Motorola MPC8260 - Manual (Page 695)

MOTOROLA Chapter 25. SCC AppleTalk Mode 25-3
Part IV. Communications Processor Module
25.3 Connecting to AppleTalk
As shown in Figure , the MPC8260 connects to LocalTalk, and, using TXD, RTS, and
RXD, is an interface for the RS-422 transceiver. The RS-422, in turn, is an interface for the
LocalTalk connector. Although it is not shown, a passive RC circuit is recommended
between the transceiver and connector.
Figure 25-2. Connecting the MPC8260 to LocalTalk
The 16
´ overspeed of a 3.686-MHz clock can be generated from an external frequency
source or from one of the baud rate generators if the resulting output frequency is close to
a multiple of the 3.686 MHz frequency. The MPC8260 asserts RTS throughout the duration
of the frame so that RTS can be used to enable the RS-422 transmit driver.
25.4 Programming the SCC in AppleTalk Mode
The AppleTalk controller is implemented by setting certain bits in the HDLC controller.
Otherwise, Chapter 21, ÒSCC HDLC Mode,Ó describes how to program the HDLC
controller. Use GSMR, PSMR, or TODR to program the AppleTalk controller.
25.4.1 Programming the GSMR
Program the GSMR as described below:
1. Set MODE to 0b0010 (AppleTalk).
2. Set DIAG to 0b00 for normal operation, with CD and CTS grounded or conÞgured
for parallel I/O. This causes CD and CTS to be internally asserted to the SCC.
3. Set RDCR and TDCR to (0b10) a 16
´ clock.
4. Set the TENC and RENC bits to 0b010 (FM0).
Two HDLC
CRC-16
Flags
Destination
Address
Data
Control
Byte
6-Bit Sync
Sequence Source
Address
Closing
Flag
16 Ones
(Abort)
RS-422
TXD
RTS
RXD
Stored in Receive Buffer
Standard HDLC frame handling
MINI-DIN 8
Stored in Transmit Buffer
SCC
RTS
TXD
Tx Data
Tx Enable
Rx Data
MPC8260
Connection