MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
25-1
Chapter 25
SCC Asynchronous HDLC Mode and IrDA
Asynchronous HDLC and IrDA uses HDLC framing techniques with UART-type characters. This
document refers to both protocols collectively as asynchronous HDLC. The asynchronous HDLC protocol
is typically used as the physical layer for point-to-point protocol
(PPP) and the infrared link access
protocol (IrLAP). Although asynchronous HDLC can be implemented in conjunction with the core, it is
more efficient and less computationally-intensive to let the CPM handle framing and transparency
functions. Additional hardware features to support infrared encoding/decoding are described in
Section 25.18, “IrDA Encoder/Decoder (SCC2 Only).”
The RFC 1549 octet stuffing/unstuffing provided by this mode supports only asynchronous transmission.
This mode cannot be used to provide octet stuffing for synchronous communication lines.
25.1
Asynchronous HDLC Features
The following list summarizes the main features of the SCC in asynchronous HDLC mode:
•
Flexible buffer structure lets all or part of a frame be sent or received
•
Separate interrupts for received frames and transmitted buffers
•
Automatic CRC generation and checking
•
Support for nonmultiplexed serial interface control signals
•
Automatic generation of opening and closing flags
•
Reception of frames with a single shared flag
•
Automatic generation and stripping of transparency characters according to RFC 1549 using
transmit and receive control character maps
•
Programmable opening flag, closing flag, and control escape characters
•
Automatic transmission of the abort sequence after a
STOP
TRANSMIT
command
•
Automatic transmission of idle characters between frames and between characters
25.2
Asynchronous HDLC Frame Transmission Processing
The SCC in asynchronous HDLC mode (asynchronous HDLC controller) works with minimal core
intervention. When the core enables the transmitter and sets TxBD[R] in the first BD of the table, the
asynchronous HDLC controller fetches data from memory and starts sending the frame. If the current
TxBD[L] is set (last buffer of a frame), the CRC and closing flag are appended. If TxBD[CM] is zero, the
transmitter updates frame status bits in the BD and clears TxBD[R]. If TxBD[I] is set, the controller sets
SCCE[TXB] so an interrupt can be generated after each buffer, after a group of buffers, or after each frame
is sent.
Summary of Contents for PowerQUICC MPC870
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