MC68000 MOTOROLA [Motorola, Inc], MC68000 Datasheet - Page 163

• Manufacturer Part Number: MC68000
• Manufacturer: MOTOROLA [Motorola, Inc]

The SCC should be disabled and re-enabled if any change is made to the SCC's parallel
I/O or serial channels physical interface configuration. The SCC does not need to be dis-
abled if only a change to a parameter RAM value is made. See 4.5.6 SCC Parameter RAM
Memory Map for a discussion of when parameter RAM values may be modified.
To save power, the SCCs may simply be disabled. Clearing the enable transmitter (ENT) bit
in the SCC mode register causes the SCC transmitter to consume the least possible power;
clearing the ENR bit causes a similar action for the SCC receiver.
The above statement on saving power is independent of a decision to use the low-power
modes (see 3.8 System Control). If a low-power mode is desired, the SCC may be disabled
before entering the low-power mode, or it may be left enabled so that an SCC interrupt may
bring the IMP out of the low-power mode. One common use of the low-power mode is to
disable the transmitter but leave the receiver enabled (i.e., in the hunt mode) so that an ar-
riving frame destined for this station will cause an interrupt, waking the IMP from its low-pow-
er mode.
The low-power mode affects the M68000 core, not the SCCs. Since the SCCs are usually
clocked at a far lower rate than the M68000 core, significant power savings may still be
achieved with the SCCs fully enabled and the M68000 core in the low-power mode.
4.5.11 UART Controller
Many applications need a simple method of communicating low-speed data between equip-
ment. The universal asynchronous receiver transmitter (UART) protocol is the de-facto stan-
dard for such communications. The term asynchronous is used because it is not necessary
to send clocking information with the data that is sent. UART links are typically 38400 baud
or less in speed and are character oriented (i.e., the smallest unit of data that can be cor-
rectly received or transmitted is a character). Typical applications of asynchronous links are
connections between terminals and computer equipment. Even in applications where syn-
chronous communication is required, the UART is often used for a local debugging port to
run board debugger software. The character format of the UART protocol is shown in Figure
4-17.
MOTOROLA
UART TCLK
UART TXD
16
START
BIT
Figure 4-17. UART Frame Format
MC68302 USER’S MANUAL
LEAST SIGNIFICANT BIT FIRST
7 OR 8 DATA BITS WITH THE
Communications Processor (CP)
ADDR.
OPTIONAL
BIT
PAR.
BIT
9/16 TO 2
STOP BITS
4-43