One of the key goals for USB4 is to retain compatibility with the existing ecosystem of USB3.2, USB 2.0 and Thunderbolt  products, and the resulting connection scales to the best mutual capability of the devices being connected. USB4 is designed to work with older versions of USB and Thunderbolt . USB4 Fabric support high throughput interconnects of 10 Gbps (for Gen 2) and 20 Gbps (for Gen 3) and supports Thunderbolt 3-compatible rates of 10.3125 Gbps (for Gen 2) and 20.625 Gbps (for Gen 3). It becomes very important to verify the Thunderbolt  backward compatibility with the designs. Though the support of USB4 Interoperability with Thunderbolt  3 (TBT3) is optional in USB4 host or USB4 peripheral device and required USB4 Hub and USB4 Based Dock but it is very essential to work in the existing ecosystem. 

Few Main features of USB4 Interoperability with Thunderbolt  3 (TBT3) Systems

• Support for Bi-Directional Pins & Retimers: TBT3 Active Cables can contain two bidirectional Re-timers which have the capability to send AT Responses on its RX channel. Router connected directly to such Retimer needs to support A Router that is connected directly to a bidirectional Re-timer shall support reception of Transactions on both TX and RX channels. 

• Bounce Mechanism: This feature is used by Router to access the Register Space of a Cable Re-timer that can only be accessed by its Link Partner.
• Asymmetric Negotiation: The Router which connects with Cable Retimers needs to follow Asymmetric TxFFE in Phase 5 of Lane Initialization. 
• USB4 Link Transitions: In TBT3 mode, the configuration of two independent Single Lane Links can be used non-transient state or Single Lane Link just using the Lane1 Adapter.

Cadence has a mature USB4 Verification IP solution that can help in the verification of USB4 designs with TBT3. Cadence has taken an active part in the Cairo group that defined the USB4 specification and has created a comprehensive Verification IP that is being used by multiple members. If you plan to have a USB4-compatible design, you can reduce the risk of adopting new technology by using our proven and mature USB4 Verification IP. Please contact your Cadence local account team, for more details.

Verification of a USB4 router design is not just about USB4 but also about the inclusion of the three other major protocols namely, USB3, DisplayPort (DP), and PCI Express (PCIe). These protocols can be simultaneously tunneled through a USB4 router. Put in simple terms, such tunneling involves the conversion of the respective native USB3, DP, or PCIe protocol traffic into the USB4 transport layer packets, which are tunneled through a USB4 fabric, and converted back into the respective original native protocol traffic.

It may sound simple but is perhaps not.

There are several aspects in a router that come into picture to carry out this task of conversion of native protocol traffic, route it to the intended destination, and then convert it back to the original form. Some of those are the USB3, DP and PCIe protocol adapters, transport mechanism using routing, flow control, paths, path set-up and teardown, control and configuration, configuration spaces.

That is not all. There are core USB4 specific logical layer intricacies as well, which carry out the tasks of ensuring that all the USB4 ports and links are working as desired to provide up to 40Gbps speed and that the USB4 traffic flows through out the fabric in the intended way. These bring on the table features like High Speed link, ordered sets, lane initialization, lane adapter state machine, low power, lane bonding, RS-FEC, side band channel, sleep and wake, error checking.

All of these put together give rise to a very large verification space against which a USB4 router design should be verified. If we were to break down this space it can be broadly put in the following major dimensions,

• Protocol Adapter Layer
  • USB3 tunneling
  • DP tunneling
  • PCIe tunneling
• Host Interface Adapter Layer
• Transport Layer
  • Flow control
  • Routing
  • Paths
• Configuration layer and control packet protocol
• Configuration spaces
• Logical Layer

The independent verification of these dimensions is not all that would qualify the design as verified. They have to be verified in various combinations of each other too. Overall, all the parts of a USB4 router system need to be working together coherently.

For example, the following diagram depicts the various layers that a USB4 router may comprise of,

A USB4 router or a domain of routers does not work on its own. There is a Connection Manager per domain, which is a software-based entity managing a domain. A router provides the various capabilities for a Connection Manager to carry out its responsibilities of managing a domain.

It would not be an exaggeration to say that the spectrum of verification of a USB4 router ranges from the very minute details of logical layer to the system-level like multiple dependencies as the whole USB4 system is brought up layer by layer, step-by-step.

Cadence has a mature Verification IP solution that can help in the verification of USB4 designs. Cadence has taken an active part in the working group that defined the USB4 specification and has created a comprehensive Verification IP that is being used by multiple members in the last two years.

If you plan to have a USB4 compatible design, you can reduce the risk of adopting a new technology by using our proven and mature USB4 Verification IP. Please contact your Cadence local account team for more details and to get connected.

USB4 can simultaneously tunnel USB3, PCIe and DisplayPort native protocol traffic through a hierarchy of USB4 routers. The key to tunneling of these protocols is routing table programmed at each ingress adapter. An entry of a routing table maps an incoming HopID, called Input/Ingress HopID to a corresponding pair of Output/Egress Adapter and Egress/Output HopID.

The responsibility of programming routing tables lies with the Connection Manager. Connection Manager, having the complete view of the hierarchy of the routers, programs the routing tables at all relevant adapter ports. Accordingly, the USB3, PCIe and DisplayPort protocol tunneled packets are routed, and reach their respective intended destinations.

The diagrammatic representation below is an example of tunneling of USB3 protocol traffic from USB4 Host Router to USB4 Peripheral Device Router through a USB4 Hub Router. The path from USB3 Host to USB3 Device is depicted by routing tables indicated at A -> B -> C -> D, and the one from USB3 Device to USB3 Host by routing tables indicated at E -> F -> G -> H . Note that the Input HopID from and Output HopID to all three protocol adapters for USB3, PCIe and DisplayPort Aux traffic, are fixed as 8, and for DisplayPort Main Link traffic are fixed as 9.

Once the native protocol traffic come into the transport layer of a USB4 router, the transport layer of it does not know to which native protocol a tunneled packet belongs to. The only way a transport layer tunneled packet is routed through the hierarchy of the routers is using the HopID values and the information programmed in the routing tables.

The figure below shows an example of tunneling of all the three USB3, PCIe and DisplayPort protocol traffic together. The transport layer tunneled packets of each of these native protocols are transported simultaneously through the routers hierarchy.

 Cadence has a mature Verification IP solution for the verification of USB3, PCIe and DisplayPort tunneling. This solution also employs the industry proven VIPs of each of these native protocols for native USB3, PCIe and DisplayPort traffic.

Verifying lane adapter state machine in a router design is quite an involved task and needs verification from several aspects including that for its link training functionality.

The diagram below shows two lane adapters connected to each other and each going through the link training process. Each training sub-state transition is contingent on conditions for both transmission and reception of relevant ordered sets needed for a transition. Until conditions for both are satisfied an adapter cannot transition to the next training sub-state.

As deduced from the lane adapter state machine section of USB4 specification, the reception condition for the next training sub-state transition is less strict than that of the transmission condition. For ex., for LOCK1 to LOCK2 transition, the reception condition requires only two SLOS symbols in a row being detected, while the transmission condition requires at least four complete SLOS1 ordered sets to be sent.

From the above conditions in the specification, it is a possibility that a lane adapter A may detect the two SLOS or TS ordered sets, being sent by the lane adapter B on the other end, in the very beginning as soon as it starts transmitting its own SLOS or TS ordered sets. On the other hand, it is also a possibility that these SLOS or TS ordered sets are not yet detected by lane adapter A even when it has met the condition of sending minimum number of SLOS or TS ordered sets.

In such a case, lane adapter A, even though it has satisfied the transmission condition cannot transition to the next sub-state because the reception condition is not yet met. Hence lane adapter A must first wait for the required number of ordered sets to be detected by it before it can go to the next sub-state. But this wait cannot be endless as there are timeouts defined in the specification, after which the training process may be re-attempted.

This interlocked way of operation also ensures that state machine of a lane adapter does not go out of sync with that of the other lane adapter. Such type of scenarios can occur whenever lane adapter state machine transitions to the training state from other states.

Cadence has a mature Verification IP solution for the verification of various aspects of the logical layer of a USB4 router design, with verification capabilities provided to do a comprehensive verification of it.

AN2784 - USB4712 Auto-FlexConnect Implementation Guidelines