Asynchronous FIFO depth calculation

Question

I was required to calculate how long it will take to fill an asynchronous FIFO. For example: Assume that module 'A' wants to send some data to the module 'B'. The frequency of module A is 80MHz. The frequency of module B is 50MHz. The burst length is 120. There are no idle cycles in both reading and writing. The FIFO depth is 20. How long it will take to fill the FIFO?

I understand that the minimum depth of the FIFO should be 45. ' Time required to write one data item=1/80MHz=12.5ns Time required to write all the data in the burst=120*12.5ns=1500ns. Time required to read one data item=1/50MHz=20ns. The number of data item can be read in a duration of 1500ns=1500/20=75 The remaining number of bytes to be stored in FIFO =120-75=45. But if the depth of FIFO is 20. How do I calculate the time to fill the FIFO?'

Question 2: I read some material. The depth of FIFO should be infinite if it is a continuous write. What is the difference between continuous and burst write? Why a finite depth of FIFO is enough for a burst to write?

Timmy Brolin · Accepted Answer · 2021-04-21 15:21:31Z

0

Assuming you are continously reading and writing on both sides. For a theoretical perfect FIFO, just solve the equation:

80000000*x-50000000*x=20
30000000*x=20
x=20/30000000
x=0.667µs

However, real-world FIFOs have clock domain crossing synchronizers, which essentially reduces the usable FIFO depth by a few entries. Often 2~4 entries are unusable. You need to examine your particular FIFO to know exactly how many clock cycles, and therefore FIFO depth, are lost due to clock domain crossing synchronization.

I have no idea what you mean by "burst" in this situation.

answered Apr 21, 2021 at 15:21

Timmy Brolin

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2 Comments

Cyan_VLSI Over a year ago

Thanks for tutoring. I am very confused about the equation. For example, 80000000*x-50000000*x=45. 30000000*=45. x=45/30000000. x=1.5us. The equation means it will takes 1.5us to fill the FIFO. The results is conflict with expectation which is 45 depth FIFO will not be full.

Timmy Brolin Over a year ago

The 45 entry FIFO will be full after 1.5us. (Actually a bit earlier, taking CDC synchronizers into acount)

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