32.6.3.4 Break Character Detection and Auto-Baud

Break character detection and auto-baud are available in this configuration:

  • Auto-baud frame format (CTRLA.FORM = 0x04 or 0x05)
  • Asynchronous mode (CTRLA.CMODE = 0)
  • 16x sample rate using fractional baud rate generation (CTRLA.SAMPR = 1)

The USART uses a break detection threshold of greater than 11 nominal bit times at the configured baud rate. At any time, if more than 11 consecutive dominant bits are detected on the bus, the USART detects a break field. When a break field is detected, the Receive Break Interrupt Flag (INTFLAG.RXBRK) is set and the USART expects the sync field character to be 0x55. This field is used to update the actual baud rate to stay synchronized. If the received sync character is not 0x55, then the Inconsistent Sync Field error flag (STATUS.ISF) is set along with the Error Interrupt Flag (INTFLAG.ERROR) and the baud rate is unchanged.

The auto-baud follows the LIN format. All LIN Frames start with a break field followed by a sync field.

Figure 32-12. LIN Break and Sync Fields

After a break field is detected and the Start bit of the sync field is detected, a counter is started. The counter is, then, incremented for the next 8 bit times of the sync field. At the end of these 8 bit times, the counter is stopped. At this moment, the 13 MSb of the counter (value divided by 8) gives the new clock divider (BAUD.BAUD) and the 3 LSb of this value (the remainder) gives the new Fractional Part (BAUD.FP).

When the sync field is received, the clock divider (BAUD.BAUD) and the Fractional Part (BAUD.FP) are updated after a synchronization delay. After the break and sync fields are received, multiple characters of data can be received.