Texas Instruments TMS320C674X manual Collision-Based Receive Buffer Flow Control

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In either case, receive flow control prevents frame reception by issuing the flow control appropriate for the current mode of operation. Receive flow control prevents reception of frames on the EMAC until all of the triggering conditions clear, at which time frames may again be received by the EMAC.

Receive flow control is enabled by the RXBUFFERFLOWEN bit in the MAC control register (MACCONTROL). The EMAC is configured for collision or IEEE 802.3X flow control using the FULLDUPLEX bit in MACCONTROL. Receive flow control is triggered when the number of free buffers in any enabled receive channel free buffer count register (RXnFREEBUFFER) is less than or equal to the receive channel flow control threshold register (RXnFLOWTHRESH) value. Receive flow control is independent of receive QOS, except that both use the free buffer values.

2.9.1.3.1Collision-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.

2.9.1.3.2IEEE 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.

38	EMAC/MDIO Module	SPRUFL5B –April 2011
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