USB4 Version 2.0 - Gen4 Link Recovery

By Cadence

June 10, 2024

USB4 Version 2.0 specification was released by the USB Promoter Group two years back. This specification enables up to 80Gbps link speed per direction in symmetric mode and 120Gbps link speed in asymmetric mode.

Here, we take an overview of the Gen4 link recovery mechanism, which is an autonomous process. It is initiated by a router when it encounters uncorrectable error events. These error events could be a timeout error, de-skew buffer error, or RS-FEC decode error.

Gen4 link recovery uses the newly defined ELT_recovery transaction of the sideband channel. This transaction is used for both Gen4 Link Recovery initiation and completion.

A router sends ELT_recovery transaction if there is an error case that requires Gen4 link recovery, or it receives ELT_recovery transaction, or ‘Initiate Gen 4 Link Recovery’ bit in PORT_CS_19 is set to ‘b1.

Gen4 link recovery preserves the current configuration space and the router states. It would also stop time-sync handshakes and transport layer scheduling, but other than that, it would stop idle packets.

An ELT_recovery transaction is sent on the sideband channel to the other router. When the other router receives this ELT_Recovery transaction, it also sends an ELT_Recovery Transaction back to the initiating router.

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