Communication Processor Module
MOTOROLA MPC823e REFERENCE MANUAL 16-319
COMMUNICATION
16
SCCs
When a station needs to transmit, it checks for activity on the LAN and when the LAN
becomes silent for a specified period, the station starts transmitting. At that time, the station
continually checks for collision on the LAN and if one is found, the station forces a jam of all
ones on its frame and stops transmitting. Collisions usually occur close to the beginning of
a frame. The station waits a random period of time, called a backoff, before trying to transmit
again. Once the backoff is complete, the station waits for silence on the LAN and then
begins retransmission on the LAN, which is called a retry. If the frame is not successfully
transmitted within 15 retries, an error occurs. The 10Mbps Ethernet provides 0.8µs per byte.
The preamble plus start frame delimiter is transmitted in 6.4µs. The minimum interframe gap
is 9.6µs and the slot time is 52µs. Therefore, you must operate the MPC823e at a minimum
frequency of 20MHz to implement Ethernet.
16.9.22.1 FEATURES. The following list summarizes the main features of a serial
communication controller in Ethernet mode:
• Performs MAC layer functions of Ethernet and IEEE 802.3
• Full-duplex operation support
• Performs framing functions
• Full collision support
• Maximum 10Mbps (10BASE-T) bit rate
• Back-to-back frame reception
• Detection of receive frames that are too long
• Multibuffer data structure
• Supports 48-bit addresses in three modes
• Up to eight parallel I/O pins can be sampled and appended to any frame
• Heartbeat indication
• Transmitter network management and diagnostics
• Receiver network management and diagnostics
• Error counters
• Internal and external loopback mode
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