Network Card User Manual

Architecture

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2.10.2 Data Transmission

The EMAC passes data to the PHY from the transmit FIFO (when enabled). Data is synchronized to the

transmit clock rate. Transmission begins when there are TXCELLTHRESH cells of 64 bytes each, or a

complete packet, in the FIFO.

2.10.2.1 Transmit Control

A jam sequence is output if a collision is detected on a transmit packet. If the collision was late (after the

first 64 bytes have been transmitted), the collision is ignored. If the collision is not late, the controller will

back off before retrying the frame transmission. When operating in full-duplex mode, the carrier sense

(EMAC_CRS) and collision-sensing (EMAC_COL) modes are disabled.

2.10.2.2 CRC Insertion

If the SOP buffer descriptor PASSCRC flag is cleared, the EMAC generates and appends a 32-bit

Ethernet CRC onto the transmitted data. For the EMAC-generated CRC case, a CRC (or placeholder) at

the end of the data is allowed but not required. The buffer byte count value should not include the CRC

bytes, if they are present.

If the SOP buffer descriptor PASSCRC flag is set, then the last four bytes of the transmit data are

transmitted as the frame CRC. The four CRC data bytes should be the last four bytes of the frame and

should be included in the buffer byte count value. The MAC performs no error checking on the outgoing

CRC.

2.10.2.3 Adaptive Performance Optimization (APO)

The EMAC incorporates adaptive performance optimization (APO) logic that may be enabled by setting

the TXPACE bit in the MAC control register (MACCONTROL). Transmission pacing to enhance

performance is enabled when the TXPACE bit is set. Adaptive performance pacing introduces delays into

the normal transmission of frames, delaying transmission attempts between stations, reducing the

probability of collisions occurring during heavy traffic (as indicated by frame deferrals and collisions),

thereby, increasing the chance of successful transmission.

When a frame is deferred, suffers a single collision, multiple collisions, or excessive collisions, the pacing

counter is loaded with an initial value of 31. When a frame is transmitted successfully (without

experiencing a deferral, single collision, multiple collision, or excessive collision), the pacing counter is

decremented by 1, down to 0.

With pacing enabled, a new frame is permitted to immediately (after one interpacket gap) attempt

transmission only if the pacing counter is 0. If the pacing counter is nonzero, the frame is delayed by the

pacing delay of approximately four interpacket gap (IPG)delays. APO only affects the IPG preceding the

first attempt at transmitting a frame; APO does not affect the back-off algorithm for retransmitted frames.

2.10.2.4 Interpacket-Gap (IPG) Enforcement

The measurement reference for the IPG of 96 bit times is changed depending on frame traffic conditions.

If a frame is successfully transmitted without collision and EMAC_CRS is deasserted within approximately

48 bit times of EMAC_TX_EN being deasserted, then 96 bit times is measured from EMAC_TX_EN. If the

frame suffered a collision or EMAC_CRS is not deasserted until more than approximately 48 bit times

after EMAC_TX_EN is deasserted, then 96 bit times (approximately, but not less) is measured from

EMAC_CRS.

2.10.2.5 Back Off

The EMAC implements the 802.3 binary exponential back-off algorithm.

Ethernet Media Access Controller (EMAC)/Management Data Input/Output SPRUFI5B–March 2009–Revised December 2010

(MDIO)

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