Output (MDIO) Module User's Guide

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Architecture

2.9.1.3.1 Collision-Based Receive Buffer Flow Control

Collision-based receive buffer flow control provides a means of preventing frame reception when the

EMAC is operating in half-duplex mode (the FULLDUPLEX bit is cleared in MACCONTROL). When

receive flow control is enabled and triggered, the EMAC generates collisions for received frames. The jam

sequence transmitted is the 12-byte sequence C3.C3.C3.C3.C3.C3.C3.C3.C3.C3.C3.C3h. The jam

sequence begins no later than approximately as the source address starts to be received. Note that these

forced collisions are not limited to a maximum of 16 consecutive collisions, and are independent of the

normal back-off algorithm.

Receive flow control does not depend on the value of the incoming frame destination address. A collision

is generated for any incoming packet, regardless of the destination address, if any EMAC enabled

channel’s free buffer register value is less than or equal to the channel’s flow threshold value.

2.9.1.3.2 IEEE 802.3x-Based Receive Buffer Flow Control

IEEE 802.3x-based receive buffer flow control provides a means of preventing frame reception when the

EMAC is operating in full-duplex mode (the FULLDUPLEX bit is set in MACCONTROL). When receive

flow control is enabled and triggered, the EMAC transmits a pause frame to request that the sending

station stop transmitting for the period indicated within the transmitted pause frame.

The EMAC transmits a pause frame to the reserved multicast address at the first available opportunity

(immediately if currently idle or following the completion of the frame currently being transmitted). The

pause frame contains the maximum possible value for the pause time (FFFFh). The EMAC counts the

receive pause frame time (decrements FF00h to 0) and retransmits an outgoing pause frame, if the count

reaches 0. When the flow control request is removed, the EMAC transmits a pause frame with a zero

pause time to cancel the pause request.

Note that transmitted pause frames are only a request to the other end station to stop transmitting.

Frames that are received during the pause interval are received normally (provided the receive FIFO is not

full).

Pause frames are transmitted if enabled and triggered, regardless of whether or not the EMAC is

observing the pause time period from an incoming pause frame.

The EMAC transmits pause frames as described below:

• The 48-bit reserved multicast destination address 01.80.C2.00.00.01h.

• The 48-bit source address (set using the MACSRCADDRLO and MACSRCADDRHI registers).

• The 16-bit length/type field containing the value 88.08h.

• The 16-bit pause opcode equal to 00.01h.

• The 16-bit pause time value of FF.FFh. A pause-quantum is 512 bit-times. Pause frames sent to

cancel a pause request have a pause time value of 00.00h.

• Zero padding to 64-byte data length (EMAC transmits only 64-byte pause frames).

• The 32-bit frame-check sequence (CRC word).

All quantities are hexadecimal and are transmitted most-significant byte first. The least-significant bit (LSB)

is transferred first in each byte.

If the RXBUFFERFLOWEN bit in MACCONTROL is cleared to 0 while the pause time is nonzero, then the

pause time is cleared to 0 and a zero count pause frame is sent.

Ethernet Media Access Controller (EMAC)/Management Data Input/Output (MDIO)42 SPRUEQ6 – December 2007

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