Auto-Baud Detect

The UART module supports automatic detection and calibration of the baud rate in the 8-bit Asynchronous and LIN modes. However, setting ABDEN to start auto-baud detection is neither necessary, nor possible in LIN mode because that mode supports auto-baud detection automatically at the beginning of every data packet. Enabling auto-baud detect with the ABDEN bit applies to the Asynchronous modes only.

When Auto-Baud Detect (ABD) is active, the clock to the BRG is reversed. Rather than the BRG clocking the incoming RX signal, the RX signal is timing the BRG. The Baud Rate Generator is used to time the period of a received 55h (ASCII “U”), which is the Sync character for the LIN bus. The unique feature of this character is that it has five falling edges, including the Start bit edge, and five rising edges, including the Stop bit edge.

In 8-bit Asynchronous mode, setting the ABDEN bit enables the auto-baud calibration sequence. The first falling edge of the RX input after ABDEN is set will start the auto-baud calibration sequence. While the ABD sequence takes place, the UART state machine is held in Idle. On the first falling edge of the receive line, the UxBRG begins counting up using the BRG counter clock, as shown in the following figure. The fifth falling edge will occur on the RX pin at the beginning of the bit 7 period. At that time, an accumulated value totaling the proper BRG period is left in the UxBRG register pair, the ABDEN bit is automatically cleared and the ABDIF interrupt flag is set. ABDIF must be cleared by software.
Figure 1. Automatic Baud Rate Calibration

RXIDL indicates that the sync input is active. RXIDL will go low on the first falling edge and go high on the fifth rising edge.

The BRG auto-baud clock is determined by the BRGS bit, as shown in the following table.
Table 1. BRG Counter Clock Rates
BRGS BRG Base Clock BRG ABD Clock
1 Fosc/4 Fosc/32
0 Fosc/16 Fosc/128

During ABD, the internal BRG register is used as a 16-bit counter. However, the UxBRG registers retain the previous BRG value until the auto-baud process is successfully completed. While calibrating the baud rate period, the internal BRG register is clocked at 1/8th the BRG base clock rate. The resulting byte measurement is the average bit time when clocked at full speed and is transferred to the UxBRG registers when complete.

Important:
1. 1.When both the WUE and ABDEN bits are set, the auto-baud detection will occur on the byte following the Break character (see the Auto Wake-on-Break section).
2. 2.It is up to the user to verify the incoming character baud rate is within the range of the selected BRG clock source. Some combinations of oscillator frequency and UART baud rates are not possible.