SCC HDLC Mode
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
Freescale Semiconductor 22-9
Data length and buffer pointer fields are described in Section 20.2, “SCC Buffer Descriptors (BDs).” Because HDLC is a frame-based protocol, RxBD[Data Length] of the last buffer of a frame contains the total number of frame bytes, including the 2 or 4 bytes for CRC. Figure 22-5 shows an example of how RxBDs are used in receiving.
4 L Last buffer in frame.
0 Not th e last buffer in frame.
1 Last buffer in frame. Indicates reception of a closing flag or an error, in which case one or more
of the CD, OV, AB, and LG bits are set. The SCC writes the number of frame octets to the data
length field.
5 F First in frame.
0 Not the first buffer in a frame.
1 First buffer in a frame.
6CM Continuous mode. Note that RxBD[E] is cleared if an error occurs during reception, regardless of
CM.
0 Normal operation.
1 RxBD[E] is not cleared by the CP after this BD is closed, allowing the associated buffer to be
overwritten next time the CP accesses it.
7 Reserved, should be cleared.
8 DE DPLL error. Set when a DPLL error occurs while this buffer is being received. DE is also set due to
a missing transition when using decoding modes in which a transition is required for every bit. Note
that when a DPLL error occurs, the frame closes and error checking halts.
9 Reserved, should be cleared.
10 LG Rx frame length violation. Set when a frame larger than the maximum defined for this channel is
recognized. Only the maximum-allowed number of bytes (MFLR) is written to the buffer. This event
is not reported until the buffer is closed, SCCE[RXF] is set, and the closing flag is received. The total
number of bytes received between flags is still written to the data length field.
11 NO Rx nonoctet aligned frame. Set when a received frame contains a number of bits not divisible by
eight.
12 AB Rx abort sequence. Set when at least seven consecutive ones are received during frame reception.
13 CR Rx CRC error. Set when a frame contains a CRC error. CRC bytes received are always written to
the Rx buffer.
14 OV Overrun. Set when a receiver overrun occurs during frame reception.
15 CD Carrier detect lost (NMSI mode only). Set when CD is negated during frame reception.

Table22-7. SCC HDLC RxBD Status and Control Field Descriptions (continued)

Bits Name Description