132 Rabbit 2000 Microprocessor12.7.5 Extra Stop Bits, Sending Parity, 9th Bit Communication SchemesSome 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, usea 9th data bit to mark the start of a message frame. The Rabbit 2000 can receive parity ormessage 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 forbyte formats that use only 7 data bits, in which case the 9th bit or parity bit is actually an8th bit. Things are a little bit messy for the transmitter software if there are 8 data bits anda 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 longstop register as described in Section B.2.3. Sending a 9th bit in the original Rabbit 2000chip 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 Patterns12.7.5.1 Parity, Extra Stop Bits with 7 Data Bit CharactersIf only 7 data bits are being sent, the problem of sending an additional parity or signal bitis 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 areto 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 lowstop bit0 70 7stop bitCharacter with 9th bit lowCharacter w/o 9th bit lowstart bitSignal shown at output pin on processor. A “1” is high.start bit0 7stop bit9th bit highCharacter w. 9th bit highThis format is notsent automatically.
