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imajeeth

SVA for fairness of Round-Robin Arbiter

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Are are any standard or recommended ways of verifying the functionality of the round-robin arbiter and more specifically checking the fairness? I'm looking for something that is scalable with the number of requestors.

 

Functionality of the arbiter is as follows:

- Say there are 3 requestors, req1, req2 and req3. The pointer points to req1 initially. Consider different cases for illustration. 

- If all 3 requests are asserted, req1 goes through and the pointer is updated to req2. 

- If req2 and req3 is asserted, then req2 will go through and pointer is updated to req3. 

- If just req3 is asserted, then req3 will go through and pointer is updated to req1(cyclic). 

 

The pointer is always updated to the next requestor that got previously granted

  - if req1 granted, pointer is updated to req2

  - if req2 got granted, pointer is updated to req3,

  - if req3 is granted, pointer is updated to req1

The pointer points to the requestor that has higher priority for the next round of requests, in order to ensure fairness in the way grants are allotted.   

 

 

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This is my solution. Not necessarily the best one. Suggestions for improvements are welcome. If any of you find any shortcomings with this method, please let me know. Open to constructive criticism!

/*
Assume there are 3 requestors namely: A,B and C with reqA, reqB and reqC being the requests
and gntA, gntB and gntC being the grants. 
Assumes the request and grant are on the same cycle
Similar sequence and assertions need to be replicated for each requestor. Can concatenate the requests and grants 
into a bit vector, in order to save some redundant code
*/
//Denotes there is request and grant for the requestor A. Does not care about other requestors 
logic requestorA_req_and_gnt;
//Denotes there is request and grant for requestor A and there is atleast one other requestor active
logic requestorA_req_and_gnt_with_other_requestors_active;
//There are requests other than requestor A
logic other_req_except_requestorA;
//There are grants other than requestor A
logic other_gnt_except_requestorA;

assign other_req_except_requestorA = reqB | reqC; //High when any one of the other requestor's are active
assign other_gnt_except_requestorA = gntB | gntC;
assign requestorA_req_and_gnt = reqA & gntA;
assign requestorA_req_and_gnt_with_other_requestors_active = reqA && other_req_except_requestorA && gntA;

//-----------------------------------------------------------------------------------------------------------------------------------------------
// Covers the case where there is request and grant on one cycle for requestorA, followed by no other requests. And then few cycles later there 
// are multiple requests(reqA and atleast one other request). In this case, requestorA should NOT be granted again
//-----------------------------------------------------------------------------------------------------------------------------------------------
sequence requestorA_req_and_gnt_followed_by_other_req_seq;
requestorA_req_and_gnt ##0 (!other_req_except_requestorA)[*1:$] ##1 reqA && other_req_except_requestorA;
endsequence

//-----------------------------------------------------------------------------------------------------------------------------------------------
//Covers the case where there are more than one requestor active(reqA and any one of the other requests) followed by cycles of no other request at all
// and again followed by more than one requestor active. First time during multiple requests, reqA is granted, so, the second time when multiple
// requests are active, gntA should not be asserted again, and one of the other requests should be granted
//-----------------------------------------------------------------------------------------------------------------------------------------------
sequence multiple_requests_with_requestorA_req_and_gnt_followed_by_other_req_seq;
requestorA_req_and_gnt_with_other_requestors_active ##1 (!other_req_except_requestorA)[*0:$] ##1 reqA && other_req_except_requestorA;
endsequence

//-----------------------------------------------------------------------------------------------------------------------------------------------
//Covers the case where there are multiple requestors active in one cycle and next cycle may or may not have reqA, but will have other requests. 
// If reqA is granted in the previous cycle, it should not be granted again in the next cycle, rather other requestor should be granted
//-----------------------------------------------------------------------------------------------------------------------------------------------
sequence back_to_back_multiple_requests_active_with_requestorA_gnted_followed_by_other_req_seq;
requestorA_req_and_gnt_with_other_requestors_active ##1 other_req_except_requestorA;
endsequence
//-----------------------------------------------------------------------------------------------------------------------------------------------
//Assertion to check if reqA do not hog the round robin arbiter
//-----------------------------------------------------------------------------------------------------------------------------------------------
assert_fairness_wrt_requestorA: assert property( @(posedge clk) disable iff(!resetn) 
(requestorA_req_and_gnt_followed_by_other_req_and_no_bts_seq or
multiple_requests_with_requestorA_req_and_gnt_followed_by_other_req_and_no_bts_seq or
back_to_back_multiple_requests_active_with_requestorA_gnted_followed_by_other_req_and_no_bts_seq
) |-> other_gnt_except_requestorA && !gntA
)
`uvm_info("ARB_ASSERT",$sformatf("Requestor A was previously granted, and not granted again, round robin scheme followed!"),UVM_LOW) 
else
`uvm_error("ARB_ASSERT_ERR",$sformatf("Requestor A was previously granted, and gets granted again, round robin scheme not followed"))

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