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Intel
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82575EB Gigabit Ethernet Controller — Flow Control
Intel
®
82575EB Gigabit Ethernet Controller 324632-003
Software Developer’s Manual and EEPROM Guide Revision: 2.1
268 January 2011
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. This allows the PHY to
automatically recover once the cable plant is repaired.
11.4.1.1 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.
11.4.2 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.
Table 86 lists the intended operation for the various settings of ASM_DIR and Pause. This information is
provided for reference only; it is the responsibility of the MAC to implement the correct function. The
PHY merely enables the two MACs to communicate their abilities to each other.
Table 86. Pause And Asymmetric Pause Settings
ASM_DIR Settings Local
(PHY Register 4d, Bit 10)
and Remote (PHY Register
5d, Bit 10)
Pause Setting -
Local (PHY Register
4d, Bit 9)
Pause Setting -
Remote (PHY
Register 5d, Bit 9)
Result
Both ASM_DIR = 1b 1b 1b Symmetric - Either side can flow control the
other
1b 0b Asymmetric - Remote can flow control local
only
0b 1b Asymmetric - Local can flow control remote
0b 0b No flow control
Either or both
ASM_DIR = 0b
1b 1b Symmetric - Either side can flow control the
other
Either or both = 0b No flow control