Round-Robin Arbiter

Medium · Finite State Machines · Interfaces & Handshakes

Implement a 4-requester round-robin arbiter. Several clients want one shared resource (a bus, a memory port, an output link). Each cycle the arbiter grants it to at most one requester. A fixed-priority arbiter would let requester 0 starve everyone else; a round-robin arbiter rotates the priority so that every requester is served fairly.

Specification

module rr_arbiter (
    input  logic       clk,
    input  logic       rst,    // synchronous, active-high
    input  logic [3:0] req,    // req[i] = 1: requester i wants the resource this cycle
    output logic [3:0] grant   // one-hot (or all zeros): grant[i] = 1 gives it to requester i
);

The arbiter keeps one piece of state: a 2-bit pointer last, the index of the requester that was granted most recently.

Priority order. In each cycle, requesters are searched in the order

last+1, last+2, last+3, last (indices modulo 4). The first one with req[i] = 1 is granted. So the requester granted most recently has the lowest priority, and the one right after it has the highest.

Grant is combinational. grant depends only on the current req and the current

last, and must reflect a change of req within the same cycle (no clock edge in between).

At most one grant. grant is one-hot when req != 0, and grant = 4'b0000 when

req = 4'b0000. A requester is never granted unless its req bit is 1.

Pointer update. On each rising edge of clk (with rst = 0): if a grant was given in

that cycle (grant != 0), last becomes the index of the granted requester. If no grant was given (req = 0), last does not change.

Reset. On a rising edge with rst = 1, last becomes 3, so right after reset the order

is 0, 1, 2, 3 (requester 0 has the highest priority).

Fairness (follows from the rules). A requester that holds its req bit high is granted

within at most 4 consecutive cycles, regardless of what the others do.

Note that a grant lasts one cycle: a requester that keeps req high after being granted competes again in the next cycle, now with the lowest priority.

Example

cycle	req (bits 3..0)	last	search order	grant
1	1111	3	0,1,2,3	0001
2	1111	0	1,2,3,0	0010
3	1111	1	2,3,0,1	0100
4	1111	2	3,0,1,2	1000
5	0101	3	0,1,2,3	0001
6	0101	0	1,2,3,0	0100
7	0000	2	–	0000 (`last` stays 2)
8	0011	2	3,0,1,2	0001

Round-Robin Arbiter

Specification

Example

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