>How often is this needed? Or does it
>depend upon the memory used??

It used to be that you needed to cycle through the 7 bit recycle address
range in 1mS with early dynamic chips. Later larger chips expanded this to
2mS for the 8 bit recycle address range. These all required the recycle
address to be externally generated, and some people went to the extent of
working out if a particular address had been accessed during the previous
recycle period for that address, and skipped that address refresh request so
that the CPU did not get held up on memory accesses.

The newer chips with built in recycle address counters I believe are set up
to require the same recycle request rate as the 256 in 2mS and the size of
the recycle address counter is invisible to the user. However as always
there is the caveat - read the data sheet for the chip! I would go as far as
to say that if you cannot find the data sheet for the particular
manufacturer that you have chips for, then finding a data sheet for an
equivalent chip from another manufacturer will give this figure accurately
enough for your purpose.

Depending on just what you are wanting to do, you may find it advantageous
to have external refresh hardware as it will take a reasonable number of
cycles of PIC processing to handle a timer interrupt to do the refresh.

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