Motorola MPC8260 Manuale Utente

MOTOROLA

Chapter 25.  SCC AppleTalk Mode  

25-3

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.

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.

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.

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