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question on fifo depth...

 
 
ekavirsrikanth@gmail.com
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      02-26-2007
i have a question that is asked in an interview i donot know whether
it is a silly question but i need know the procedure to slove this
type of questions.

its about fifo depth where i am writing into fifo asynchronously with
25 writes per second and reading form fifo with 25 reads per second
synchronously. then what the depth of this fifo?
cases:

1. asynchronulsy writing means.........
2. when bust of data is coming ........

regards
srik

 
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Pieter Hulshoff
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      02-26-2007
http://www.velocityreviews.com/forums/(E-Mail Removed) wrote:
> i have a question that is asked in an interview i donot know whether
> it is a silly question but i need know the procedure to slove this
> type of questions.
>
> its about fifo depth where i am writing into fifo asynchronously with
> 25 writes per second and reading form fifo with 25 reads per second
> synchronously. then what the depth of this fifo?
> cases:
>
> 1. asynchronulsy writing means.........
> 2. when bust of data is coming ........


I believe that with asynchronously writing in this context they mean
that the write clock is not the same as the read clock. This implies
that information passing from read to write or vise versa (like
read/write pointers to detect the filling of the FIFO) needs to cross
clock boundaries. This is usually done using Gray encoding/decoding, and
of course the normal 2 FF delay for proper clock boundary crossing.
Because of this you will also need to include a small error margin when
calculating FIFO full and empty conditions.

For the answer to your question, point 2 is quite important: Are
read/write actions at regular intervals or can they come in bursts? With
"when burst of data is coming" they seem to imply that data can be
written in bursts, meaning no data coming in for a large time, and then
a lot of data coming in in a short amount of time. The size of the
bursts (read as well as write) greatly influences the size of the
elastic store.

Kind regards,

Pieter Hulshoff
 
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