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Hardware interview practice

Check per-channel link credits

HardComputer ArchitectureSystemVerilog

A two-channel link starts with four transmit credits per channel. Sending consumes one credit; a returned credit restores one. Write a SystemVerilog checker for credit accounting and protocol errors.

Starting point

Question code

input logic clk, rst_n;
input logic tx_fire, tx_ch;
input logic cr_valid, cr_ch;
output logic error;
Reviewed example

Work through one case

Input
Channel 1 has two credits and observes tx_fire and cr_valid for channel 1 on the same edge.
Expected output
Channel 1 remains at two credits and error remains clear.

At a non-boundary count, the legal send consumes exactly the credit restored by the same-channel return.

What to cover

Requirements

  1. Track an integer available[2], initialized to four for each channel on reset and constrained to 0..4.
  2. A tx_fire consumes one from tx_ch; if its pre-edge count is zero, leave the count unchanged and set sticky error.
  3. A return without a same-channel send adds one; if its pre-edge count is four, leave that count unchanged and set sticky error.
  4. For a same-channel send and return: counts 1..3 stay unchanged; at 0 flag the send and apply the return to reach 1; at 4 flag the return and apply the send to reach 3.
Continue practicing
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