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L—Last in frame
This bit is set by the ASYNC HDLC controller when this buffer is the last in a frame. This
implies the reception of a closing flag or reception of an error, in which case one or more
of the BRK, CD, OV, and AB bits are set. The ASYNC HDLC controller will write the num-
ber of frame octets to the data length field.
0— This buffer is not the last in a frame.
1— This buffer is the last in a frame.
F—First in frame
This bit is set by the ASYNC-HDLC controller when this buffer is the first in a frame.
0— The buffer is not the first in a frame.
1— The buffer is the first in a frame.
CM—Continuous Mode
0— Normal operation.
1— The E-bit is not cleared by the CP after this BD is closed, allowing the associated
data buffer to be overwritten automatically when the CP next accesses this BD.
However, the E-bit will be cleared if an error (other than CRC error) occurs during
reception, regardless of the CM bit.
BRK—Break Character Received
The current frame was closed because a Break Character was received.
AB—Rx Abort Sequence
The ASYNC HDLC Abort Sequence or a framing error was received to terminate this
frame.
CR—Rx CRC Error
This frame contains a CRC error. The received CRC bytes are always written to the re-
ceive buffer.
OV—Overrun
A receiver overrun occurred during frame reception.
CD—Carrier Detect Lost
The carrier detect signal was negated during frame reception.
Data Length
Data length is the number of octets written by the CP into this BD's data buffer. It is written
by the CP once as the BD is closed.
When this BD is the last BD in a frame (L=1), the data length contains the total number of
frame octets (including the 2 bytes for CRC).
If the received frame has a length (plus CRC) which is an exact
multiple of MRBLR, the BD with the "L" bit set will not actually
MOTOROLA
Freescale Semiconductor, Inc.
Note
For More Information On This Product,
Go to: www.freescale.com
Asynchronous HDLC
17
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