Figure D-26. Using Cd (Sync) In The Nmsi Transmit Case - Motorola MC68302 User Manual

Since the L bit is set, once the frame ends, the synchronization process must occur once
again. Also, to force resynchronizations instead of waiting for the transmission to finish, a
STOP TRANSMIT command can be given, followed by a RESTART TRANSMIT command;
however, the STOP TRANSMIT command will abort the current buffer.
RCLK
(I/O)
TCLK
(I/O)
TXD
(OUTPUT)
RTS
(OUTPUT)
CTS DON'T
(INPUT) CARE
CD (SYNC)
(INPUT)
EXSYN = 1
NTSYN = 1
DIAG1-DIAG0 BITS = 11
L = 1 IN THE Tx BD
ONLY ONE Tx BD IS READY

Figure D-26. Using CD (Sync) In the NMSI Transmit Case

Figure D-27 shows how CD (sync) is used in the NMSI receive case. Setting the EXSYN bit
causes CD (sync) to control the reception of data. CD (sync) should be latched low on the
rising clock (RCLK) of the second bit of the frame. (Latching CD (sync) during the 2nd bit of
the frame allows external BISYNC sync detection logic, which also uses EXSYN, extra time
to present the external sync to the SCC.) Once synchronization is achieved, it will never be
lost unless an ENTER HUNT MODE command is given, a receive overrun occurs, or the
receiver is disabled and re-enabled (ENR bit is cleared, ENTER HUNT MODE command is
issued, and ENR is set). Once synchronization is lost, a new frame can be resynchronized
using CD (sync).
Notice that we have been discussing the receive and transmit cases separately. The receive
and transmit halves of the SCC really are separate and distinct; however, in transparent
mode, the receive and transmit halves of the SCC share the CD (sync) pin, which is not true
in normal NMSI.
MOTOROLA
6.5 TCLKS
DATA READY; WAITING FOR CD
CD BEING LATCHED HERE ON
RISING RCLK CAUSES DATA TO
BEGIN TRANSMITTING IN 6.5
BIT TIMES.
MC68302 USER'S MANUAL
MC68302 Applications
FIRST BIT TRANSMITTED LAST BIT
NO EFFECT; ONLY THE FALLING
EDGE IS DETECTED.
D-55