Connecting To Appletalk; Programming The Scc In Appletalk Mode; Programming The Gsmr - Freescale Semiconductor MPC850 User Manual

24.3 Connecting to AppleTalk

As shown in Figure 24-2, the MPC850 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.
SCC
6-Bit Sync Two HDLC
TXD
Sequence Flags
RTS
Standard HDLC frame handling
Figure 24-2. Connecting the MPC850to 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 MPC850 asserts RTS throughout the duration
of the frame so that RTS can be used to enable the RS-422 transmit driver.

24.4 Programming the SCC in AppleTalk Mode

The AppleTalk controller is implemented by setting certain bits in the HDLC controller.
Otherwise, Chapter 23, "SCC HDLC Mode," describes how to program the HDLC
controller. Use GSMR, PSMR, or TODR to program the AppleTalk controller.

24.4.1 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 configured
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.
MPC850
Tx Data
TXD
Tx Enable
RTS
Rx Data
RXD
Stored in Receive Buffer
Stored in Transmit Buffer
Destination Source Control
Address Address
Chapter 24. SCC AppleTalk Mode
RS-422
Data
Byte
Connecting to AppleTalk
MINI-DIN 8
Connection
Closing 16 Ones
CRC-16
Flag (Abort)