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Chapter 22
Serial Communications Controllers
220
220
The MPC860 has four serial communications controllers (SCC), which can be conÞgured
independently to implement different protocols for bridging functions, routers, and
gateways, and to interface with a wide variety of standard WANs, LANs, and proprietary
networks. An SCC has many physical interface options such as interfacing to TDM buses,
ISDN buses, and standard modem interfaces.
The SCCs are independent from the physical interface, but SCC logic formats and
manipulates data from the physical interface. Furthermore, the choice of protocol is
independent from the choice of interface. An SCC is described in terms of the protocol it
runs. When an SCC is programmed to a certain protocol or mode, it implements
functionality that corresponds to parts of the protocolÕs link layer (layer 2 of the OSI
reference model). Many SCC functions are common to protocols of the following
controllers:
¥ UART, described in Chapter 23, ÒSCC UART Mode.Ó
¥ HDLC and HDLC bus, described in Chapter 24, ÒSCC HDLC Mode.Ó
¥ IrDA or asynchronous HDLC, described in Chapter 26, ÒSCC Asynchronous HDLC
Mode and IrDA.Ó
¥ AppleTalk/LocalTalk, described in Chapter 25, ÒSCC AppleTalk Mode.Ó
¥ BISYNC, described in Chapter 27, ÒSCC BISYNC Mode.Ó
¥ Transparent, described in Chapter 29, ÒSCC Transparent Mode.Ó
¥ Ethernet, described in Chapter 28, ÒSCC Ethernet Mode.Ó
Although the selected protocol usually applies to both the SCC transmitter and receiver, one
half of an SCC can run transparent operations while the other half runs a standard protocol
(except Ethernet).
Each Rx and Tx internal clock can be programmed with either an external or internal
source. Internal clocks originate from one of four baud rate generators (BRGs) or one of
eight external clock pins; see Section 21.2.4.3, ÒSI Clock Route Register (SICR),Ó for each
SCCÕs available clock sources. These clocks can be as fast as a 1:2 ratio of the system clock.
(For example, an SCC internal clock can run at 12.5 MHz in a 25-MHz system.) However,
an SCCÕs ability to support a sustained bit stream depends on the protocol as well as other
factors. See Appendix B, ÒSerial Communications Performance.Ó
MOTOROLA
Chapter 22. Serial Communications Controllers
22-1
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