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ETHERNET COMMUNICATION MODULE
Transmit packet processor
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Receive buffer
descriptor field
definitions
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Transmit packet processor
Transmit frames are transferred from system memory to the transmit packet
processor into a 256-byte TX_FIFO. Because various parts of the transmit frame can
Field Description
W
WRAP bit, which, when set, tells the RX_RD logic that this is the last buffer
descriptor in the ring. In this situation, the next buffer descriptor is found using the
appropriate Buffer Descriptor Pointer register.
When the
WRAP bit is not set, the next buffer descriptor is found using an offset of
0x10 from the current buffer descriptor pointer.
I When set, tells the RX_RD logic to set RXBUFC in the Ethernet Interrupt Status
register after the frame has been transferred to system memory.
E
ENABLE bit, which, when set, tells the RX_RD logic that this buffer descriptor is
enabled. When a new frame is received, pools that do not have the
ENABLE bit set
in their next buffer descriptor are skipped when deciding in which pool to put the
frame.
The receive processor can use up to four different-sized receive buffers in system
memory.
Note:
To enable a pool that is currently disabled, change the
ENABLE bit from 0 to 1 and
reinitialize the buffer descriptors pointed to by the Buffer Descriptor Pointer
register:
1 Set the ERXINIT bit in the Ethernet General Control Register 1.
7 Wait for RXINIT to be set in the Ethernet General Status register.
Change the ENABLE bit only while the receive packet processor is idle.
Buffer pointer 32-bit pointer to the start of the buffer in system memory. This pointer must be
aligned on a 32-bit boundary.
Status Lower 16 bits of the Ethernet Receive Status register. The status is taken from the
receive status FIFO and added to the buffer descriptor after the last word of the
frame is written to system memory.
F When set, indicates the buffer is full. The
RX_RD logic sets this bit after filling a
buffer. The system software clears this bit, as required, to free the buffer for future
use. When a new frame is received, pools that have the F bit set in their next buffer
descriptor are skipped when deciding in which pool to put the frame.
Buffer length This is a dual use field:
When the buffer descriptor is read from system memory, buffer length
indicates the maximum sized frame, in bytes, that can be stored in this buffer
ring.
When the RX_RD logic writes the descriptor back from the receive status FIFO
into system memory at the end of the frame, the buffer length is the actual
frame length, in bytes.Only the lower 11 bits of this field are valid, since the
maximum legal frame size for Ethernet is 1522 bytes.