Re: Quartus II infered latches

On Aug 12, 7:49 am, jacko <jackokr...@gmail.com> wrote:
> On 11 Aug, 18:16, Mike Treseler <mtrese...@gmail.com> wrote:
>
> I'm not complaining about the packing, it does a fine job. With
> further testing and wrapping the carry in CARRY primitives, it has
> proved that 29 of 32 CARRY_SUM primitives were ignored, and the size
> went up about 20LEs and the speed dropped to 13MHz. Wierd!
>
> The chain is definatly detected. I was just wondering why there is
> less inputs allowed on a chained LE. A replacement of input or output
> by fast chain signal is understandable, but I don't get the 3 input
> limit (one of them being a chain itself).

The Altera devices I have used (ACEX 1K) are limited to 3 inputs when
used in adders because they split the LUT into one 3-LUT for the sum
and another 3-LUT for the carry. This could either be because they
want to save some of the area used by the carry chain, or because the
carry chain is patented. I expect it is the latter.

Rick

rickman