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Communication Processor Module
16-228
MPC823 USER’S MANUAL
MOTOROLA
SCC2
COMMUNICATION
16
PROCESSOR
MODULE
16.9.16.1 FEATURES.The following list summarizes the main features of the SCC2 in
HDLC mode:
Flexible data buffers with multiple buffers per frame
Separate interrupts for frames and buffers (receive and transmit)
Received frames threshold to reduce interrupt overhead
Can be used with the SCC2 DPLL
Four address comparison registers with mask
Maintenance of five 16-bit error counters
Flag/abort/idle generation or detection
Zero insertion/deletion
16- or 32-bit CRC-CCITT generation/checking
Detection of nonoctet aligned frames
Detection of frames that are too long
Programmable flags (0–15) between successive frames
Automatic retransmission in case of collision
16.9.16.2 SCC2 HDLC CHANNEL FRAME TRANSMISSION PROCESS.The HDLC
transmitter is designed to work with little or no intervention from the core. When the core
enables one of the transmitters, it starts transmitting flags or idles as programmed in the
PSMR–SCC2 HDLC register. The SCC2 HDLC controller polls the first buffer descriptor in
the transmit channel buffer descriptor table. When there is a frame to transmit, the SCC2
HDLC controller fetches the data from memory and starts transmitting the frame after it
transmits the minimum number of flags that you specify between frames. When the end of
the current buffer descriptor has been reached and the last buffer in the frame bit is set, the
CRC and closing flag are appended. In HDLC mode, the least-significant bit of each octet
and the most-significant bit of the CRC are transmitted first. An HDLC frame is illustrated in
After a closing flag is transmitted, the SCC2 HDLC controller writes the frame status bits into
the buffer descriptor and clears the R bit. When you reach the end of the current buffer
descriptor and the last bit is not set, only the R bit is cleared. In either mode, an interrupt can
be issued if the I bit in the TX buffer descriptor is set. The SCC2 HDLC controller then
proceeds to the next TX buffer descriptor in the table. This method allows you to be
interrupted after each buffer, a specific buffer, or each frame.
OPENING FLAG
ADDRESS
CONTROL
INFORMATION
(OPTIONAL)
CRC
CLOSING FLAG
8 BITS
16 BITS
8 BITS
8N BITS
16 BITS
8 BITS
Figure 16-76. SCC2 HDLC Framing Structure