Synthesis

[abdel21]

WARNING:Xst:1989 - Unit <mem_ctrl>: instances <Mcompar__cmp_eq0041>, <Mcompar__cmp_eq0045> of unit <LPM_COMPARE_1> are equivalent, second instance is removed
 

WARNING:Xst:1988 - Unit <L2>: instances <Mcompar__cmp_le0000>, <Mcompar__cmp_gt0000> of unit <LPM_COMPARE_3> and unit <LPM_COMPARE_7> are dual, second instance is removed

Why do these warning appears and how can I find out  where the problem is, that is, how could I identify what part of my VHDL code is wrong?

Or at least, if anyone could tell me how to avoid seconds instances from being removed, that is, keep both firsts and seconds instances in my synthesized design.

[gszakacs]

In the synthesis properties, turn off "Equivalent register removal".

This isn't necessarily a problem.  Often you'll find the "equivalent registers" have been

added during synthesis because you also have "register duplication" turned on.

I'm not at my work computer right now, so the above quoted terms may not be exactly

right.  In any case they are in the synthesis properties and both are on by default.

Under most conditions you can safely ignore these warnings.  Where you may run into problems

is removal of registers that were duplicated in order to push into the IOB's, or registers

duplicated to speed up the clock by reducing the fanout.

[htsvn]

Hello, The keep attribute should be indeed what you are looking for, assuming you are using XST as a synthesizer, e.g. attribute KEEP : string; attribute KEEP of ring_delay1 : signal is "true"; Other synthesizers may require different attributes. Rgds.

[yangliiris]

or  add "save" flag in your desing  to keep them from been trimed