User’s Manual
207
B.2.3 Serial Port Changes
Two features were added to the Rabbit 2000 serial port hardware in revisions A–C to
improve and simplify asynchronous serial and clocked serial communication.
Asynchronous Serial Port
In the asynchronous transmission mode, serial data are transmitted in the following order.
In the original Rabbit 2000 it was difficult to transmit the additional stop bit. This could
only be done by inserting a time delay before the next byte was transmitted. An additional
register, the
long stop register
, was added in revisions A–C. The register serves as an alter-
nate data-out register, and data stored in this register will be transmitted with 2 stop bits
(high level at the Tx pin). This simplifies implementing “9th bit” protocols as well as
sending parity for compatibility with legacy systems. With the new register, data may be
conveniently transmitted with either a “1” or “0” bit inserted following the last data bit,
and that bit will then be followed by a stop bit.
Section 12.6 and Section 12.7 provide additional information about asynchronous serial
data transmission.
The Serial Port x Long Stop Register (SxLR) is only present in revisions A–C.
Table B-6. Asycnchronous Serial Data Transmission Order
Start
Data Bits
Stop Bit or
Special Flag
Stop Bit
start bit
7 or 8 data bits
stop bit
—
stop bit
stop bit
address flag
stop bit
parity bit
stop bit
Table B-7. Long Stop Register, All Ports
Serial Port x Long Stop Register
(SALR)
(Address = 0xC2)
(SBLR)
(Address = 0xD2)
(SCLR)
(Address = 0xE2)
(SDLR)
(Address = 0xF2)
Bit(s)
Value
Description
7:0
Read
Returns the contents of the receive buffer.
Write
Loads the transmit buffer with an address byte, marked with a “one” address bit,
for transmission.
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