The physical layer interfaces the data link layer with signaling technology for link data interchange, and is subdivided into the logical sub-block and the electrical sub-block.
- The logical sub-block frames and de-frames TLPs and DLLPs. It also implements the link training and status state machine (LTSSM), which handles link initialization, training, and maintenance. Scrambling and descrambling of data (for Gen1/Gen2/Gen3/Gen4/Gen5 operation) is also performed in this sub-block.
- The electrical sub-block defines the input and output buffer characteristics that interface the device to the PCIe link. The physical layer also supports lane reversal (for multi-lane designs) and lane polarity inversion, as required by the PCI Express Base Specification 5.0 (https://www.pcisig.com/specifications).
Data exchange with the other components on the link occurs over the serial lines of one or more gigabit transceivers (GTs), which expose parallel interfaces at lower clock frequencies to the PCIe controller. For Gen1, Gen2, Gen3, Gen4, and Gen5 operation, the physical layer is up-configuration capable in the downstream port mode only.