USB PD 3.1 (protocol layer)

An independent clean-room USB Power Delivery 3.1 protocol-layer VIP: a Source BFM that holds its own capability list and drives a modelled VBUS rail, a separately written Sink BFM that keeps ITS OWN copy of that list and lives with whatever voltage arrives, and a passive protocol checker (“usbpd/, make usbpd_oss, 25/25 checks). Seven rules — GoodCRC echo and presence, MessageID progression across Soft_Reset, Object Position in range (including the reserved position 0), requested current within the selected PDO, PS_RDY only after Accept, and Request object count — each with a test that fires it. SCOPE: the protocol layer and the policy engine above it. The BMC-encoded CC wire (biphase mark coding, 4b5b, the packet CRC) is the PD PHY and is NOT modelled; messages are typed transactions. Chunked Extended messages, PPS/APDO regulation, EPR and cable-plug SOP’/SOP” discovery are out of scope and the package header says so rather than listing them. This is the one VIP in the catalog whose failure mode is physical: a mis-negotiated contract puts real voltage on a real pin, and its sharpest test is a NEGATIVE one. T9 re-advertises the capability list while the sink’s policy decision is already taken: Object Position 2 was 9 V when the sink chose it and is 20 V when the source honours it. Every field is legal — position in range, current within the PDO, IDs correct, GoodCRCs present — so the passive checker is blind by construction, and a 9 V-rated sink takes 20 V. The wire carries an INDEX into a list and no record of which version the sink was reading; the protection is a policy-engine obligation no monitor can enforce.

Key Features

• Source and Sink policy engines, each reading only its own capability list
• PDO/RDO encode and decode (fixed supply), Object Position selection by index
• Explicit-contract AMS: Source_Capabilities → Request → Accept → PS_RDY, plus Reject/Soft_Reset
• Passive checker, 7 rules, each with an injected fault that fires exactly it
• A modelled VBUS rail and a sink input rating, so overvoltage is observable
• The stale-capability-list blind spot named and asserted, not papered over

Standards & Compliance

USB-IF USB Power Delivery Specification v3.1 — protocol layer