Texas Instruments TMS320C674X manual Transmit Flow Control, Speed, Duplex, and Pause Frame Support

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2.9.2.6Transmit Flow Control

Incoming pause frames are acted upon, when enabled, to prevent the EMAC from transmitting any further frames. Incoming pause frames are only acted upon when the FULLDUPLEX and TXFLOWEN bits in the MAC control register (MACCONTROL) 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 receive multicast/broadcast/promiscuous channel enable register (RXMBPENABLE) is set. The TXFLOWEN and FULLDUPLEX bits affect whether or not MAC control frames are acted upon, but they have no affect upon whether or not 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:

•TXFLOWEN bit is set in MACCONTROL

•The frame’s length is 64 to RXMAXLEN 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:

•If the destination address is not equal to the reserved multicast address or any enabled or disabled unicast address, then the transmit pause timer immediately expires, or

•If the new pause time value is 0, then the transmit pause timer immediately expires, else

•The EMAC transmit pause timer immediately is set 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 nonzero pause time has finished being received (MII_RXDV going inactive). No transmission begins until the pause timer has expired (the EMAC may transmit pause frames in order 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 or not.

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

•The 48-bit source address of the transmitting device.

•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.

2.9.2.7Speed, Duplex, and Pause Frame Support

The MAC operates at 10 Mbps or 100 Mbps, in half-duplex or full-duplex mode, and with or without pause frame support as configured by the host.

40	EMAC/MDIO Module	SPRUFL5B –April 2011
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