Altera's altsyncram MAXIMUM_DEPTH

Hi Manfred,
    There is no real need to restrict it. There have been several requests
to relax this condition and we will get to it in a future release.

- Subroto Datta
Altera Corp.

"Manfred M&#4294967295;cke" <manfred.getmuecke@ridgmxof.thisat> wrote in message
news:oprzq27uthgdoir8@news.inode.at...
> Hi Subroto,
>
> I would like to renew my question: Why do you restrict FIFO depths to
> powers of two? I can't see the need for that.
>
> Regards, Manfred
>

Hi Manfred,

> I would like to renew my question: Why do you restrict FIFO depths to
> powers of two? I can't see the need for that.

The dual-clock FIFO internally uses a Gray counter, which is fairly trivial
to write for a power of two, plus the fact that counter rollover happens
with a single-bit transition as well.

The Gray counter greatly reduces the risk of the Other Side (the one in in
the different clock domain) seeing inaccurate counter values: the count is
either the same, or only one bit has changed. For a normal counter, due to
variations in the delay path between the various counter bits, part of the
logic in the other clock domain might see a number of counter bits still
having the old value, and a number that has the new value, resulting in a
nonsense value. When there's a large difference between reader and writer
clock frequencies, there may be not a single bit transition, but at least
the number of transitions is minimized over time.

I haven't studied Gray counters deeply enough to see whether it's feasible,
or even possible to write a Gray counter generator algorithm that can
_efficiently_ do single-bit-transition counter rollover on an arbitrary
(though pre-computed) value. If this is possible without going into long
combinatorial chains (which would reduce operating frequency) it should
definitely be feasible to remove this power-of-two restriction.

For the single-clock version - hey, why not?

Just my $.02



Ben Twijnstra