28Verification

Fix a countdown that never reports done

Problem

Intended behavior. Countdown waits a programmable number of cycles. On an edge with load high, count takes the value of cycles. On every other edge, a nonzero count goes down by one and a zero count stays 0. left shows count, and done is high exactly when count is 0. After a load of 5, done rises five edges after the load edge. Reset clears count, so done is high after reset.

Current behavior. The starter compiles, loads correctly, and starts counting down, but after a load of 5 done never rises. The bench's wait loop runs until chiseltest's idle-cycle timeout stops it.

Your task. Find why the count never reaches 0 and fix the design. Keep the ports and the public bench.

Read and predict

The bench sets a 20-cycle timeout, checks done after reset, loads 5, checks left right after the load edge, and then steps until done reads 1. After each step it prints WAIT n left=.... It catches the TimeoutException and prints its message on a TIMEOUT line, so the run ends normally either way.

Before running, write the WAIT lines a correct countdown prints and the number on its DONE line. Then run the starter and compare.

Diagnose

The timeout message only says that 20 cycles passed without an input-changing poke. Peeking at outputs or poking the same input value does not restart that count. It does not say why the countdown stalled. The WAIT lines do: find the first line where left stops changing, and note the value it is stuck at. Then evaluate the condition that guards the decrement at that value.

Also decide which side is wrong, the bench or the design. The contract says done follows count reaching 0, so a longer timeout would only make the same hang take longer.

Completion check

• After a load of n, done rises exactly n edges after the load edge, for small values, for 1, and for the maximum 255.
• A load of 0 keeps done high, and left never wraps below 0.
• load takes priority over counting: loading again while counting restarts from the new value, and a held load keeps reloading.
• Raising done one value early is not a fix. left itself has to reach 0.

Hints

Variation

In the fixed design, lower the timeout to 4 and predict whether the run still passes. Then explain how the right timeout depends on the longest count the bench loads.