Datasheet

Synch
Start
Bit
Stop
Bit
0 1 2 3 4 5 6 7
8
Bit Times
Break
Delimiter
Synch
USCI Operation: UART Mode www.ti.com
15.3.4 Automatic Baud Rate Detection
When UCMODEx = 11 UART mode with automatic baud rate detection is selected. For automatic baud
rate detection, a data frame is preceded by a synchronization sequence that consists of a break and a
synch field. A break is detected when 11 or more continuous zeros (spaces) are received. If the length of
the break exceeds 22 bit times the break timeout error flag UCBTOE is set. The synch field follows the
break as shown in Figure 15-5.
Figure 15-5. Auto Baud Rate Detection - Break/Synch Sequence
For LIN conformance the character format should be set to 8 data bits, LSB first, no parity and one stop
bit. No address bit is available.
The synch field consists of the data 055h inside a byte field as shown in Figure 15-6. The synchronization
is based on the time measurement between the first falling edge and the last falling edge of the pattern.
The transmit baud rate generator is used for the measurement if automatic baud rate detection is enabled
by setting UCABDEN. Otherwise, the pattern is received but not measured. The result of the
measurement is transferred into the baud rate control registers UCAxBR0, UCAxBR1, and UCAxMCTL. If
the length of the synch field exceeds the measurable time the synch timeout error flag UCSTOE is set.
Figure 15-6. Auto Baud Rate Detection - Synch Field
The UCDORM bit is used to control data reception in this mode. When UCDORM is set, all characters are
received but not transferred into the UCAxRXBUF, and interrupts are not generated. When a break/synch
field is detected the UCBRK flag is set. The character following the break/synch field is transferred into
UCAxRXBUF and the UCAxRXIFG interrupt flag is set. Any applicable error flag is also set. If the
UCBRKIE bit is set, reception of the break/synch sets the UCAxRXIFG. The UCBRK bit is reset by user
software or by reading the receive buffer UCAxRXBUF.
When a break/synch field is received, user software must reset UCDORM to continue receiving data. If
UCDORM remains set, only the character after the next reception of a break/synch field will be received.
The UCDORM bit is not modified by the USCI hardware automatically.
When UCDORM = 0 all received characters will set the receive interrupt flag UCAxRXIFG. If UCDORM is
cleared during the reception of a character the receive interrupt flag will be set after the reception is
complete.
The automatic baud rate detection mode can be used in a full-duplex communication system with some
restrictions. The USCI can not transmit data while receiving the break/sync field and if a 0h byte with
framing error is received any data transmitted during this time gets corrupted. The latter case can be
discovered by checking the received data and the UCFE bit.
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Universal Serial Communication Interface, UART Mode SLAU144J–December 2004–Revised July 2013
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