25.8.4 Start Frame Detection

The USART start frame detector can wake up the MCU from Power-down and Standby sleep mode when it detects a start bit.

When a high-to-low transition is detected on RxDn, the internal 8MHz oscillator is powered up and the USART clock is enabled. After start-up the rest of the data frame can be received, provided that the baud rate is slow enough in relation to the internal 8MHz oscillator start-up time. Start-up time of the internal 8MHz oscillator varies with supply voltage and temperature.

The USART start frame detection works both in asynchronous and synchronous modes. It is enabled by writing the Start Frame Detection Enable bit (SFDE). If the USART Start Interrupt Enable (RXSIE) bit is set, the USART Receive Start Interrupt is generated immediately when start is detected.

When using the feature without start interrupt, the start detection logic activates the internal 8MHz oscillator and the USART clock while the frame is being received, only. Other clocks remain stopped until the Receive Complete Interrupt wakes up the MCU.

The maximum baud rate in synchronous mode depends on the sleep mode the device is woken up from, as follows:

  • Idle sleep mode: system clock frequency divided by four
  • Standby or Power-down: 500kbps

The maximum baud rate in asynchronous mode depends on the sleep mode the device is woken up from, as follows:

  • Idle sleep mode: the same as in active mode
    Table 25-4. Maximum Total Baudrate Error in Normal Speed Mode
    BaudrateFrame Size
    5678910
    0 - 28.8kbps+6.67
-5.88+5.79
-5.08+5.11
-4.48+4.58
-4.00+4.14
-3.61+3.78
-3.30
    38.4kbps+6.63
-5.88+5.75
-5.08+5.08
-4.48+4.55
-4.00+4.12
-3.61+3.76
-3.30
    57.6kbps+6.10
-5.88+5.30
-5.08+4.69
-4.48+4.20
-4.00+3.80
-3.61+3.47
-3.30
    76.8kbps+5.59
-5.88+4.85
-5.08+4.29
-4.48+3.85
-4.00+3.48
-3.61+3.18
-3.30
    115.2kbps+4.57
-5.88+3.97
-5.08+3.51
-4.48+3.15
-4.00+2.86
-3.61+2.61
-3.30
    Table 25-5. Maximum Total Baudrate Error in Double Speed Mode
    BaudrateFrame Size
    5678910
    0 - 57.6kbps+5.66
-4.00+4.92
-3.45+4.35
-3.03+3.90
-2.70+3.53
-2.44+3.23
-2.22
    76.8kbps+5.59
-4.00+4.85
-3.45+4.29
-3.03+3.85
-2.70+3.48
-2.44+3.18
-2.22
    115.2kbps+4.57
-4.00+3.97
-3.45+3.51
-3.03+3.15
-2.70+2.86
-2.44+2.61
-2.22