11-6
MCF5272 User’s Manual
FEC Frame Transmission
•
After bit time 21, the data sequence is monitored for a valid start-of-frame delimiter
(SFD) of 11. If a 00 is detected, the frame is rejected. When a 11 is detected, the
PA/SFD sequence is complete.
•
In MII mode, the receiver checks for at least one byte matching the SFD. Zero or
more PA bytes may occur, but if a 00 bit sequence is detected before the SFD byte,
the frame is ignored.
•
After the first 8 bytes of the frame have been passed to the receive FIFO, the FEC
performs address recognition on the frame.
As soon as a collision window (512 bits) of data is received, and if address recognition has
not rejected the frame, the FEC starts transferring the incoming frame to the receive buffer
descriptor’s (RxBD’s) associated data buffer. If the frame is a runt (due to collision) or is
rejected by address recognition, no receive buffers are filled. Thus, no collision frames are
presented to the user except late collisions, which indicate serious LAN problems. When
the data buffer has been filled, the FEC clears RxBD[E] and generates an RXB interrupt (if
RBIEN is asserted in I_MASK register). If the incoming frame exceeds the length of the
data buffer, the FEC fetches the next RxBD in the table and, if it is empty, continues
transferring the rest of the frame to this BD’s associated data buffer.
The RxBD length is determined in the R_BUFF_SIZE value in the R_BUFF_SIZE register.
The user should program R_BUFF_SIZE to be at least 128 bytes. R_BUFF_SIZE must be
quad-word (16-byte) aligned.
During reception, the FEC checks for a frame that is either too short or too long. When the
frame ends (carrier sense is negated), the receive CRC field is checked and written to the
data buffer. The data length written to the last BD in the Ethernet frame is the length of the
entire frame. Frames shorter than 64 bytes are discarded automatically with no system bus
impact.
When the receive frame is complete, the FEC sets RxBD[L], writes the other frame status
bits into the RxBD and clears RxBD[E]. The FEC next generates a maskable interrupt
(I_EVENT[RFINT], maskable by I_MASK[RFIEN]), indicating that a frame has been
received and is in memory. The FEC then waits for a new frame. The FEC receives serial
data lsb first.
11.4.2 CAM Interface
In addition to the FEC address recognition logic, an external CAM may be used for frame
reject with no additional pins other than those in the MII interface. This CAM interface is
documented in an application note titled “Using Motorola’s Fast Static RAM CAMs with
the MPC860T’s Media Independent Interface,” located at the following URL:
http://www.mot.com/SPS/RISC/netcomm/aesop/mpc8XX/860/860tcam.pdf.
Summary of Contents for DigitalDNA ColdFire MCF5272
Page 1: ...MCF5272UM D Rev 0 02 2001 MCF5272 ColdFire Integrated Microprocessor User s Manual ...
Page 38: ...xxxviii MCF5272 User s Manual TABLES Table Number Title Page Number ...
Page 58: ...1 10 MCF5272 User s Manual MCF5272 Specific Features ...
Page 90: ...2 42 MCF5272 User s Manual Exception Processing Overview ...
Page 96: ...3 6 MCF5272 User s Manual MAC Instruction Execution Timings ...
Page 158: ...5 46 MCF5272 User s Manual Motorola Recommended BDM Pinout ...
Page 184: ...7 12 MCF5272 User s Manual Interrupt Controller Registers ...
Page 338: ...13 44 MCF5272 User s Manual Application Examples ...
Page 414: ...18 6 MCF5272 User s Manual PWM Programming Model ...
Page 452: ...19 38 MCF5272 User s Manual Power Supply Pins ...
Page 482: ...20 30 MCF5272 User s Manual Reset Operation ...
Page 492: ...21 10 MCF5272 User s Manual Non IEEE 1149 1 Operation ...