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Forums > Functional Verification > Assertion-Based Verification and Formal Analysis

Subject: How to check data integrity with assertions?
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bijitm
Posts: 5
Online: User is Offline
1/13/2008 7:49 PM  
I am verifying a bus system. Apart from verifying simple point to point connectivity checks, I am also planning to do a data integrity check. What is the best way to do that with assertions in a pure formal environment?
If I write a set of all-0s and all-1s and then read it back, does that do a proper integrity check? Please suggest any better way to do this.
jmueller
Posts: 14
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1/14/2008 1:09 AM  
Hi bijitm,

there is an example for data integrity in a fifo: If data enters the FIFO then “sooner or later” the same data exits the FIFO:

output_data_integrity : assert forall dvalue in {0:255} :
always ( (push && data_in == dvalue) ->
eventually! (pop && data_out == dvalue));

Two key aspects here are liveness (eventually) and non deterministic constants (forall). For a bus system this concept is also applicable, but maybe you are able to predict exactly when the output should show the data coming in (e.g. same or next cycle instead of eventually). The "push" in a bus system represents the control signal condition indicating a transfer coming in and the "pop" goint out.

In fully symmetric datapath a valid abstraction is to use only one bit instead of the full data vector, that can reduce complexity for the formal prove.

Joerg.
weberrm
Posts: 0
Online: User is Offline
1/14/2008 6:01 AM  
Hi!

Below is anoter way to do it without the forall.

logic [W-1:0] dvalue;
assume_stable_dvalue: assume always(stable(dvalue))@(posedge clk);

assert_data_integrity: assert always(
  push & (data_in==dvalue) ->eventually!
  pop & (data_out==dvalue)
)@(posedge clk);


This technique uses a non-deterministic constant, dvalue, which takes on and holds all values for the proof.

Depending on the tool version you are using I've had clocking problems with the forall.  Also, to protect all values, instead of 255 you would need something like ((2**W)-1) in the forall solution.  The forall solution does have the advantatge of working in simulation.  However, the questioner said the verification would be done formally.
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Forums > Functional Verification > Assertion-Based Verification and Formal Analysis > How to check data integrity with assertions?


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