user manual

Software Developer’s Manual 193
PHY Functionality and Features
11.7.3 10BASE-T
For 10BASE-T links, the PHY and its link partner begin exchanging Normal Link Pulses (NLPs).
The PHY transmits an NLP every 16 ms, and expects to receive one every 10 to 20 ms. The link is
maintained as long as normal link pulses are received.
11.8 Link Enhancements
The PHY offers two enhanced link functions, each of which are discussed in the sections that
follow:
• SmartSpeed
• Flow Control
11.8.1 SmartSpeed
SmartSpeed is an enhancement to Auto-Negotiation that enables the PHY to react intelligently to
network conditions that prohibit establishment of a 1000BASE-T link, such as cable problems.
Such problems might enable Auto-Negotiation to complete, but then inhibit completion of the
training phase. Normally, if a 1000BASE-T link fails, the PHY returns to the Auto-Negotiation
state with the same speed settings indefinitely. With SmartSpeed enabled, after five failed attempts,
the PHY automatically downgrades the highest ability it advertises to the next lower speed: from
1000 to 100 to 10. Once a link is established, and if it is later broken, the PHY automatically
upgrades the capabilities advertised to the original setting.
11.8.1.1 Using SmartSpeed
SmartSpeed is enabled by setting PHY register 16d, bit 7 to 1b. When SmartSpeed downgrades the
PHY advertised capabilities, it sets bit 5 of PHY register 19. When link is established, its speed is
indicated in PHY register 17, bits 15:14. SmartSpeed automatically resets the highest-level Auto-
Negotiation abilities advertised, if link is established and then lost for more than two seconds.
11.8.2 Flow Control
Flow control enables congested nodes to pause traffic. MACs indicate their ability to implement
flow control during Auto-Negotiation.
The PHY transparently supports MAC-to-MAC advertisement of flow control through its Auto-
Negotiation process. Prior to Auto-Negotiation, the MAC indicates its flow control capabilities via
PHY register 4d, bit 10 (Pause) and PHY register 4d, bit 11 (ASM_DIR). After Auto-Negotiation,
the link partner’s flow control capabilities are indicated in PHY register 5d, bits 11:10.