AN1120 Ethernet Theory of Operation However, with ubiquitous deployment, internet Author: M. Simmons connectivity, high data rates and limitless range Microchip Technology Inc. expansibility, Ethernet can accommodate nearly all wired communications requirements. Potential INTRODUCTION applications include: Remote sensing and monitoring This document specifies the theory and operation of Remote command, control and firmware updating the Ethernet technology found in PIC MCUs with Bulk data transfer integrated Ethernet and in stand-alone Ethernet Live streaming audio, video and media controllers. Public data acquisition (date/time, stock quotes, Ethernet technology contains acronyms and terms news releases, etc.) defined in Table 1. THEORY OF OPERATION APPLICATIONS Ethernet is a data link and physical layer protocol Ethernet is an asynchronous Carrier Sense Multiple defined by the IEEE 802.3 specification. It comes in Access with Collision Detect (CSMA/CD) many flavors, defined by maximum bit rate, mode of protocol/interface, with a payload size of 46-1500 octets. transmission and physical transmission medium. With data rates of tens to hundreds of megabits/second, Maximum Bit Rate (Mbits/s): 10, 100, 1000, etc. it is generally not well suited for low-power applications. Mode of Transmission: Broadband, Baseband Physical Transmission Medium: Coax, Fiber, UTP, etc. TABLE 1: ETHERNET GLOSSARY Term Definition CRC Cyclic Redundancy Check: Type of checksum algorithm used when computing the FCS for all Ethernet frames and the hash table key for hash table filtering of receive packets. DA Destination Address: The 6-octet destination address field of an Ethernet frame. ESD End-of-Stream Delimiter: In 100 Mb/s operation, the ESD is transmitted after the FCS (during the inter-frame gap) to denote the end of the frame. FCS Frame Check Sequence: The 4-octet field at the end of an Ethernet frame that holds the error detection checksum for that frame. IP Internet Protocol: Refers either to IPv4 or IPv6. LAN Local Area Network or Large Area Network. MAC Media Access Control: The block responsible for implementing the Media Access Control functions of the Ethernet specification. MAC Address A 6-octet number representing the physical address of the node(s) on an Ethernet network. Every Ethernet frame contains both a source and destination address, both of which are MAC addresses. MDI Medium Dependent Interface or Management Data Input. MDO Management Data Output. MDIO Management Data Input/Output. MII Media Independent Interface: Standard 4-bit interface between the MAC and the PHY for communicating TX and RX frame data. In 10 Mb/s mode, the MII runs at 2.5 MHz in 100 Mb/s mode, it runs at 25 MHz. MIIM MII Management: Set of MII sideband signals used for accessing the PHY registers. 2008 Microchip Technology Inc. DS01120A-page 1AN1120 TABLE 1: ETHERNET GLOSSARY (CONTINUED) Term Definition OUI Organizationally Unique Identifier: The upper three octets of a MAC address are referred to as the OUI, and typically are assigned to an organization or company. Microchips OUI is 00-04-A3h. Octet In Ethernet terms, one 8-bit byte. Packet Buffer The physical or virtual memory where all transmit and receive packets (frames) are stored. PHY The block that implements the Ethernet physical layer. RAM Random Access Memory (normally volatile memory). Receive Buffer Logical portion of the packet buffer used to store received packets. RX Receive. SA Source Address: The 6-octet source address field of an Ethernet frame. SFD Start Frame Delimiter: The single octet field of an Ethernet frame that marks the start of a frame. SPI Serial Peripheral Interface. SSD Start-of-Stream Delimiter: In 100 Mb/s Ethernet, the first octet of the preamble is known as the SSD and is encoded differently from the rest of the preamble. Station Address The Station Address is the MAC address of the Ethernet node. It is typically compared against the destination address in a received Ethernet frame to determine if the frame should be received or not. On the transmit side, it is typically transmitted as the source address of an Ethernet frame. Transmit Buffer Logical portion of the packet buffer used to store packets to be transmitted. TX Transmit. RMII Reduced Media Independent Interface: A 2-bit version of the MII. SMII Serial Media Independent Interface: A 1-bit version of the MII. NRZI Non-Return-to-Zero Inverted: A binary code in which a logical one is represented by a signal transition and a logical zero is represented by the lack of a transition. DS01120A-page 2 2008 Microchip Technology Inc.