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Q190FreeSystemVerilog

Detect a starving arbiter requester

Difficulty
Medium
Topic
Arbiters
Language
SV
Interview prompt

Question

Add a watchdog that flags any port that remains continuously asserted but ungranted for more than K sampled cycles.

Candidate starting point

Implementation scaffold

class StarvationWatchdog #(int N = 8);
  local longint unsigned wait_count[N];
  local bit starving[N];
  local bit starvation_alert;
  local int threshold;

  function new(int threshold);
    if (N <= 0 || threshold < 0) $fatal(1, "invalid watchdog");
    this.threshold = threshold;
    foreach (wait_count[i]) begin
      wait_count[i] = 0;
      starving[i] = 0;
    end
    starvation_alert = 0;
  endfunction

  function void sample(int grant_idx, bit [N-1:0] reqs);
    // Implement here: sample.
  endfunction

  function bit port_starving(int port);
    // Implement here: port_starving.
  endfunction

  function bit any_starving();
    // Implement here: any_starving.
  endfunction
endclass
Reviewed example

Trace one case

Input
K=2; port 1 requests without a grant on sampled cycles 0, 1 and 2, then wins on cycle 3
Expected output
wait counts = 1, 2, 3 with starvation asserted after cycle 2; cycle 3 clears count and flag

Continuous ungranted assertion increments through cycles with no winner, and an accepted grant resets that port's watchdog state.

What to cover

Requirements

  1. Maintain a wait count and starvation flag per port.
  2. Increment a port only while it requests and does not receive the accepted grant.
  3. Clear a port's state when it wins or stops requesting.
  4. Set a global alert when any per-port count exceeds K.
  5. Count asserted requesters during cycles in which there is no grant.
  6. Expose both per-port status and the global alert.
Continue practicing
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