Motorola MCF5281 Inter-Packet Gap IPG Time, Collision Handling, Internal and External Loopback

Functional Description

To transmit a pause frame, the FEC must operate in full-duplex mode and the user must assert flow control pause (TCR[TFC_PAUSE]). On assertion of transmit flow control pause (TCR[TFC_PAUSE]), the transmitter asserts TCR[GTS] internally. When the transmission of data frames stops, the EIR[GRA] (graceful stop complete) interrupt asserts. Following EIR[GRA] assertion, the pause frame is transmitted. On completion of pause frame transmission, flow control pause (TCR[TFC_PAUSE]) and TCR[GTS] are deasserted internally.

The user must specify the desired pause duration in the OPD register.

Note that when the transmitter is paused due to receiver/microcontroller pause frame detection, transmit flow control pause (TCR[TFC_PAUSE]) still may be asserted and will cause the transmission of a single pause frame. In this case, the EIR[GRA] interrupt will not be asserted.

17.4.11 Inter-Packet Gap (IPG) Time

The minimum inter-packet gap time for back-to-back transmission is 96 bit times. After completing a transmission or after the backoff algorithm completes, the transmitter waits for carrier sense to be negated before starting its 96 bit time IPG counter. Frame transmission may begin 96 bit times after carrier sense is negated if it stays negated for at least 60 bit times. If carrier sense asserts during the last 36 bit times, it will be ignored and a collision will occur.

The receiver receives back-to-back frames with a minimum spacing of at least 28 bit times. If an inter-packet gap between receive frames is less than 28 bit times, the following frame may be discarded by the receiver.

17.4.12 Collision Handling

If a collision occurs during frame transmission, the Ethernet controller will continue the transmission for at least 32 bit times, transmitting a JAM pattern consisting of 32 ones. If the collision occurs during the preamble sequence, the JAM pattern will be sent after the end of the preamble sequence.

If a collision occurs within 512 bit times, the retry process is initiated. The transmitter waits a random number of slot times. A slot time is 512 bit times. If a collision occurs after 512 bit times, then no retransmission is performed and the end of frame buffer is closed with a Late Collision (LC) error indication.

17.4.13 Internal and External Loopback

Both internal and external loopback are supported by the Ethernet controller. In loopback mode, both of the FIFOs are used and the FEC actually operates in a full-duplex fashion.

MOTOROLA Chapter 17. Fast Ethernet Controller (FEC) 17-17