Datasheet

• 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.
24.6.18 802.1Qav Support - Credit-based Shaping
A credit-based shaping algorithm is available on the two highest priority queues and is defined in the
standard 802.1Qav: Forwarding and Queuing Enhancements for Time-Sensitive Streams. This allows
traffic on these queues to be limited and to allow other queues to transmit.
Traffic shaping is enabled via the CBS (Credit Based Shaping) Control register. This enables a counter
which stores the amount of transmit 'credit', measured in bytes that a particular queue has. A queue may
only transmit if it has non-negative credit. If a queue has data to send, but is held off from doing as
another queue is transmitting, then credit will accumulate in the credit counter at the rate defined in the
IdleSlope register (CBSISQx) for that queue.
portTransmitRate is the transmission rate, in bits per second, that the underlying MAC service that
supports transmission through the Port provides. The value of this parameter is determined by the
operation of the MAC. IdleSlope is the rate of change of increasing credit when waiting to transmit and
must be less than the value of the portTransmitRate.
IdleSlope is the rate of change of credit when waiting to transmit and must be less than the value of the
portTransmitRate.
The max value of IdleSlope (or sendSlope) is (portTransmitRate / bits_per_MII_Clock).
In case of 100 Mbps, maximum IdleSlope = (100 Mbps / 4) = 0x17D7840.
When this queue is transmitting the credit counter is decremented at the rate of sendSlope which is
defined as (portTransmitRate - IdleSlope). A queue can accumulate negative credit when transmitting
which will hold off any other transfers from that queue until credit returns to a non-negative value. No
transfers are halted when a queue's credit becomes negative; it will accumulate negative credit until the
transfer completes.
The highest priority queue always has priority regardless of which queue has the most credit.
24.6.19 PHY Interface
Different PHY interfaces are supported by the Ethernet MAC:
• MII
• RMII
The MII interface is provided for 10/100 operation and uses txd[3:0] and rxd[3:0]. The RMII interface is
provided for 10/100 operation and uses txd[1:0] and rxd[1:0].
SAM D5x/E5x Family Data Sheet
GMAC - Ethernet MAC
© 2019 Microchip Technology Inc.
Datasheet
DS60001507E-page 506