The objective of USB4 protocol to achieve high speed signal transmission and thereby providing high data bandwidths for protocol tunneling would not have been possible without USB4 re-timer. Like several other serial protocols where the generation-by-generation higher link speeds are being targeted, USB4 system too needs re-timers, whether On-board or in Active cables, to support 40Gbps link speed.
Re-timers are protocol aware components on the serial lines which recover the incoming data and transmit a fresh copy of it from its other side. USB4 Re-timers between two routers take part in the lane initialization process, do their own receiver equalization, bit synchronization and clock recovery and switching. Once done, they forward the traffic from their receiver to their transmitter.
Re-timers play important role not only in data forwarding but also during low power entry and exits.
USB4 Specification allows up to six re-timers in between two routers. More the re-timers, more intricate the link functioning becomes, especially for low power exits. During low power exit each re-timer needs to switch to transmitting various type of signals like Low Frequency Periodic Signaling, Electrical Idle and Low Power Ordered Sets, in one or both directions depending on the type of low power exit.
For the Thunderbolt3 (TBT3) compatibility of a USB4 system, re-timer also have specific functionality of being bi-directional re-timer in TBT3 active cable. This means that the sideband channel of a re-timer can be bi-directional when it is operating in a TBT3-compatible link.
One of an important aspect of USB4 router design verification is to verify it with Re-timers too. For this there must be one or more re-timer verification components connected between the router design and the router verification component.
Similarly, for a re-timer design verification also, it is important to have it verified with multiple re-timers between two router verification components.
Cadence has a mature Verification IP solution, including re-timer aspects, for the verification of various aspects of a USB4 router or a re-timer design, with verification capabilities provided to do a comprehensive verification of these.