Is there a maximum preamble length for Ethernet packets when using the CRC feature of the RocketIO? How is the start of the CRC delimited?
The IEEE 802.3 Ethernet specification specifically states that the preamble is 7 bytes long. Once the CRC circuitry detects the start of packet character (/S/, /K27.7/), it knows a frame is coming and starts a counter. If it does not see the start of frame delimiter (D5) within the first 8 bytes (this is true when the user extended the preamble), it will flag an error. Thus, the start of frame delimiter initiates the CRC computation.