====== Energy-Efficient Ethernet (EEE) ======

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|desc=Introduce ''Energy-Efficient Ethernet'' specified in **IEEE 802.3**.}}

===== Overview =====
The EEE capability combines the IEEE 802.3 **Media Access Control (MAC) Sublayer** with a family of **Physical Layers** defined to support operation in the ''Low Power Idle (LPI)'' mode. When the LPI mode is enabled, systems on both sides of the link can save power during periods of low link utilization. The key features of EEE are listed below:
  * Provides a **protocol** (The next sections describe the protocols that run on the [[#MII|MII]] and [[#GMII|GMII]].) to coordinate transitions to or from a lower level of power consumption.
  * Without changing the link status.
  * Without dropping or corrupting frames.
  * The transition time in to and out of the lower level of power consumption is kept **small enough to be transparent** to upper layer protocols and applications.

<callout type="warning">
EEE only takes effect when both the MAC and PHY support EEE.
</callout>

===== MII =====

==== Transmit direction LPI transition ====
  * When the transmit LPI state diagram is in state ''LPI_ASSERTED'', the LPI client requests the PHY to transition to the LPI state by **deasserting ''TX_EN''**, **asserting ''TX_ER''**, and setting **''TXD<3:0>'' to ''0b0001''**.
  * The LPI client requests the PHY to transition **out of** the LPI state by **deasserting ''TX_ER'' and ''TXD''**
{{:network:ethernet:802-3:media:img:20230628-070859.png?direct&600}}

==== Receive direction LPI transition ====
  * When the PHY receives signals from the link partner to indicate transition **into** the low power state, it indicates this to the LPI client by **asserting ''RX_ER''** and setting **''RXD<3:0>'' to ''0b0001''** while keeping **''RX_DV'' deasserted**.
  * When the PHY receives signals from the link partner to indicate transition **out of** the low power state, it indicates this to the LPI client by **deasserting ''RX_ER''** and returning to a normal interframe state.
{{:network:ethernet:802-3:media:img:20230628-070919.png?direct&600}}

===== GMII =====
The GMII has the same LPI mechanism as the [[#MII|MII]], except that ''TXD<7:0>'' and ''RXD<7:0>'' are set to ''0x01''.

===== Note =====
  * When PHY or MAC loopback is enabled, the MAC cannot sense the LPI signals it sends. <wrap suspect>I guess this is because RS maps the xMII to the PLS, not the LPI client, when loopback is enabled.</wrap>

===== Reference =====
  * //**IEEE 802.3-2022** Clause 78 - Energy-Efficient Ethernet (EEE)//
  * //**IEEE 802.3-2022** Clause 22.2.2 - MII signal functional specifications//
  * //**IEEE 802.3-2022** Clause 35.2.2 - GMII signal functional specifications//