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 start-up time of the oscillators varies with supply voltage and temperature. For details on oscillator start-up time characteristics, refer to the Electrical Characteristics section.

If a false Start bit is detected, the device will, if another wake-up source has not been triggered, go back into the Standby sleep mode.

The Start-of-Frame detection works in Asynchronous mode only. It is enabled by writing the Start-of-Frame Detection Enable (SFDEN) bit in the USARTn.CTRLB register. If a Start bit is detected while the device is in Standby sleep mode, the USART Receive Start Interrupt Flag (RXSIF) bit is set.

The USART Receive Complete Interrupt Flag (RXCIF) bit and the RXSIF bit share the same interrupt line, but each has its dedicated interrupt settings. The table below shows the USART Start Frame Detection modes, depending on the interrupt setting.

Table 1. USART Start Frame Detection Modes
SFDEN RXSIF Interrupt RXCIF Interrupt Comment
0 x x Standard mode
1 Disabled Disabled Only the oscillator is powered during the frame reception. If the interrupts are disabled and buffer overflow is ignored, all incoming frames will be lost
1 Disabled Enabled System/all clocks are awakened on Receive Complete interrupt
1 Enabled x System/all clocks are awakened when a Start bit is detected
Note: The SLEEP instruction will not shut down the oscillator if there is ongoing communication.