egress rates are not identical.

If the two clocks are derived from the same source clock then barry's observation is valid and I would add additional depth to match the amount of CDC that is used to transfer the address pointers and flags.

 

[fragnen]

The two clocks are asynchronous to each other.

The write clock is running at 100 MHz. The read clock is running at 200 MHz. Data is coming at the Data input pin of the asynchronous FIFO at a rate of 80 data per 100 write clock cycles. Data is read from the asynchronous FIFO at a rate of 4 data per 10 read clock cycles.

--- Updated Sep 30, 2022 ---

If the two clocks are derived from the same source clock then barry's observation is valid and I would add additional depth to match the amount of CDC that is used to transfer the address pointers and flags.

How much will be the additional depth?

 

[barry]

The two clocks are asynchronous to each other.

The write clock is running at 100 MHz. The read clock is running at 200 MHz. Data is coming at the Data input pin of the asynchronous FIFO at a rate of 80 data per 100 write clock cycles. Data is read from the asynchronous FIFO at a rate of 4 data per 10 read clock cycles.

--- Updated Sep 30, 2022 ---

How much will be the additional depth?

You just repeated yourself while completely ignoring what people are telling you. To summarize:
1) If the two clocks are synchronous then your FIFO only needs to be deep enough to accommodate latency.
2) If the two clocks are asynchronous you will eventually overrun or underrun the FIFO, no matter how big it is.

 

[fragnen]

You just repeated yourself while completely ignoring what people are telling you. To summarize:
1) If the two clocks are synchronous then your FIFO only needs to be deep enough to accommodate latency.

How to calculate such depth to accommodate latency and how much will be the depth? Which latency are you referring?

 

[barry]

latency between start of write and start of read. latency due to internal CDC requirements (post #3)

 

[dpaul]

@barry,
"You just repeated yourself while completely ignoring what people are telling you. To summarize:"
Because this is a question from fragnen!

Dear members,
please consider these previous posts from the OP while trying to answer this question/thread-

https://www.edaboard.com/threads/minimun-depth-of-an-asynchronous-fifo.395522/#post-1698572

https://www.edaboard.com/threads/designing-asynchronous-fifo-with-odd-number-depth.395513/

https://www.edaboard.com/threads/asynchronous-fifo-and-its-depth.399641/

I did not compare in depth the above previous posts from fragnen to this post, but I have a feeling that the same question is being asked.

 

[fragnen]

latency between start of write and start of read. latency due to internal CDC requirements (post #3)

How to calculate that?

 

[barry]

How to calculate that

I’m done with you. Find a different hobby or career or whatever it is you’re doing.