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ASIC.FYI

Q216FreeSystemVerilog

Predict weighted round-robin grants

Difficulty
Hard
Topic
Arbiters
Language
SV
Interview prompt

Question

Extend the arbiter so port i has configurable weight W[i] and may receive up to that many consecutive grants before priority moves to the next active requester.

Starting point

Question code

class WRRPredictor #(int N = 8);
  function void set_weights(int values[N]);
  function int predict_next(bit [N-1:0] reqs);
  function void update_state(int grant_idx, bit [N-1:0] reqs);
  function void reset();
endclass
Reviewed example

Trace one case

Input
weights=[2,1,3]; all three requesters remain asserted for six accepted grants
Expected output
grant sequence=[0,0,1,2,2,2]

Each port consumes its complete positive epoch before service rotates, exactly matching the configured weights.

What to cover

Requirements

  1. Require a positive effective weight for every port.
  2. Track the current priority port and its remaining grants in the current epoch.
  3. Skip inactive requesters and return -1 when all requests are inactive.
  4. Begin a fresh weight epoch when service moves to another port.
  5. Consume one unit only after an accepted grant.
  6. Define weight reconfiguration as starting a fresh epoch at the current priority.
Continue practicing
Q292 · ArbitersPredict a strict round-robin grantQ293 · ArbitersSelect round-robin winners with rotate and isolateQ182 · ArbitersDetect a starving arbiter requester