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Universal Asynchronous Receiver/Transmitter
MOTOROLA
MC68328 USER’S MANUAL 12/9/97
11-3
UART
11
PRELIMINARY
RTS—The request-to-send pin is an output that serves two purposes. Normally, the
receiver indicates that it is ready to receive data by asserting this pin (low). This pin
would be connected to the far-end transmitter’s CTS pin. When the receiver detects a
pending overrun, it negates this pin. For other applications, this pin can serve as a
general-purpose output controlled by the RTS bit in the UART receiver register.
GPIO—The general-purpose input/output bidirectional pin has several functions. It can
be a general-purpose input that
t can post interrupts on any transition,
t is controlled by the GPIO bit in the UART baud control register,
t can serve as the source of the clock to the baud rate generator, or
t can output the bit clock at the selected baud rate.
11.3 SERIAL OPERATION
The UART module has two operating modes—NRZ and infra-red. NRZ mode is usually
associated with RS-232. Each character is transmitted as a frame delimited by a Start bit (0)
at the beginning and a Stop bit (1) at the end. Data bits are transmitted least -significant bit
first and each bit is presented for a full bit time. If parity is used, the parity bit is transmitted
after the most-significant bit. Figure 11-2 illustrates an 8 bit ASCII “A” character (41 hex) with
odd parity.
Infra-red operation uses character frames like NRZ mode, but instead of driving ones and
zeros for a full bit time, zeros are transmitted as 3/16 bit time pulses and ones remains low.
The polarity of the transmitted pulses and the expected receive pulses can be inverted so
that a direct connection can be made to external infrared transceiver modules that use active
low pulses. Figure 11-3 illustrated a character in infra-red mode.
Figure 11-2. NRZ ASCII “A” Character with Odd Parity
Figure 11-3. IrDA ASCII “A” Character with Odd Parity
START
BIT
0
BIT
1
BIT
2
BIT
3
BIT
4
BIT
5
BIT
6
BIT
7
PARITY
BIT
STOP
BIT
START
BIT
0
BIT
1
BIT
2
BIT
3
BIT
4
BIT
5
BIT
6
BIT
7
PARITY
BIT
STOP
BIT