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Datasheet 35
Networking Silicon — 82555
8.0 Auto-Negotiation Functionality
The 82555 supports Auto-Negotiation. Auto-Negotiation is a scheme of auto-configuration
designed to manage interoperability in multifunctional LAN environments. It allows two stations
with “N” different modes of communication to establish a common mode of operation. At power-
up, Auto-Negotiation automatically establishes a link that takes advantage of an Auto-Negotiation
capable device. An Auto-Negotiation capable device can detect and automatically configure its
port to take maximum advantage of common modes of operation without user intervention or prior
knowledge by either station. The possible common modes of operation are: 100BASE-TX,
100BASE-TX Full Duplex, 100BASE-T4, 10BASE-T, and 10BASE-T Full Duplex.
8.1 Description
Auto-Negotiation selects the fastest operating mode (in other words, the highest common
denominator) available to hardware at both ends of the cable. A PHY’s capability is encoded by
bursts of link pulses called Fast Link Pulses (FLPs). Connection is established by FLP exchange
and handshake during link initialization time. Once the link is established by this handshake, the
native link pulse scheme resumes (that is, 10BASE-T or 100BASE-TX link pulses). A reset or
management renegotiate command (through the MDI interface) will restart the process. To enable
Auto-Negotiation, bit 12 of the MDI Control Register must be set. If the 82555 cannot perform
Auto-Negotiation, it will set this bit to 0b and determine the speed using Parallel Detection.
The 82555 supports four technologies: 100BASE-Tx Full and Half Duplex and 10BASE-T Full
and Half Duplex. Since only one technology can be used at a time (after every renegotiate
command), a prioritization scheme must be used to ensure that the highest common denominator
ability is chosen.
Table 4 lists the technology ability field bit assignments. Each bit in this table is
set according to what the PHY is capable of supporting. In the case of the 82555, bits 0, 1, 2, and 3
are set. Table 5 lists the priority of each of the technologies.
Table 4. Technology Ability Field Bit Assignments
Bit Setting Technology
0 10BASE-T Half Duplex
1 10BASE-T Full Duplex
2 100BASE-T Half Duplex
3 100BASE-T Full Duplex
4 100BASE-T4
5 Pause (Flow Control)
6 Reserved
7 Reserved
Table 5. Technology Priority
Priority Technology
1 100BASE-TX Full Duplex
2 100BASE-T4
3 100BASE-TX Half Duplex