Fertigation Frame Guard

Vela Growers runs a fertigation system: a computer doses fertilizer and acid into the greenhouse irrigation water. Recipe frames travel to the dosing engine over 300 m of noisy serial cable, past 200 lamp igniters. Last April a corrupted frame told the acid pump to run 9 times longer than intended. The dosing hardware obeyed, and the acid overdose killed three rows of seedlings. The corrective action lands on your desk.

Two habits of the sending computer matter. Its watchdog restarts transmissions in the middle of anything. And every few seconds it sends a heartbeat: a frame with LEN of 0. The old guard prototype locked up on the first heartbeat of the season. That is why the greenhouse still runs unguarded.

Your task: build a guard block between the line decoder and the dosing engine, so that no frame reaches the engine without the guard's approval. The decoder hands you the framing: sof pulses at the start of a frame, and dv pulses once for each received byte on d. Check each frame's checksum as the bytes stream past. Pulse one verdict per frame: ok when the sum matches, bad when it does not.

Interface

Behavior

• After sof, the first dv byte is LEN, the next LEN dv bytes are payload, and the following dv byte is the checksum
• The checksum is valid when it equals (LEN + sum of payload bytes) mod 256; the length byte is inside the sum
• The cycle after the checksum byte is sampled, exactly one of ok or bad pulses for exactly one cycle; both are 0 at all other times
• LEN = 0 is legal (heartbeat): the checksum byte follows the length byte directly and must equal LEN
• An sof at any point abandons the frame in progress with no verdict and starts a fresh frame
• Nothing from the abandoned frame may leak into the new frame's sum
• A dv coinciding with sof is ignored: the byte belongs to the marker, and the next dv byte is the new frame's LEN
• dv bytes outside any frame are ignored; gaps of any length between bytes are legal
• A new sof may arrive on the very cycle a verdict pulses
• rst = 1 at a rising edge returns the guard to idle, verdict pulses low, frame in progress forgotten

What the bench checks

• The testbench checks ok and bad on every cycle
• Good and bad frames verify that the checksum includes LEN and that each verdict lasts one cycle
• Valid frames cover arbitrary gaps, zero-length heartbeats, and an eight-byte payload
• Wrapped checksum sums are accepted modulo 256, while an off-by-one checksum after the wrap is rejected
• A new sof mid-payload or where a checksum was expected must abandon the old frame without a verdict
• A byte arriving with sof and stray bytes outside a frame must be ignored
• Reset mid-frame must cancel it, and a new sof during a verdict pulse must start the next frame cleanly

Constraints

Do not add ports.