The MAC transmitter can operate in either half-duplex or full-duplex mode, and transmits frames in accordance with the Ethernet IEEE Std 802.3. In half-duplex mode, the CSMA/CD protocol is followed.
TX frame assembly starts by adding the preamble and the start frame delimiter. The packets are taken from the TXFIFO.
For short packets, padding is added to take the frame length to 60 bytes. CRC is calculated using an order 32-bit polynomial. This is inverted and appended to the end of the frame taking the frame length to a minimum of 64 bytes. If the [No_CRC] descriptor bit is set =1 of the last buffer descriptor of a TX frame, neither pad nor CRC are appended. The [No_CRC] bit can also be set through the FIFO.
In full-duplex mode (at all data rates), frames are transmitted immediately. Back-to-back frames are transmitted at least 96-bit times apart to check the inter-packet gap.
In half-duplex mode, the transmitter checks carrier sense. If asserted, the transmitter waits for a signal to become inactive, and then starts transmission after the inter-packet gap of 96-bit times.
Collisions in Half-duplex Mode
If the collision signal is asserted during transmission, the transmitter transmits a jam sequence of 32 bits taken from the data register and then retries transmission after the backoff time has elapsed. If the collision occurs during either the preamble or SFD, then these fields are completed prior to generation of the jam sequence.
The backoff time is based on an XOR of the 10 least significant bits of the data coming from the packet buffer and a 10-bit value from the pseudo-random number generator. The number of bits that are actually used depends on the number of collisions seen. After the first collision, one bit is used to determine the backoff time. After the second collision, two bits are used. This continues up to a maximum of 10 bits for the 10th through 16th collision. When a frame transmits without a collision, the number of bits used for a collision starts back at 1.
After 16 collisions in a row, an error is indicated and no further TX attempts are made, which is in accordance with the truncated binary exponential backoff algorithm.
In 10/100 Mb/s mode, both collisions and late collisions are treated identically (backoff and retry are performed up to 16 times). When operating in 1000 Mb/s mode, late collisions are treated as an exception and the transmission is aborted without a retry. This condition is reported in the transmit buffer descriptor word [1] (late collision, bit [26]) and also in the transmit status register (late collision, bit [7]).
An interrupt can also be generated (if enabled) when this exception occurs, and bit [5] in the interrupt status register is set.
When bit [28] is set in the network configuration register, the IPG can be stretched beyond 96 bits depending on the length of the previously transmitted frame and the value written to the stretch_ratio register. The least significant 8 bits of the stretch_ratio register multiply the previous frame length (including preamble) and the next significant 8 bits (+1 so as not to get a divide by zero) divide the frame length to generate the IPG.
IPG stretch only works in full-duplex mode and when bit [28] is set in the network configuration register. The stretch_ratio register cannot be used to shrink the IPG below 96 bits.