Marvell 88e1515 PHY loopback mode test

Xavier On the platform 88E1512 PHY chip ,88E1512 The description of the chip register manual is more detailed .

First understand the basic concept of ：

  The physical layer of Gigabit Ethernet

The physical layer of Gigabit Ethernet is divided into physical coding sublayer PCS（Physical Coding Sublayer）、 Physical media connection sublayer PMA（Physical Medium Attachment） Sublayers related to physical media PMD（Physical Medium Dependent） Three layers , As shown in the figure below ：

among PCS Sub layer leader 8b10b code , It can take from GMII Received by mouth 8 Bit parallel data is converted into 10 Bit parallel data output . because 10 Bits of data can effectively reduce the DC component , Reduce the bit error rate , In addition, use 8b10b Coding is convenient for extracting clock from data and first synchronization . You can put PCS Look at both ends GMII Interface and TBI Interface .

PMA The sublayer will further PCS The coding results of the sublayer are transmitted to various physical media , It is mainly responsible for completing serial parallel conversion .PCS Layer by 125M The rate of parallel transmission 10 Bit code to PMA layer , from PMA Layer conversion to 1.25Gbps Send the serial data stream of , So that we can actually get 1Gbps Gigabit Ethernet transmission rate . You can put PMA The two ends of the sublayer are regarded as TBI Interface and SGMII Interface .

PMD The sublayer will complete the interface for various actual physical media , Complete the real physical connection . because 1000BASE-X Support multiple physical media , Such as optical fiber and shielded twisted pair , Their physical interfaces will obviously not be the same . Some need photoelectric conversion , Some have to complete the conversion from imbalance to balance .PMD The layer will specify these specific connectors .

88E1512 Four kinds of loopback Pattern

1.System Interface Loopback

 2.Synchronous SERDES Loopback

3.Line Loopback

 4.External Loopback

  Let's use it first System Interface Loopback, Relatively simple and direct configuration 0_0.14 = 1

  I use open source phytool Tool configuration BMCR Of bit14 GitHub - wkz/phytool: Linux MDIO register accesshttps://github.com/wkz/phytool This tool also supports C45 and C42 agreement

First use ethtool Check the current status ：

Read phy The value of the current register

  Set to loopback Pattern

Loopback speed is determined by Registers 21_2.{6,13}