UART/Autobaud Functional Description
12-48
12.5.10.2
TX
With XOFF1, two characters are transmitted when the RX FIFO has passed
the programmed trigger level TCR(3:0).
With XON1, two characters are transmitted when the RX FIFO reaches the
trigger level programmed via TCR(7:4).
After an XOFF character has been sent, if software flow control is disabled, the
module transmits XON characters automatically to enable normal transmis-
sion to proceed.
The transmission of XOFF/XON follows the exact same protocol as transmis-
sion of an ordinary byte from the FIFO. This means that even if the word length
is set to be 5, 6, or 7 characters, the 5, 6, or 7 least significant bits of
XOFF1,2/XON1,2 are transmitted. (The transmission of 5, 6, or 7 bits of a char-
acter is seldom done, but this functionality is included to maintain compatibility
with earlier designs.)
It is assumed that software flow control and hardware flow control are never
enabled simultaneously.
12.5.11
Autobauding Mode
In autobaud mode, UART can extract transfer characteristics (speed, length
and parity) from an AT command. These characteristics are used to receive
data following an AT and to send data.
Here are valid AT commands:
AT DATA <CR>
at DATA
<CR>
A
/a/
A line break during the acquisition of the sequence AT is not recognized and
echo functionality is not implemented in hardware.
A/ and a/ are not used to extract characteristics, but they must be recognized
because of their special meaning. They instruct the software to repeat the last
received AT command; therefore, an a/ always comes after an AT and transfer
characteristics are not expected to change between an AT and an a/.
When a valid AT is received, it and all subsequent data are saved into the
FIFO, including final CR (0x0D). Then the autobaud state machine waits for
the next valid AT command. If an a/ (A/) is received, the a/ (A/) is saved into
the FIFO and the state machine waits for next valid AT command.