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EMAC Functional Architecture
2.10.2.5 Back Off
The EMAC implements the 802.3 binary exponential back-off algorithm.
2.10.2.6 Transmit Flow Control
When enabled, incoming pause frames are acted upon to prevent the EMAC from transmitting any further
frames. Incoming pause frames are only acted upon when the FULLDUPLEX and TXFLOWEN bits in the
MACCONTROL register are set. Pause frames are not acted upon in half-duplex mode. Pause frame
action is taken if enabled, but normally the frame is filtered and not transferred to memory. MAC control
frames are transferred to memory, if the RXCMFEN bit in the RXMBPENABLE register is set. The
TXFLOWEN and FULLDUPLEX bits affect whether MAC control frames are acted upon, but they have no
effect upon whether MAC control frames are transferred to memory or filtered.
Pause frames are a subset of MAC control frames with an opcode field of 0001h. Incoming pause frames
are only acted upon by the EMAC if the following conditions occur:
• The TXFLOWEN bit is set in the MACCONTROL register.
• The frame's length is between 64 bytes and RXM AXLEN bytes inclusive.
• The frame contains no CRC error or align/code errors.
The pause time value from valid frames is extracted from the two bytes following the opcode. The pause
time is loaded into the EMAC transmit pause timer and the transmit pause time period begins.
If a valid pause frame is received during the transmit pause time period of a previous transmit pause
frame, then either the destination address is not equal to the reserved multicast address or any enabled or
disabled unicast address, and the transmit pause timer immediately expires; or the new pause time value
is 0, and the transmit pause timer immediately expires. Otherwise, the EMAC transmit pause timer is set
immediately to the new pause frame pause time value. (Any remaining pause time from the previous
pause frame is discarded.)
If the TXFLOWEN bit in MACCONTROL is cleared, then the pause timer immediately expires.
The EMAC does not start the transmission of a new data frame any sooner than 512-bit times after a
pause frame with a non-zero pause time has finished being received (MRXDV going inactive). No
transmission begins until the pause timer has expired (the EMAC may transmit pause frames to initiate
outgoing flow control). Any frame already in transmission when a pause frame is received is completed
and unaffected.
Incoming pause frames consist of:
• A 48-bit destination address equal to one of the following:
– The reserved multicast destination address 01.80.C2.00.00.01h
– Any EMAC 48-bit unicast address. Pause frames are accepted, regardless of whether the channel
is enabled.
• The 48-bit source address of the transmitting device
• 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. A pause-quantum is 512 bit-times
• Padding to 64-byte data length
• 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.
The padding is required to make up the frame to a minimum of 64 bytes. The standard allows pause
frames longer than 64 bytes to be discarded or interpreted as valid pause frames. The EMAC recognizes
any pause frame between 64 bytes and RXMAXLEN bytes in length.
57
SPRUEF8F–March 2006–Revised November 2010 C6472/TCI6486 EMAC/MDIO
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