Auto-Baud Detect

The EUSART module supports automatic detection and calibration of the baud rate.

In the Auto-Baud Detect (ABD) mode, 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 ā€œUā€) which is the Sync character for the LIN bus. The unique feature of this character is that it has five rising edges including the Stop bit edge.

Setting the ABDEN bit of the BAUDxCON register starts the auto-baud calibration sequence. While the ABD sequence takes place, the EUSART state machine is held in Idle. On the first rising edge of the receive line, after the Start bit, the SPxBRG begins counting up using the BRG counter clock as shown in Figure 1. The fifth rising edge will occur on the RXx pin at the end of the eighth bit period. At that time, an accumulated value totaling the proper BRG period is left in the SPxBRGH, SPxBRGL register pair, the ABDEN bit is automatically cleared and the RCxIF interrupt flag is set. The value in the RCxREG needs to be read to clear the RCxIF interrupt. RCxREG content should be discarded. When calibrating for modes that do not use the SPxBRGH register the user can verify that the SPxBRGL register did not overflow by checking for 00h in the SPxBRGH register.

The BRG auto-baud clock is determined by the BRG16 and BRGH bits as shown in Table 1. During ABD, both the SPxBRGH and SPxBRGL registers are used as a 16-bit counter, independent of the BRG16 bit setting. While calibrating the baud rate period, the SPxBRGH and SPxBRGL registers are clocked at 1/8th the BRG base clock rate. The resulting byte measurement is the average bit time when clocked at full speed.

  1. 1.If the WUE bit is set with the ABDEN bit, auto-baud detection will occur on the byte following the Break character (see Auto-Wake-up on Break).
  2. 2.It is up to the user to determine that the incoming character baud rate is within the range of the selected BRG clock source. Some combinations of oscillator frequency and EUSART baud rates are not possible.
  3. 3.During the auto-baud process, the auto-baud counter starts counting at one. Upon completion of the auto-baud sequence, to achieve maximum accuracy, subtract 1 from the SPxBRGH:SPxBRGL register pair.
Table 1. BRG Counter Clock Rates
BRG16 BRGH BRG Base Clock BRG ABD Clock
1 1 FOSC/4 FOSC/32
1 0 FOSC/16 FOSC/128
0 1 FOSC/16 FOSC/128
0 0 FOSC/64 FOSC/512
Note: During the ABD sequence, SPxBRGL and SPxBRGH registers are both used as a 16-bit counter, independent of the BRG16 setting.
Figure 1. Automatic Baud Rate Calibration