17Implementation

Close a ready-valid valve in both directions

Problem

Intended behavior. Valve sits on a Decoupled link between a producer (in) and a consumer (out). While open is high it is transparent: the producer's offer reaches the consumer, and the consumer's ready reaches the producer. While open is low, nothing passes. In every cycle the producer and the consumer must agree: a value leaves the producer exactly when the consumer receives it.

Current state. The forward half is written. out.bits follows the producer directly, while out.valid is gated by open. The backward half is a placeholder that holds in.ready low, so the producer never sees a transfer and keeps offering the same value.

Your task. Replace the TODO assignment so in.ready reports the cycles in which the offered value really passes through. Keep the ports and the forward assignments unchanged.

Read and predict

The public run plays both ends. The producer offers 1, then 2, and so on, moving to the next value only after a cycle with sent=1. The consumer's ready and the open input follow fixed patterns. got shows the value the consumer received in that cycle, or -.

Run the starter first. Predict how often the consumer receives 1, and why that is worse than receiving nothing. Then, for a correct valve, predict the sent column: it must equal 1 in exactly the cycles where got is not -, and the consumer must receive 1, 2, 3, 4 once each.

Find the rule

A transfer on a ready-valid link happens when valid and ready are high in the same cycle. On the consumer side, valid is already gated by open. Ask what the producer must see so that its own fire is true in exactly the same cycles as the consumer's fire.

Run and explain

Run the repaired valve. The sent column must be 1 on exactly the lines where got shows a value, and got must read 1, 2, 3, 4 in order. Then look at cycle 2, where open is low while the consumer is ready, and explain what your in.ready reports there and why.

Completion check

When open is high, in.ready follows the consumer's ready.
When open is low, the producer never sees a transfer, even if the consumer is ready.
In every cycle, in.fire and out.fire are equal: no value is lost and none is duplicated.
Passing ready straight through is not enough; check a cycle where open is low and the consumer is ready.

Hints

Hint 1

The starter's duplicate 1s come from a consumer-side transfer the producer never heard about. The opposite mistake loses values instead.

Hint 2

The backward signal needs the same gate as the forward one. Combine the consumer's ready with open.

Concept

Decoupled carries signals in both directions, so any logic placed on the link must treat both. Gating only valid makes the two ends disagree about which cycles transferred data. The fire of each side has to describe the same event.

Variation

Suppose open also had to wait for a value: the valve opens only while in.bits is even. Predict which parts of the forward and backward logic change, and which values from the public stream would reach the consumer.