Rabbit 6000 User’s Manual
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200
20. S
ERIAL
P
ORTS
E – F
20.1 Overview
Serial Ports E and F are identical to each other, and their asynchronous operation is identical to
that of Serial Ports A – D except for the source of the data clock, and the transmit, receive, and
clock pins. Each serial port can be used in the asynchronous or the HDLC mode with an internal
or external (HDLC mode) clock.
In the asynchronous mode, either 7 or 8 data bits can be transferred, and both a parity bit and/or
ninth data bit can be appended as well. Parity and the ninth data bits are also detected when they
are received. The asynchronous mode is full-duplex.
The transmit and receive buffers of Serial Ports E and F have 4 bytes each; this reduces the inter-
rupt overhead requirements because an interrupt does not have to be generated as often. A serial
port interrupt may be generated whenever at least one byte is available in the receive buffer or
whenever a byte is shifted out of the transmit buffer.
The status of each serial port is available in its Serial Port Status Register (SxSR), and contains
information 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).
Serial Ports E and F support the HDLC mode with either an internal or an external clock; separate
pins are used for the transmit and receive clocks. The HDLC packet flag encapsulation, flag
escapes, and CRC calculation and check are handled automatically by the processor. The serial
port can detect end-of-frame, short-frame, and CRC errors. Interrupts are generated by the recep-
tion of an end-of-frame, at the end of a transmission of a CRC, by an abort sequence, or by a clos-
ing flag. Transmit and receive operations are essentially automatic.
The standard CRC-CCITT polynomial (
x
16
+
x
12
+
x
5
+ 1) is implemented for the CRC, with the
generator and checker preset to all ones.
It is possible to send packets with or without a CRC appended. It is also possible to select whether
an abort or flag will be transmitted if the transmitter underflows. A packet under transition can be
aborted and the abort pattern sent. The idle condition of the line can be flags or all ones.
Several types of data encoding are available in HDLC mode: NRZ, NRZI, biphase-level (Man-
chester), biphase-space (FM0), and biphase-mark (FM1). IrDA-compliant RZI encoding is also
available in HDLC mode; it reduces the bit widths to ¼ the normal width, which allows the serial-
port signal to be connected directly to an IrDA transceiver.
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