Motorola MPC8260 - Manual (Page 903)

MOTOROLA Chapter 31. FCC HDLC Controller 31-1
Chapter 31
FCC HDLC Controller
310
310
Layer 2 of the seven-layer OSI model is the data link layer (DLL), in which HDLC is one
of the most common protocols. The framing structure of HDLC is shown in Figure 31-1.
HDLC uses a zero insertion/deletion process (commonly known as bit stufÞng) to ensure
that the bit pattern of the delimiter ßag does not occur in the Þelds between ßags. The
HDLC frame is synchronous and therefore relies on the physical layer for a method of
clocking and of synchronizing the transmitter/receiver.
Because the layer 2 frame can be transmitted over a point-to-point link, a broadcast
network, or a packet-and-circuit switched system, an address Þeld is needed for the frame's
destination address. The length of this Þeld is commonly 0, 8, or 16 bits, depending on the
data link layer protocol. For instance, SDLC and LAPB use an 8-bit address and SS#7 has
no address Þeld because it is used always in point-to-point signaling links. LAPD further
divides its 16-bit address into different Þelds to specify various access points within one
device. It also deÞnes a broadcast address. Some HDLC-type protocols also permit
extended addressing beyond 16 bits.
The 8- or 16-bit control Þeld provides a ßow-control number and deÞnes the frame type
(control or data). The exact use and structure of this Þeld depends upon the protocol using
the frame. Data is transmitted in the data Þeld, which can vary in length depending upon
the protocol using the frame. Layer 3 frames are carried in this data Þeld.
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 transmitted Þrst and the msb of the CRC is transmitted Þrst.
When GFMR[MODE] selects HDLC mode, that FCC functions as an HDLC controller.
When an FCC in HDLC mode is used with a nonmultiplexed modem interface, the FCC
outputs are connected directly to the external pins. Modem signals can be supported
through the appropriate port pins. The receive and transmit clocks can be supplied either
externally or from the bank of baud-rate generators. The HDLC controller can also be
connected to one of the TDM channels of the serial interface and used with the TSA. The
HDLC controller consists of separate transmit and receive sections whose operations are
asynchronous with the core and can either be synchronous or asynchronous with other
FCCs. The user can allocate external buffer descriptors (BDs) for receive and transmit tasks
so many frames can be sent or received without core intervention.