Rabbit 6000 User’s Manual
digi.com
184
19. S
ERIAL
P
ORTS
A – D
19.1 Overview
Serial Ports A, B, C, and D are identical, except for the source of the data clock and the transmit, receive,
and clock pins. In addition to being used as a regular serial port, Serial Port A can be used to bootstrap the
processor. Each serial port can be used in the asynchronous or the clocked serial mode with an internal or
external clock.
In the asynchronous mode, either 7 or 8 data bits can be transferred, and a parity bit and/or a ninth data bit
can be appended as well. Parity and the ninth data bits are also detected when they are received. The asyn-
chronous mode is full-duplex, while the clocked mode can be half or full-duplex.
The transmit and receive buffers have 4 bytes each. A serial port interrupt is generated whenever at least
one byte is available in the receive buffer, whenever a byte is shifted out of the transmit buffer, and when-
ever the transmit buffer is empty. Each serial port has a separate interrupt vector.
The status of each serial port is available in its Serial Port Status Register (SxSR), and contains informa-
tion on whether a received byte is available, the receive buffer was overrun, a parity error was received,
the transmit buffer is empty or busy sending a byte, and the state of the ninth data bit (whether it is an
address bit or a stop bit).
All four common SPI clock modes are supported, and the bit order of the data may be either MSB or LSB
first. The transmit and receive operations are under program control as well.
Figure 19.1 Serial Ports A – D Operation in Clocked Serial Mode
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