The physical layer is responsible for the transmission of the raw bit stream over the PHYsical transport medium and is the lowest layer within the OSI network model. With high-speed interfaces, such as the serial protocols USB 2.0, PCI Express, SATA, and DDR2, the PHY serves as a bridge between the digital and modulated parts of the interface. The trend is to integrate these mixed-signal interfaces into systems-on-a-chip (SoCs) that are manufactured in digital logic deep sub-micron technologies with channel lengths of 65-nm and 45-nm. These technologies are tuned for digital integration and pose a number of challenges to the mixed-signal circuit designer, such as low operating voltages that reduce head room for analog circuits.

After an introduction to circuit and process trends in deep sub-micron technologies, this paper presents a complete protocol solution using the high speed memory DDR2 interface as an example. This is followed by a description of the implementation challenges of integrating intellectual property (IP) into a system-on-a-chip (SoC). The paper concludes with a proposal for production testing a high-speed serial PHYs.