Asynchronous Fifo depth calculation and burst

[unni5567]

Dears,

I need to design a asynchronous fifo , data rate is 90Kbps , and data width is 64bit . writing freq is 15M and reading 10M .
could you help me to calculate the Depth of the fifo.
How can we calculate the parameter burst, from this information .
Please help me ppm of a write and read frequency sources has a severe impact on my fifo ?

Thanks and Regards
Unnikrishnan KM

 

[TrickyDicky]

when you say asynchronous, do you mean dual clocked?

 

[unni5567]

Yes , two clocks are there write clk and read clock.
Could you help me in this regard.

 

[TrickyDicky]

it really depends when you are going to read and write.
at 1400 word/s, thats not really a lot of data.

But with a slow read clock than write clock, you cannot put data in on every clock cycle.

 

[permute]

two clear factors come up -- the longest sustained burst for writing, and the longest sustain holdoff for reading. these are given by the source/destination of the data.

eg, if you write 1MB at a time, then wait for several minutes, you will have a low average data rate. but the fifo will need to be large. Likewise, if the read process must wait for several minutes, you will be able to fill up a large fifo before any reads occur. If both conditions occur, you would need a 1MB buffer.

When the data rate is approximately uniform, the fifo sizes can be minimized. in this case, to the point where a minimum sized fifo is ok. eg, a 256 word 18kb BRAM for an FPGA, or a 16 word distributed ram.

 

[unni5567]

could you please make it clear, or could you point out any papers/books that could help me in this regard .
ppm difference of clocks would be affecting the fifo depths again, I believe.

 

[TrickyDicky]

it is the data rate thats important, not the PPM difference in the clock.
Port A could be clocked 300Mhz with port B at 10MHz, aslong as port A isnt written very often, you only have to look at how big the burst is to see how deep you need the FIFO.

 

[omara007]

what if the read burst size is different from write burst size, i.e. read is 32bits while write is 64bits ?