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I

When set, tells the 

TX_WR

 logic to set 

TXBUFC

 in the Ethernet Interrupt Status 

register (see page 385) when the buffer is closed due to a normal channel 
completion.

Buffer pointer

32-bit pointer to the start of the buffer in system memory. This pointer can be 
aligned on any byte of a 32-bit word.

Status

Lower 16 bits of the Ethernet Transmit Status register. The status is returned 
from the Ethernet MAC at the end of the frame and written into the last buffer 
descriptor of the frame. 

L

When set, tells the 

TX_WR

 logic that this buffer descriptor is the last descriptor 

that completes an entire frame. This bit allows multiple descriptors to be 
chained together to make up a frame.

F

When set, indicates the buffer is full. The 

TX_WR

 logic clears this bit after 

emptying a buffer. The system software sets this bit as required, to signal that 
the buffer is ready for transmission. If the 

TX_WR

 logic detects that this bit is 

not set when the buffer descriptor is read, it does one of two things:

 

If a frame is not in progress, the 

TX_WR

 logic sets the 

TXIDLE

 bit in the 

Ethernet Interrupt Status register.

 

If a frame is in progress, the 

TXBUFNR

 bit in the Ethernet Interrupt Status 

register is set.

In either case, the 

TX_WR

 logic stops processing frames until 

TCLER

 (clear 

transmit logic) in Ethernet General Control Register #2 is toggled from low to 
high.

TXBUFNR

 is set only for frames that consist of multiple buffer descriptors and 

contain a descriptor — not the first descriptor — that does not have the F bit set 
after frame transmission has begun.

Buffer length

This is a dual use field:

 

When the buffer descriptor is read from the TX buffer descriptor RAM, 

buffer length indicates the length of the buffer, in bytes. The 

TX_WR

 logic 

uses this information to identify the end of the buffer. For proper operation 
of the 

TX_WR

 logic, all transmit frames must be at least 34 bytes in length.

 

When the 

TX_WR

 logic updates the buffer descriptor at the end of the 

frame, it writes the length of the frame, in bytes, into this field for the last 
buffer descriptor of the frame. 

If the MAC is configured to add the CRC to the frame (that is, CRCEN in 
MAC Configuration Register #2 is set to 1), this field will include the four 
bytes of CRC. This field is set to 0x000 for jumbo frames that are aborted 
(see "TXAJ" on page 346)

Only the lower 11 bits of this field are valid, since the maximum legal frame size 
for Ethernet is 1522 bytes.