by Weighted round robin arbiter allows each requestor a share of the common resource which is relative to its predefined weight. For example, if two requestors have the weight of 3 and 7 respectively, then over a long period of time, assuming both requestors always have their request asserted, the first one will get 30% of the grants and the second will get 70% of the total grants.
A weighted round robin arbiter design with a total of 3 requests, i.e., req[2:0], is shown in the Verilog code below.
The design consists of 2 parts: a credit mask logic and a normal round robin arbiter. The credit mask logic generates the masked version of requests and feeds them into round robin arbiter. The round robin arbiter produces the final grant.
The credit mask logic keeps one credit counter for each request, indicating number of credits used by the request so far, and the initial values of the credit counters are all 0. The corresponding credit counter increments when the arbiter grants the request, and the request will be masked out if it reaches its threshold. All credit counters will be reset to 0 when none of the asserted requests have credits, and the next cycle starts.
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module wrr_arb #( parameter MAX_THRE = 8) ( input clk, input rst_n, input [$clog2(MAX_THRE+1)-1:0] max_thre, input [2:0] req, output logic [2:0] grant ); logic [2:0][$clog2(MAX_THRE+1)-1:0] crd_cnt; logic [2:0][$clog2(MAX_THRE+1)-1:0] crd_cnt_next; logic [2:0][$clog2(MAX_THRE+1)-1:0] crd_cnt_incr; logic [2:0] has_crd; logic [2:0] mask_req; logic replenish; // when none of the asserted requests have credits, replenish all credit counters assign replenish = ((req & has_crd) == '0); genvar i; generate for (i = 0; i < 3; i = i + 1) begin assign crd_cnt_incr[i] = crd_cnt[i] + 1'b1; assign has_crd[i] = (crd_cnt_incr[i] <= max_thre); // credit mask logic generates masked version of requests assign mask_req[i] = (has_crd[i] | replenish) & req[i]; // next credit counter value always_comb begin crd_cnt_next[i] = crd_cnt[i]; if (replenish) if (grant[i]) crd_cnt_next[i] = 1; else crd_cnt_next[i] = 0; else if (grant[i]) crd_cnt_next[i] = crd_cnt_incr[i]; end // only update the credit counters when replenish or being granted always_ff @(posedge clk or negedge rst_n) if (~rst_n) crd_cnt[i] <= '0; else if (replenish | grant[i]) crd_cnt[i] <= crd_cnt_next[i]; end endgenerate // the masked version of requests will be fed into normal round robin arbiter rrb_arb u_rrb_arb (.clk, .rst_n, .req(mask_req), .grant); endmodule: wrr_arb |
Hi, I would suggest you changing max_thre to be an array to indicate the different weights for each request, because the code shown here doesn’t really reflect weighted grant behavior.
Here is what I modified
input [2:0][$clog2(MAX_THRE+1)-1:0] max_thre,
…
assign has_crd[i] = (crd_cnt_incr[i] <= max_thre[i]);
For example, if max_thre is [2,3,5], it turns out to be the req[2] get 5/10 grants, and req0 and req[1] gained 2/10 and 3/10 respectively.
Hey can u explain by examples for the case of 1:3:5 weights in the above code
same as rrb_arb in the last line u need to add
assign grant = (mask_req == ‘0) ? raw_grant : mask_grant;
to make it work for continuous requests like of this type req=3’b001 for a long period of time.
In the line number 30 u need to make
assign has_cred[i] = crd_cnt_incr[i] <= max_thre[i];
Hello,
I believe the decode for “replenish” must factor an additional term that ensures a request is present. Otherwise, lack of requests will replenish the credit counters.
Current version:
assign replenish = ((req & has_crd) == ‘0);
Modified version:
assign replenish = (req != ‘b0) && ((req & has_crd) == ‘0);
Does this look reasonable?
Regards,
Adithya
Hey Adithya,
Why should we bother about requests at all, when we are planning to replenish credit counters.
replenish = (has_crd == ‘0); should suffice, I believe. Let me know, if I am wrong.
Great explanation.
I have one question though!
Why should we replenish when you have no requests at all. Shouldn’t ‘everyone’s credit being saturated’ be enough to say we are good to replenish?