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The first word of the Tx BD contains status and control bits. Bits 15–10 are prepared by the
user before transmission; bits 1–0 are set by the HDLC controller after the buffer has been
transmitted. Bit 15 is set by the user when the buffer and BD have been prepared and is
cleared by the HDLC controller after the frame has been transmitted.
R—Ready
0 = This buffer is not currently ready for transmission. The user is free to manipulate
this BD (or its associated buffer). The HDLC controller clears this bit after the buffer
has been fully transmitted or after an error condition has been encountered.
1 = The data buffer, which has been prepared for transmission by the user, has not yet
transmitted. No fields of this BD may be written by the user once this bit is set.
X—External Buffer
0 = The buffer associated with this BD is in internal dual-port RAM.
1 = The buffer associated with this BD is in external memory.
W—Wrap (Final BD in Table)
0 = This is not the last BD in the Tx BD table.
1 = This is the last BD in the Tx BD table. After this buffer has been used, the HDLC
controller will transmit data from the first BD in the table.
The user is required to set the wrap bit in one of the first eight
BDs; otherwise, errant behavior may occur.
I—Interrupt
0 = No interrupt is generated after this buffer has been serviced.
1 = Either TXB or TXE in the HDLC event register will be set when this buffer has been
serviced by the HDLC controller, which can cause an interrupt.
L—Last
0 = This is not the last buffer in the frame.
1 = This is the last buffer in the current frame.
TC—Tx CRC
This bit is valid only when the last (L) bit is set.
0 = Transmit the closing flag after the last data byte. This setting can be used for test-
ing purposes to send a "bad" CRC after the data.
1 = Transmit the CRC sequence after the last data byte.
Bits 9–2—Reserved for future use.
The following status bits are written by the HDLC controller after it has finished transmitting
the associated data buffer.
MOTOROLA
NOTE
MC68302 USER'S MANUAL
Communications Processor (CP)
4-79
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