![](http://datasheet.mmic.net.cn/290000/XPC801ZP25_datasheet_16187868/XPC801ZP25_374.png)
Serial Communication Modules
MOTOROLA
MPC801 USER’S MANUAL
16-3
16
Ready to Send (RTS)—This output pin serves two basic purposes. Normally, the
receiver indicates that it is ready to receive data by asserting this pin (low), which would
be connected to the end transmitter’s CTS pin. When the receiver detects a pending
overrun, it negates this pin. However, for other applications, the RTS pin can be a
general-purpose output controlled by the RTS pin in the global register.
General-Purpose Input/Output (GPIO)—This bidirectional pin serves several purposes.
It can be a general-purpose input that posts interrupts on any transition or it can be a
general-purpose output controlled by the GPIO bit in the baud register. It can also serve
as the clock source to the baud rate generator and output the bit clock at the selected
baud rate.
16.2.1 Sub-Block Description
This UART module is easy to use from both a hardware and software perspective because
five working registers perform all possible status and control functions. In addition, all
register bits can be read and most of them are read/write. The modem control signals are
flexible.The CTS pin is an input that can either provide hardware flow-control to the
transmitter or be used as a general-purpose input.
A maskable interrupt is posted on each transition of this signal. The RTS pin is an output
from the receiver that indicates when the receiver FIFO is not full. This bit can be configured
as a general-purpose output. A GPIO pin is provided that can be used to bring an external
bit-clock into the module or it can be used as a general-purpose input with a maskable
interrupt posted on each transition. It can even be configured as an output that provides a
bit-clock or signal under software control. The UART consists of four submodules—the
transmitter, receiver, baud rate generator, and global controller interface.
16.2.1.1 THE TRANSMITTER.
PowerPC
core and transmits it serially. While the FIFO is empty, the transmitter outputs a
continuous IDLE bit (1 in NRZ mode and 0 in IrDA mode). When a character is available for
transmission, the START, STOP, and PARITY (if enabled) bits are added to the character
and it is serially shifted at the selected bit rate. The transmitter posts a maskable interrupt
when it is ready for parallel data. Three interrupt sources are available. If you want to take
full advantage of the 8-byte FIFO, you should enable the FEM interrupt. In the interrupt
service routine, the FIFO should be interrogated after each byte is loaded.
The transmitter accepts a parallel character from the
If there is space available and the TXAVEN bit of the control register is set, more data is
loaded into the FIFO. Another interrupt from the transmitter will not be generated until the
FIFO has completely emptied. If your software has a large interrupt service latency, the
FHAL of the transmitter register interrupt can be used. In this case, an interrupt is generated
when the occupancy in the FIFO is less than half of its size. If you do not need the FIFO,
you should use the TXAVEN interrupt of the control register. An interrupt is generated when
at least one space is available in the FIFO.