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Peripheral Architecture
2.9.2Data 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.
2.9.2.1Transmit 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
2.9.2.2CRC Insertion
If the SOP buffer descriptor PASSCRC flag is cleared, the EMAC generates and appends a
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.
2.9.2.3Adaptive 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
2.9.2.4Interpacket-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 MCRS is deasserted within approximately 48 bit times of MTXEN being deasserted, then 96 bit times is measured from MTXEN. If the frame suffered a collision or MCRS is not deasserted until more than approximately 48 bit times after MTXEN is deasserted, then 96 bit times (approximately, but not less) is measured from MCRS.
2.9.2.5Back Off
The EMAC implements the 802.3 binary exponential
SPRU941A
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