24.6.17 Energy Efficient Ethernet Support

Features
  • Energy Efficient Ethernet according to IEEE 802.3az
  • A system’s transmit path can enter a low power mode if there is nothing to transmit.
  • A PHY can detect whether its link partner’s transmit path is in low power mode, and configure its own receive path to enter low power mode.
  • Link remains up during lower power mode and no frames are dropped.
  • Asymmetric, one direction can be in low power mode while the other is transmitting normally.
  • LPI (Low Power Idle) signaling is used to control entry and exit to and from low power modes.
    Note: LPI signaling can only take place if both sides have indicated support for it through auto-negotiation.

Operation

  • Low power control is done at the MII (reconciliation sublayer).
  • As an architectural convenience in writing the 802.3az it is assumed that transmission is deferred by asserting carrier sense - in practice it will not be done this way. This system will know when it has nothing to transmit and only enter low power mode when it is not transmitting.
  • LPI should not be requested unless the link has been up for at least one second.
  • LPI is signaled on the MII transmit path by asserting 0x01 on txd with tx_en low and tx_er high.
  • A PHY on seeing LPI requested on the MII will send the sleep signal before going quiet. After going quiet it will periodically emit refresh signals.
  • The sleep, quiet and refresh periods are defined in 802.3az, Table 78-2.
  • LPI mode ends by transmitting normal idle for the wake time. There is a default time for this but it can be adjusted in software using the Link Layer Discovery Protocol (LLDP) described in 802.3az, Clause 79.
  • LPI is indicated at the receive side when sleep and refresh signaling has been detected.