
Communications Processor (CP)
MOTOROLA
MC68302 USER’S MANUAL
4-45
(RTS). Other modem lines such as data set ready (DSR) and data terminal ready (DTR) can
be supported through the parallel I/O pins.
The UART consists of separate transmit and receive sections whose operations are asyn-
chronous with the M68000 core and may be either synchronous or asynchronous with re-
spect to each other. Each clock can be supplied either from the baud rate generator or from
the external pins.
The UART key features are as follows:
Flexible Message-Oriented Data Buffers
Multidrop Operation
Receiver Wakeup on IDLE Line or Address Mode
Eight Control Character Comparison Registers
Two Address Comparison Registers
Four 16-Bit Error Counters
Programmable Data Length (7 or 8 Bits)
Programmable 1 or 2 Stop Bits with Fractional Stop Bits
Even/Odd/Force/No Parity Generation
Even/Odd/No Parity Check
Frame Error, Noise Error, Break, and IDLE Detection
Transmits Preamble and Break Sequences
Freeze Transmission Option
Maintenance of Four 16-Bit Error Counters
Provides Asynchronous Link for DDCMP Use
Flow Control Character Transmission Supported
4.5.11.1 Normal Asynchronous Mode
In the normal asynchronous mode, the receive shift register receives the incoming data on
the RXD pin. The length and the format of the serial word in bits are defined by the control
bits in the UART mode register. The order of reception is as follows:
Start Bit
Seven or Eight Data Bits with the Least Significant Bit First
Address/Data Bit (Optional)
Parity Bit (Optional)
Stop Bits
The receiver samples each bit of the incoming data three times around its center. The value
of the bit is determined by the majority of those samples. If all the samples do not agree, a
noise indication counter is incremented. When a complete character has been clocked in,
the contents of the shift register are transferred to the UART receive data register. If there
is an error in this character, then the appropriate error bits will be set by the IMP.