6.4.2 MAC Transmit Block
The MAC transmitter operates in half duplex mode and transmits frames in accordance with the Ethernet IEEE 802.3 standard. In half duplex mode, the CSMA/CD protocol of the IEEE 802.3 specification is followed.
A small input buffer receives data through the FIFO interface which will extract data in 32-bit form. All subsequent processing prior to the final output is performed in bytes.
Transmit data is output to the integrated 10BASE‑T1S PHY using the internal MII interface.
Frame assembly starts by adding the preamble and the start frame delimiter. Data is taken from the transmit FIFO interface a word at a time.
If necessary, padding is added to take the frame length to 60 bytes. A 32-bit CRC is calculated, inverted, and appended to the end of the frame taking the frame length to a minimum of 64 bytes.
In half duplex mode, the transmitter checks carrier sense. If asserted, the transmitter waits for the signal to become inactive and then begins transmission after the interframe gap of 96 bit times. If the collision signal is asserted during transmission, the transmitter will transmit a jam sequence of 32 bits taken from the data register and then retry transmission after the back off time has elapsed. If the collision occurs during either the preamble or Start Frame Delimiter (SFD), then these fields will be completed prior to generation of the jam sequence.
The back off time is based on a pseudo random binary sequence (PRBS) generator seeded from the least significant 32 bits of the MAC address as configured in the Specific Address Bottom 1 (MAC_SAB1) register. The number of bits of the PRBS output used depends on the number of collisions seen. After the first collision 1 bit is used, then the second 2 bits and so on up to the maximum of 10 bits. All 10 bits are used above ten collisions. An error will be indicated and no further attempts will be made if 16 consecutive attempts cause collision. This operation is compliant with the description in Clause 4.2.3.2.5 of the IEEE 802.3 standard which refers to the truncated binary exponential back off algorithm. Collision back off and retry will be performed up to 16 times.