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Forum: FPGA, VHDL & Verilog FPGA: i get a different answer in modelsim and in device


Author: Amine Amine (aminemr)
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I'm new to VHDL and require some help. I'm Interfacing with Artix 7 to 
make a simple modular multiplier but i have some problems. When I 
synthesize my code i get some warnings which i think are wrong so i 
ignored them. when I run post route simulation i got the results as 
required but when i burned the code on hardware i get a different answer 
for the same value that i check in the stimulus.
Can some one please help me.

the warnings that i get are:

Xst:737 - Found 1-bit latch for signal <zt<1>>. Latches may be generated 
from incomplete case or if statements. We do not recommend the use of 
latches in FPGA/CPLD designs, as they may lead to timing problems.

Xst:1710 - FF/Latch <C_b_out_30> (without init value) has a constant 
value of 0 in block <Montgomery_multiplier>. This FF/Latch will be 
trimmed during the optimization process.

Xst:1895 - Due to other FF/Latch trimming, FF/Latch <z8_19> (without 
init value) has a constant value of 0 in block <Montgomery_multiplier>. 
This FF/Latch will be trimmed during the optimization process.

Xst:3001 - This design contains one or more registers or latches with an 
active
asynchronous set and asynchronous reset. While this circuit can be 
built,
it creates a sub-optimal implementation in terms of area, power and
performance. For a more optimal implementation Xilinx highly recommends
one of the following:
1) Remove either the set or reset from all registers and latches if
not needed for required functionality
2) Modify the code in order to produce a synchronous set
and/or reset (both is preferred)
3) Use the -async_to_sync option to transform the asynchronous
set/reset to synchronous operation
(timing simulation highly recommended when using this option)

Author: guest (Guest)
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There is a very simple design rule concerning latches:

Do not use latches.

The problems you are seeing are very likely a direct result of having 
latches. Correct your design until you have no latch warnings.

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