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Peripheral Architecture
2.10.7Receive Frame Classification
Received frames are proper (good) frames, if they are between 64 bytes and the value in the receive maximum length register (RXMAXLEN) bytes in length (inclusive) and contain no code, align, or CRC errors.
Received frames are long frames, if their frame count exceeds the value in RXMAXLEN. The RXMAXLEN reset (default) value is 5EEh (1518 in decimal). Long received frames are either oversized or jabber frames. Long frames with no errors are oversized frames; long frames with CRC, code, or alignment errors are jabber frames.
Received frames are short frames, if their frame count is less than 64 bytes. Short frames that address match and contain no errors are undersized frames; short frames with CRC, code, or alignment errors are fragment frames. If the frame length is less than or equal to 20, then the frame CRC is passed, regardless of whether the RXPASSCRC bit is set or cleared in the receive multicast/broadcast/promiscuous channel enable register (RXMBPENABLE).
A received long packet always contains RXMAXLEN number of bytes transferred to memory (if the RXCEFEN bit is set in RXMBPENABLE), regardless of the value of the RXPASSCRC bit. Following is an example with RXMAXLEN set to 1518:
∙If the frame length is 1518, then the packet is not a long packet and there are 1514 or 1518 bytes transferred to memory depending on the value of the RXPASSCRC bit.
∙If the frame length is 1519, there are 1518 bytes transferred to memory regardless of the RXPASSCRC bit value. The last three bytes are the first three CRC bytes.
∙If the frame length is 1520, there are 1518 bytes transferred to memory regardless of the RXPASSCRC bit value. The last two bytes are the first two CRC bytes.
∙If the frame length is 1521, there are 1518 bytes transferred to memory regardless of the RXPASSCRC bit value. The last byte is the first CRC byte.
∙If the frame length is 1522, there are 1518 bytes transferred to memory. The last byte is the last data byte.
2.10.8Promiscuous Receive Mode
When the promiscuous receive mode is enabled by setting the RXCAFEN bit in the receive multicast/broadcast/promiscuous channel enable register (RXMBPENABLE), nonaddress matching frames that would normally be filtered are transferred to the promiscuous channel. Address matching frames that would normally be filtered due to errors are transferred to the address match channel when the RXCAFEN and RXCEFEN bits in RXMBPENABLE are set. A frame is considered to be an address matching frame only if it is enabled to be received on a unicast, multicast, or broadcast channel. Frames received to disabled unicast, multicast, or broadcast channels are considered nonaddress matching.
MAC control frames address match only if the RXCMFEN bit in RXMBPENABLE is set. The RXCEFEN and RXCSFEN bits in RXMBPENABLE determine whether error frames are transferred to memory or not, but they do not determine whether error frames are address matching or not. Short frames are a special type of error frames.
A single channel is selected as the promiscuous channel by the RXPROMCH bit in RXMBPENABLE. The promiscuous receive mode is enabled by the RXCMFEN, RXCEFEN, RXCSFEN, and RXCAFEN bits in RXMBPENABLE. Table 4 shows the effects of the promiscuous enable bits. Proper frames are frames that are between 64 bytes and the value in the receive maximum length register (RXMAXLEN) bytes in length inclusive and contain no code, align, or CRC errors.
SPRU941A
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