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SCCs on the MC68302 can do this very efficiently because of their sophisticated DMA ca-
pability, and very little MC68000 core intervention is required.
Third, some applications require the switching of data without interfering with the protocol
encoding itself. For instance, in a multiplexer, data from a high-speed time-multiplexed serial
stream is multiplexed into multiple low-speed data streams. In this case, the idea is to switch
the data path but not alter the protocol encoded on that data path.
Finally, some applications require a special protocol that does not fall under the category of
HDLC, UART, etc. In some instances, transparent mode can be used; however, care should
be taken to understand the capabilities of transparent mode before trying this. The most im-
portant issue is how this new protocol recognizes its frames on the receiving end. Transpar-
ent mode on the MC68302 was designed to work well receiving continuous streams of data
(no gaps in the data exist over time). This is different from receiving transparent frames; al-
though there is some support for this, it is limited on the MC68302.
D.8.3 Physical Interface to Accompany Transparent Mode
Before discussing the details of transparent mode timing, we need to chose the physical in-
terface to go with the transparent mode. The timings associated with transparent mode differ
based on the physical interface chosen.
The MC68302 supports the following four physical interfaces:
• Nonmultiplexed Serial Interface—NMSI
• Pulse Code Modulation Highway—PCM
• Interchip Digital Link—IDL
• General Circuit Interface—GCI
You will probably chose either an NMSI or PCM highway interface, unless you are designing
an ISDN-based system. If you are designing an ISDN-based system, you will probably
choose either an IDL or GCI interface. The following paragraphs discuss all the interfaces,
but special attention is given to the NMSI.
The following discussion assumes some knowledge of the inter-
faces. For more applications information on these interfaces, re-
fer to 4.4 Serial Channels Physical Interface.
The most commonly used physical interface on the MC68302 is the nonmultiplexed serial
interface (NMSI). The NMSI consists of seven basic modem (RS-232) signals: TXD, TCLK,
RXD, RCLK, RTS, CTS, and CD. Each of the three SCCs can have its own set of these sig-
nals, as shown in Figure D-21.
MOTOROLA
NOTE
MC68302 USER'S MANUAL
MC68302 Applications
D-47
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