WARNING:Xst:2170

sneha22 · ‎01-17-2011

HI,

I am getting this warning though I have not created too many case states .There are only 4 case statements.

WARNING:Xst:2170 - Unit RS232_TX : the following signal(s) form a combinatorial loop: next_state<3>.

WARNING:Xst:2170 - Unit RS232_TX : the following signal(s) form a combinatorial loop: next_state<2>.

WARNING:Xst:2170 - Unit RS232_TX : the following signal(s) form a combinatorial loop: next_state<1>.

WARNING:Xst:2170 - Unit RS232_TX : the following signal(s) form a combinatorial loop: next_state<0>.

What danger does it pose to the block of the code?

I dont know why i am getting this warning?

awillen · ‎11-29-2007

The warnings you get have nothing to do with you having too many states (what makes you think that?). The real reason is that you did not use proper synchronous digital design techniques: your signal next_state is read and written to in a combinational circuit.

To see how a state machine has to be coded, open the Language Temples ==> Verilog/VHDL ==> Synthesis constructs ==> Coding examples ==> State machines.

Better yet, buy a book on your favorite HDL and read it thoroughly.

Adrian

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sneha22 · ‎01-17-2011

solved it..

bassman59 · ‎02-25-2008

sneha22 wrote:
solved it..

And your solution was ... ??

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Yes, I do this for a living.

awillen · ‎11-29-2007

bassman59 wrote:

sneha22 wrote:
solved it..

And your solution was ... ??

...probably not to create a combinational loop.

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sneha22 · ‎01-17-2011

i was bymistake changing the state of the fsm outside the clk process..silly one ...

melika · ‎10-20-2011

hello,
i have this warning in ISE 10.1, but it does not write any signals that form a combinatorial loop??!!
how i can find that signals?
thanks.

WARNING Xst :2170 :the following signal(s) form a combinatorial loop:

mrovin · ‎12-27-2012

If you don't see the signal involved in the combinational loop, it is just because you are viewing the design summary window. But if you switch to console or warnings panes (at the bottom of the main window) you will see the same messages but with the list of involved signals. Hope this helps.