MPC5553/MPC5554 Microcontroller Reference Manual, Rev. 5
Freescale Semiconductor
14-47
then the frame non-octet aligned (NO) error is reported in the RxBD. If there is no CRC error, then no error
is reported.
14.4.14.2.3 CRC Error
When a CRC error occurs with no dribble bits, the FEC closes the buffer and sets the CR bit in the RxBD.
CRC checking cannot be disabled, but the CRC error can be ignored if checking is not required.
14.4.14.2.4 Frame Length Violation
When the receive frame length exceeds MAX_FL bytes the BABR interrupt will be generated, and the LG
bit in the end of frame RxBD will be set. The frame is not truncated unless the frame length exceeds 2047
bytes).
14.4.14.2.5 Truncation
When the receive frame length exceeds 2047 bytes the frame is truncated, and the TR bit is set in the
RxBD.
14.5
Buffer Descriptors
This section provides a description of the operation of the driver/DMA via the buffer descriptors. It is
followed by a detailed description of the receive and transmit descriptor fields.
14.5.1
Driver/DMA Operation with Buffer Descriptors
The data for the FEC frames must reside in memory external to the FEC. The data for a frame is placed in
one or more buffers. Associated with each buffer is a buffer descriptor (BD) which contains a starting
address (pointer), data length, and status/control information (which contains the current state for the
buffer). To permit maximum user flexibility, the BDs are also located in external memory and are read in
by the FEC DMA engine.
Software “produces” buffers by allocating/initializing memory and initializing buffer descriptors. Setting
the RxBD[E] or TxBD[R] bit “produces” the buffer. Software writing to either the TDAR or RDAR tells
the FEC that a buffer has been placed in external memory for the transmit or receive data traffic,
respectively. The hardware reads the BDs and “consumes” the buffers after they have been produced. After
the data DMA is complete and the buffer descriptor status bits have been written by the DMA engine, the
RxBD[E] or TxBD[R] bit will be cleared by hardware to signal that the buffer has been “consumed.”
Software may poll the BDs to detect when the buffers have been consumed or may rely on the buffer/frame
interrupts. These buffers may then be processed by the driver and returned to the free list.
The ECR[ETHER_EN] signal operates as a reset to the BD/DMA logic. When ECR[ETHER_EN] is
deasserted the DMA engine BD pointers are reset to point to the starting transmit and receive BDs. The
buffer descriptors are not initialized by hardware during reset. At least one transmit and receive buffer
descriptor must be initialized by software before the ECR[ETHER_EN] bit is set.
The buffer descriptors operate as two separate rings. ERDSR defines the starting address for receive BDs
and ETDSR defines the starting address for transmit BDs. The last buffer descriptor in each ring is defined
by the wrap (W) bit. When set, W indicates that the next descriptor in the ring is at the location pointed to
by ERDSR and ETDSR for the receive and transmit rings, respectively. Buffer descriptor rings must start
on a 32-bit boundary; however, it is recommended they are made 128-bit aligned.
Summary of Contents for MPC5553
Page 5: ...MPC5553 MPC5554 Microcontroller Reference Manual Rev 5 2 Freescale Semiconductor...
Page 21: ...MPC5553 MPC5554 Microcontroller Reference Manual Rev 5 xvi Freescale Semiconductor...
Page 47: ...MPC5553 MPC5554 Microcontroller Reference Manual Rev 5 1 26 Freescale Semiconductor...
Page 163: ...MPC5553 MPC5554 Microcontroller Reference Manual Rev 5 4 20 Freescale Semiconductor...
Page 179: ...MPC5553 MPC5554 Microcontroller Reference Manual Rev 5 5 16 Freescale Semiconductor...
Page 561: ...MPC5553 MPC5554 Microcontroller Reference Manual Rev 5 13 38 Freescale Semiconductor...
Page 615: ...MPC5553 MPC5554 Microcontroller Reference Manual Rev 5 14 54 Freescale Semiconductor...
Page 707: ...MPC5553 MPC5554 Microcontroller Reference Manual Rev 5 17 68 Freescale Semiconductor...
Page 755: ...MPC5553 MPC5554 Microcontroller Reference Manual Rev 5 18 48 Freescale Semiconductor...
Page 873: ...MPC5553 MPC5554 Microcontroller Reference Manual Rev 5 19 118 Freescale Semiconductor...
Page 984: ...MPC5553 MPC5554 Microcontroller Reference Manual Rev 5 Freescale Semiconductor 21 41...
Page 985: ...MPC5553 MPC5554 Microcontroller Reference Manual Rev 5 21 42 Freescale Semiconductor...
Page 1019: ...MPC5553 MPC5554 Microcontroller Reference Manual Rev 5 22 34 Freescale Semiconductor...
Page 1129: ...MPC5553 MPC5554 Microcontroller Reference Manual Rev 5 25 90 Freescale Semiconductor...
