看mpc8548的手册有这么一句：
The MIIMCFG register is written by the user to configure all MII management operations. Note that MII management hardware is shared by all eTSECs. Thus, only through the MIIM registers of eTSEC1 can external PHYs be accessed and configured.
最初很是想不通，原来MII接口里并不包含MDC和MDIO两根线。就是说可能很多组MII(或RMMI、GMMI)都共享一组MDC、MDIO。这就意味着最多只能引出30个网口。因为phy地址范围的限制，而且phy地址0x0和0x1f不能使用。MAC接交换机时，需要注意phy地址是否会冲突。
关于phy，总是听硬件设计人员说起，我并没概念
PHY is an abbreviation for the physical layer of the OSI model.
An instantiation of PHY connects a link layer device (often called a MAC) to a physical medium such as an optical fiber or copper cable. A PHY device typically includes a Physical Coding Sublayer (PCS) and a Physical Medium Dependent (PMD) layer. The PCS encodes and decodes the data that is transmitted and received. The purpose of the encoding is to make it easier for the receiver to recover the signal.
[edit]Example uses
Wi-Fi: The PHY portion consists of the RF, mixed-signal and analog portions, that are often called transceivers, and the digital baseband portion that place high demand on the digital signal processing (DSP) and communication algorithm processing, including channel codes. It is common that these PHY portions are integrated with the media access control (MAC) layer in System-on-a-chip (SOC) implementations. Other similar wireless applications are 3G/4G/LTE, WiMAX, UWB etc.
Ethernet: A PHY chip (PHYceiver) is commonly found on Ethernet devices. Its purpose is physical, analog signal access to the link. It is usually used in conjunction with an Media Independent Interface (MII) chip or interfaced to a microcontroller that takes care of the higher layer functions.
Universal Serial Bus (USB): A PHY chip is integrated into most USB controllers in hosts or embedded systems and provides the bridge between the digital and modulated parts of the interface.
IrDA: The Infrared Data Associations IrDA specification includes an IrPHY specification for the physical layer of the data transport.
Serial ATA (SATA): Serial ATA controllers like the VIA Technologies VT6421 use a PHY.
SDRAM chip interfaces
Flash memory chip interfaces
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