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Rabbit 2000 Microprocessor User’s Manual
12.7.5 Extra Stop Bits, Sending Parity, 9th Bit Communication Schemes
Some systems may require two stop bits. In some cases, it may be necessary to send a par-
ity bit. Certain systems, such as some 8051-based multidrop communications systems, use
a 9th data bit to mark the start of a message frame. The Rabbit 2000 can receive parity or
message formats that contain a 9th bit without problem. Transmitting messages with par-
ity or messages that always contain a 9th bit is also possible. It is quite easy to do so for
byte formats that use only 7 data bits, in which case the 9th bit or parity bit is actually an
8th bit. Things are a little bit messy for the transmitter software if there are 8 data bits and
a 9th parity or signaling bit is needed. Sending a 9th low bit is supported by hardware.
Sending a 9th bit is easier with revisions A–C of the Rabbit 2000 chip, which have a long
stop register as described in
Section B.2.3. Sending a 9th bit in the original Rabbit 2000
chip as a high value required delaying the transmission of the next character by 1 baud,
effectively providing the 9th bit high and a stop bit, which is the same as two stop bits.
Figure 12-8 illustrates the standard asynchronous serial output patterns.
Figure 12-8. Asynchronous Serial Output Patterns
12.7.5.1 Parity, Extra Stop Bits with 7 Data Bit Characters
If only 7 data bits are being sent, the problem of sending an additional parity or signal bit
is easily solved by sending 8 bits and always setting bit 7 (the eighth bit) of the byte to "1"
or “0” depending on what is desired. No special precautions are needed if two stop bits are
to be received. If parity is received with 7 data bits, receive the data as 8 bits, and the par-
ity will be in the high bit of the byte.
start bit
data bits
9th bit low
stop bit
07
stop bit
Character with 9th bit low
Character w/o 9th bit low
start bit
Signal shown at output pin on processor. A “1” is high.
start bit
07
stop bit
9th bit high
Character w. 9th bit high
This format is not
sent automatically.