The physical layer defines the PHY portion of a port and the physical connection between a downstream facing port (on a host or hub) and the upstream facing port on a device. The SuperSpeed physical connection is comprised of two differential data pairs, one transmit path and one receive path (see Figure). The nominal signaling data rate is 5 Gbps. The electrical aspects of each path are characterized as a transmitter, channel, and receiver; these
collectively represent a unidirectional differential link. Each differential link is AC-coupled with capacitors located on the transmitter side of the differential link. The channel includes the electrical characteristics of the cables and connectors.
At an electrical level, each differential link is initialized by enabling its receiver termination. The transmitter is responsible for detecting the far end receiver termination as an indication of a bus connection and informing the link layer so the connect status can be factored into link operation and management.
When receiver termination is present but no signaling is occurring on the differential link, it is
considered to be in the electrical idle state. When in this state, low frequency periodic signaling (LFPS) is used to signal initialization and power management information. The LFPS is relatively simple to generate and detect and uses very little power.
Each PHY has its own clock domain with Spread Spectrum Clocking (SSC) modulation. The
USB 3.0 cable does not include a reference clock so the clock domains on each end of the physical connection are not explicitly connected. Bit-level timing synchronization relies on the local receiver aligning its bit recovery clock to the remote transmitter’s clock by phase-locking to the signal transitions in the received bit stream.
The receiver needs enough transitions to reliably recover clock and data from the bit stream. To assure that adequate transitions occur in the bit stream independent of the data content being transmitted, the transmitter encodes data and control characters into symbols using an 8b/10b code. Control symbols are used to achieve byte alignment and are used for framing data and managing the link. Special characteristics make control symbols uniquely identifiable from data symbols. A number of techniques are employed to improve channel performance. For example, to avoid overdriving and improve eye margin at the receiver, transmitter de-emphasis may be applied when multiple bits of the same polarity are sent. Also, equalization may be used in the receiver with the characteristics of the equalization profile being established adaptively as part of link training. Signal (timing, jitter tolerance, etc.) and electrical (DC characteristics, channel capacitance, etc.) performance of SuperSpeed links are defined with compliance requirements specified in terms of transmit and receive signaling eyes. The physical layer receives 8-bit data from the link layer and scrambles the data to reduce EMI emissions. It then encodes the scrambled 8-bit data into 10-bit symbols for transmission over the physical connection. The resultant data are sent at a rate that includes spread spectrum to further lower the EMI emissions. The bit stream is recovered from the differential link by the receiver, assembled into 10-bit symbols, decoded and descrambled, producing 8-bit data that are then sent to the link layer for further processing
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