MC68000 MOTOROLA [Motorola, Inc], MC68000 Datasheet - Page 164

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

Communications Processor (CP)
Since the transmitter and receiver work asynchronously, there is no need to connect trans-
mit and receive clocks. Instead, the receiver over-samples the incoming data stream by a
factor of 16 and uses some of these samples to determine the bit value. Traditionally, the
middle three of the 16 samples are used. Two UARTs can communicate using a system like
this if parameters such as the parity scheme and character length are the same for both
transmitter and receiver.
When data is not transmitted in the UART protocol, a continuous stream of ones is transmit-
ted. This is called the idle condition. Since the start bit is always a zero, the receiver can
detect when real data is once again present on the line. The UART also specifies an all-ze-
ros character, called a break, which is used to abort a character transfer sequence.
Many different protocols have been defined that use asynchronous characters, but the most
popular of these is the RS-232 standard. RS-232 specifies standard baud rates, handshak-
ing protocols, and mechanical/electrical details. Another popular standard using the same
character format is RS-485, which defines a balanced line system allowing longer cables
than RS-232 links. Synchronous protocols like HDLC or DDCMP are sometimes defined to
run over asynchronous links. Other protocols like PROFIBUS (see Appendix C RISC Micro-
code from RAM) extend the UART protocol to include LAN-oriented features such as token
passing.
All the standards provide handshaking signals, but some systems require just three physical
lines: transmit data, receive data, and ground.
Many proprietary standards have been built around the asynchronous character frame, and
some even implement a multidrop configuration. In multidrop systems, more than two sta-
tions may be present on a network, with each station having a specific address. Frames
composed of many characters may be broadcast, with the first character acting as a desti-
nation address. To allow this procedure, the UART frame is extended by one bit to distin-
guish between an address character and the normal data characters.
By appropriately setting the SCC mode register, any of the SCC channels may be config-
ured to function as a UART. The UART controller provides standard serial I/O using asyn-
chronous character-oriented (start-stop) protocols. The UART may be used to communicate
with other existing UART devices. Also, in conjunction with another SCC channel, it may be
used in either ISDN terminal adaptor or X.25 packet assembly and disassembly (PAD) ap-
plications.
The UART provides a port for serial communication to other microprocessors, terminals,
etc., either locally or through modems. It includes facilities for communication using stan-
dard asynchronous bit rates and protocols. The UART supports a multidrop mode for mas-
ter/slave operation with wakeup capability on either an idle line or an address bit.
The UART uses a seven-pin interface in NMSI mode. It transmits data from memory (inter-
nal or external) to the TXD line and receives data from the RXD line into memory. The seven
dedicated serial interface pins are transmit data (TXD), receive data (RXD), receive clock
(RCLK), transmit clock (TCLK), carrier detect (CD), clear to send (CTS), and request to send
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MC68302 USER’S MANUAL
MOTOROLA