Freescale Semiconductor MPC8260 - Manual (Page 729)

MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
Freescale Semiconductor 22-1
Chapter 22
SCC HDLC Mode
High-level data link control (HDLC) is one of the most common protocols in the data link layer, layer 2
of the OSI model. Many other common layer 2 protocols, such as SDLC, SS#7, AppleTalk, LAPB, and
LAPD, are based on HDLC and its framing structure in particular.
Figure 22-1 shows the HDLC framing
structure.
HDLC uses a zero insertion/deletion process (bit-stuffing) to ensure that a data bit pattern matching the
delimiter flag does not occur in a field between flags. The HDLC frame is synchronous and relies on the
physical layer for clocking and synchronization of the transmitter/receiver.
An address field is needed to carry the frame's destination address because the layer 2 frame can be sent
over point-to-point links, broadcast networks, packet-switched or circuit-switched systems. An address
field is commonly 0, 8, or 16 bits, depending on the data link layer protocol. SDLC and LAPB use an 8-bit
address. SS#7 has no address field because it is always used in point-to-point signaling links. LAPD
divides its 16-bit address into different fields to specify various access points within one device. LAPD
also defines a broadcast address. Some HDLC-type protocols permit addressing beyond 16 bits.
The 8- or 16-bit control field provides a flow control number and defines the frame type (control or data).
The exact use and structure of this field depends on the protocol using the frame. The length of the data in
the data field depends on the frame protocol. Layer 3 frames are carried in this data field. Error control is
implemented by appending a cyclic redundancy check (CRC) to the frame, which in most protocols is 16
bits long but can be as long as 32 bits. In HDLC, the lsb of each octet is sent first; the msb of the CRC is
sent first.
HDLC mode is selected for an SCC by writing GSMR_L[MODE] = 0b0000. In a nonmultiplexed modem
interface, SCC outputs connect directly to external pins. Modem signals can be supported through port C.
The Rx and Tx clocks can be supplied from either the bank of baud rate generators, by the DPLL, or
externally. An SCC can also be connected through the TDM channels of the serial interface (SI). In HDLC
mode, an SCC becomes an HDLC controller, and consists of separate transmit and receive sections whose
operations are asynchronous with the core and can either be synchronous or asynchronous with respect to
other SCCs.
22.1 SCC HDLC Features
The main features of an SCC in HDLC mode are follows:
• Flexible buffers with multiple buffers per frame
• Separate interrupts for frames and buffers (Rx and Tx)
• Received-frames threshold to reduce interrupt overhead
• Can be used with the SCC DPLL