changing values in a fifo

Patrick,

The solution I proposed would address changes to all occurances in the FIFO.
See, the FIFO only stores the address of an entry in the CAM/RAM.  If the
value changes, the address remains the same, just the value in the CAM/RAM
gets changed.  Unfortunately, Altera's CAM Megafunctions don't allow direct
reading of the CAM entries by address thus necessitating the RAM.  The CAM
is simply used to determine whether a value has been submitted before or
not.  The RAM stores the same value as the CAM except you can read it out by
indexing into the RAM using the FIFO output.

Blake

"Patrick Klacka" <pklacka@trexenterprises.com> wrote in message
news:501364cc437d0c00562c49ae102caf01@news.teranews.com...
> Thank you everyone for your suggestions thus far. It's clear that more
> details are needed in order to better explore this problem. Please see
> http://www.trexenterprises.com/~pklacka/fifo.html for implementations in C
> code.
>
> It seems that all the solutions favor one change to a particular value,
but
> do not consider the case of the changed value being changed. For example,
> the value of 5 is changed to 6. Now a 5 is retrieved from the fifo. We
know
> through the Blake's CAM implementation and Jim's SyncRAM implementation
that
> the value of 5 will be transformed to a 6. Now the 6 is changed to a 7.
When
> we retrieve a 6 from the fifo, we get a 7 as intented. But should a 5 come
> off the fifo, we still get a 6, unless some sort of recursion is used. The
> recursion is what I am attempting to avoid. If all the 5's in the fifo
could
> be changed to 6's before the 6's get changed to 7's, then I have a working
> solution.
>
> Here is another idea that may be impossible to implement, but it might
shed
> some more light on what I am trying to achieve. Consider a full fifo that
> expects a value to be inserted every time you remove a value. So there is
a
> constant flow from one side to the other. (I think this was correctly
> likened to a vector shift-register) Could there be another fifo of the
exact
> same length that flowed in the opposite direction which contains the
compare
> and the change values. Each time a new value is inserted/removed from the
> primary fifo, the secondary fifo compares register values at the same
> location in the primary fifo, makes the appropriate change if necessary,
> then shifts itself with a new value. Using this idea, all of the 5's will
be
> changed to 6's before the 6's get changed to 7's, but I think it uses the
> same amount of resources as my original solution.
>
>

"Patrick Klacka" <pklacka@trexenterprises.com> writes:


Build with FFs? What's the depth X width?

Homann
-- 
Magnus Homann, M.Sc. CS & E
d0asta@dtek.chalmers.se

Patrick Klacka wrote:
> Thank you everyone for your suggestions thus far. It's clear that more
> details are needed in order to better explore this problem. Please see
> http://www.trexenterprises.com/~pklacka/fifo.html for implementations in C
> code.
> 
> It seems that all the solutions favor one change to a particular value, but
> do not consider the case of the changed value being changed. For example,
> the value of 5 is changed to 6. Now a 5 is retrieved from the fifo. We know
> through the Blake's CAM implementation and Jim's SyncRAM implementation that
> the value of 5 will be transformed to a 6. Now the 6 is changed to a 7. When
> we retrieve a 6 from the fifo, we get a 7 as intented. But should a 5 come
> off the fifo, we still get a 6, unless some sort of recursion is used. The
> recursion is what I am attempting to avoid. If all the 5's in the fifo could
> be changed to 6's before the 6's get changed to 7's, then I have a working
> solution.

  We need some more info on peak/average Read and Change stream rates, 
and likely 'change depth'.
  You can either change on read, and iterate until the change stack
stops changes, which will be HW frugal, but will have a finite settling 
time. ( similar to a linked-list in SW )
  Or, you can change all in-situ, but that is also likely to have a 
finite settling time, as the delta-list will have to arrive 
sequentially, but maybe at a much lower average rate.
  This will read faster, but consume much more HW.
So it's like any parallel/serial trade off: HW vs bandwidth.

  If the change-list stream rate is slow, you could do a combination of 
table correct {fast, but one-deep), and interleaved scan change,
(slower, but will recurse).
  Thus the FIFO becomes a bit like video memory, where one task 
scans/changes and the other reads.
  It would need "pause" ability, for when/if too many changes arrive
for the change paths to settle.

-jg

jim granville <no.spam@designtools.co.nz> wrote...

>   We need some more info on peak/average Read and Change stream rates, 
> and likely 'change depth'.

data is 8 bits, fifo is 512 entries, reading from the fifo happens
once every 10 clock cycles, a change happens approx. every 10th time,
and the average change depth is 2.

>   You can either change on read, and iterate until the change stack
> stops changes, which will be HW frugal, but will have a finite settling 
> time. ( similar to a linked-list in SW )

Yes, this is what I am currently doing. For situations where change
depth is 2 or less, this works fine. For change depths of 3 or more, I
have to buffer the incoming data.

>   Or, you can change all in-situ, but that is also likely to have a 
> finite settling time, as the delta-list will have to arrive 
> sequentially, but maybe at a much lower average rate.
>   This will read faster, but consume much more HW.
> So it's like any parallel/serial trade off: HW vs bandwidth.

I'm hoping for some clever solution that uses this method -
unfortunately the best I could come up with involved too much hardware
- leaving no room for anything else. If there was a way to implement
this smarter, I'm interested!
 
>   If the change-list stream rate is slow, you could do a combination of 
> table correct {fast, but one-deep), and interleaved scan change,
> (slower, but will recurse).
>   Thus the FIFO becomes a bit like video memory, where one task 
> scans/changes and the other reads.
>   It would need "pause" ability, for when/if too many changes arrive
> for the change paths to settle.

I've considered this approach, and if you can figure out a way to make
it work, that would be great. The problem I had was that many values
could change to the same value, eliminating the possibility of using
another linked-list (reverse direction from the initial solution) to
iterate through the changes in the lookup-table.