25.3.4.9 Start-of-Frame Detection
The Start-of-Frame Detection feature enables the USART to wake up from Standby sleep mode upon data reception.
When a high-to-low transition is detected on the RXD pin, the oscillator is powered up and the USART peripheral clock is enabled. After start-up, the rest of the data frame can be received, provided that the baud rate is slow enough concerning the oscillator start-up time. The oscillators start-up time varies with supply voltage and temperature. For more information on oscillator start-up time characteristics, refer to the Electrical Characteristics section.
If a false Start bit is detected and no other wake-up source has been triggered, the device will return to Standby sleep mode.
The Start-of-Frame detection feature operates only in Asynchronous mode. It is enabled by setting the Start-of-Frame Detection Enable (SFDEN) bit in the Control E (USARTn.CTRLE) register. If a Start bit is detected while the device is in Standby sleep mode, the USART Receive Start (RXS) interrupt flag is set.
The USART Receive Complete (RXC) interrupt flag and the RXS bit share the same interrupt line, but each have dedicated interrupt settings. The table below shows the USART Start Frame Detection modes, depending on the interrupt setting.
| SFDEN | RXSIF Interrupt | RXCIF Interrupt | Comment |
|---|---|---|---|
0 | x | x | Standard mode |
1 | Disabled | Disabled | Only the oscillator is powered during frame reception. If interrupts are disabled and buffer overflow is ignored, all incoming frames will be lost. |
1
| Disabled | Enabled | System and all clocks are awakened when a Receive Complete interrupt occurs |
1
| Enabled | x | System and all clocks are awakened when a Start bit is detected |
SLEEP instruction
will not shut down the oscillator if communication is ongoing.