8.2.6.4 MAC Transmit Block

The MAC transmitter can operate in either Half Duplex or Full 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 can be output using the RMII/RGMII interface.

Frame assembly starts by adding 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. 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 bit is set in the second word of the last buffer descriptor of a transmit frame, neither pad nor CRC are appended. The no CRC bit can also be set through the FIFO interface.

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 guarantee the interframe gap.

In Half Duplex mode, the transmitter checks carrier sense. If asserted, the transmitter waits for the signal to become inactive, and then starts 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 backoff 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 backoff time is based on an XOR of the 10 least significant bits of the data coming from the transmit FIFO interface and a 10-bit pseudo random number generator. The number of bits 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 backoff algorithm.

In 10/100 mode, both collisions and late collisions are treated identically, and backoff and retry will be performed up to 16 times. When operating in Gigabit mode, late collisions are treated as an exception and transmission is aborted, without 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 will be set.

When operating in Gigabit mode (half duplex) both carrier extension and frame bursting are performed in accordance with the IEEE 802.3 standard. For frames less than 512 bytes carrier extension is used to ensure the minimum slot time is not violated.

Frame bursting is used by the transmitter in Gigabit mode (half duplex) when more than one frame is queued for transmission. The first frame of a burst must be carrier extended (if necessary) to ensure the minimum slot time of 512 bytes is achieved, after which all subsequent frames within the burst must only satisfy the minimum frame length of 64 bytes or greater. Each interframe gap within the burst is filled by the transmitter with carrier extensions, thus ensuring control of the medium is not given up. Several frames may be transmitted up to the burst limit of 65,536 bytes. The transmitter relinquishes control of the medium when there are no more frames queued for transmission or the burst limit is exceeded.

In Gigabit mode any collisions occurring after the minimum slot time for the first frame within a burst are treated as a late collision. The burst is terminated upon this event.

In all modes of operation, if the transmit DMA underruns, a bad CRC is automatically appended using the same mechanism as jam insertion and the GTXER signal is asserted. For a properly configured system this should never occur ; it is also impossible if configured to use the DMA with packet buffers, as the complete frame is buffered in local packet buffer memory.

When bit 28 is set in the Network Configuration register, the Inter Packet Gap (IPG) may be stretched beyond 96 bits depending on the length of the previously transmitted frame and the value written to the IPG Stretch register (GMAC_IPGS). The least significant 8 bits of the IPG Stretch register multiply the previous frame length (including preamble). 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 IPG Stretch register cannot be used to shrink the IPG below 96 bits.