Forum: FPGA, VHDL & Verilog Safe FSM design

Author: SparkyT (Guest)
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Hi all,
I am using Libero 11.5 SP3, including Synplify Pro.
I need some help on how to synthesize safe FSMs.

In synplify, in the VHDL Compiler options, there is a High reliability 
where i can tick 'Preserve and decode Unreachable States'. Once this is 
ticked, I can see the RTL/Technology view with a state-error-detect box. 
But once i flash the design, it fails. (I have a memory check test to 
see this). This tick, applies to the whole design, more than 10 FSM's, 
and counters.
Once un-ticked, the design passes the memory test.
So i started looking into attibutes. So far I used the
attribute syn_safe_case : boolean;                            -- enables/disables the safe case option
attribute syn_safe_case of RTL : architecture is true;              -- turns off sequential optimizations for counters, FSM, and sequential logic
attribute syn_encoding : string;                            -- Overrides the default FSM Compiler encoding for a state machine and applies the specified encoding
attribute syn_encoding of curr_st : signal is "safe";                   -- default encoding and adds reset logic

either together or on their own, they all pass the memory test.  I 
definetely see different logic being synthesized, but cant see the 
state-error-detect box, anywhere.

How can I know for sure that i synthesized a safe FSM?
How do you synthesize yours?

Thank you in advance

Author: Lothar Miller (lkmiller) (Moderator)
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SparkyT wrote:
> I need some help on how to synthesize safe FSMs.
Why do you need a "safe FSM"?
A "safe FSM" only ensures, that no undefined state can be reached 
(or better: a undefined state can be reached, but it will recover from 
that and fall back to a "default state").
But a "safe FSM" does not guaranty that the state transistions are 
in the correct order!!

> How do you synthesize yours?
I never ever used the "safe implementation" of FSM (And i have running 
some thousands of FSM all around the world).
You must simply know what leads to problems (clock domain crossing and 
unsynched inputs), and avoid things like this:

And try that with google translator:
Beitrag "WHEN OTHERS in einer FSM"
The last sentence in the last post concludes it very precisely... ;-)

: Edited by Moderator
Author: SparkyT (Guest)
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so if I understand well, you are saying that a safe design, would allow 
the FSM to escape a 'lock' if an invalid state occurs after a 
transition, but would not help to prevent the bad transition in the 
first place. And since we are not talking about space applications, 
clock domain crossing and unsynched inputs are the real problems to 
solve the illegal transition, rather than the escape of the illigal 
Am I correct?

Author: Lothar Miller (lkmiller) (Moderator)
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SparkyT wrote:
> Am I correct?
Yes, you got it.
Any problem with occasional failures that I had on earth was due to any 
kind of sync problems, and those cannot be fixed with a "safe FSM"...

Author: SparkyT (Guest)
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Thanks a lot for the information.


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