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Hardware interview practice

Four-requester round-robin arbiter

MediumRTL DesignSystemVerilog

Four clients share a one-cycle service slot. Design a synthesizable combinational round-robin grant and sequential priority pointer with deterministic reset and idle behavior.

Starting point

Question code

input logic clk, rst_n;
input logic [3:0] req;
output logic [3:0] grant;
Reviewed example

Work through one case

Input
After reset, req=4'b0101 for two cycles
Expected output
First grant=0001; next grant=0100

The pointer starts at requester 0 and then advances to requester 1 after the first grant.

What to cover

Requirements

  1. grant is combinational, one-hot-or-zero, and selects the first asserted requester encountered from the current priority pointer with wraparound.
  2. On a rising edge with a nonzero grant, move the pointer to the requester immediately after the granted one.
  3. If req is zero, drive grant zero and leave the pointer unchanged.
  4. Whenever rst_n is 0, drive grant=0; on a rising edge with rst_n=0, set requester 0 as the first priority.
Continue practicing
RTL DesignDesign and verify a round-robin arbiterArbitersArbitrate fairly between two requesters