62.6.6 MAC Receive Block

All processing within the MAC receive block is implemented using a 16-bit data path. The MAC receive block checks for valid preamble, FCS, alignment and length, presents received frames to the FIFO interface and stores the frame destination address for use by the address checking block.

If, during the frame reception, the frame is found to be too long, a bad frame indication is sent to the FIFO interface. The receiver logic ceases to send data to memory as soon as this condition occurs.

At end of frame reception the receive block indicates to the DMA block whether the frame is good or bad. The DMA block will recover the current receive buffer if the frame was bad.

Ethernet frames are normally stored in DMA memory complete with the FCS. Setting the FCS remove bit in the network configuration (bit 17) causes frames to be stored without their corresponding FCS. The reported frame length field is reduced by four bytes to reflect this operation.

The receive block signals to the register block to increment the alignment, CRC (FCS), short frame, long frame, jabber or receive symbol errors when any of these exception conditions occur.

If bit 26 is set in the network configuration, CRC errors will be ignored and CRC errored frames will not be discarded, though the Frame Check Sequence Errors statistic register will still be incremented. Additionally, if not enabled for Jumbo Frames mode, then bit 13 of the receiver descriptor word 1 will be updated to indicate the FCS validity for the particular frame. This is useful for applications such as EtherCAT, where individual frames with FCS errors must be identified.

Received frames can be checked for length field error by setting the Length Field Error Frame Discard bit of the Network Configuration register (bit 16). When this bit is set, the receiver compares a frame's measured length with the length field (bytes 13 and 14) extracted from the frame. The frame is discarded if the measured length is shorter. This checking procedure is for received frames between 64 bytes and 1518 bytes in length.

Each discarded frame is counted in the 10-bit Length Field Frame Error statistics register. Frames where the length field is greater than or equal to 0x0600 hex will not be checked.

When operating in Gigabit mode (half duplex), the receiver will discard frames which do not meet the minimal slot time of 512 bytes. If a burst is detected, the first frame is checked to ensure it meets the slot time, but all subsequent frames of the burst are checked to ensure they meet the minimum frame size of 64 bytes.

In Gigabit mode (half duplex), carrier extension errors are detected by the receiver during the minimum slot time, and the frame discarded. An error of this nature causes the Receive Symbol Errors statistic register to be incremented. Carrier extension errors occurring during the inter packet gap period are ignored and have no effect on the statistics.