This register is not used in Master SPI Mode (UMSEL0[1:0] = 11)
| Bit7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| RXSIE | RXS | SFDE | |||||
| AccessR/W | R/W | R/W | |||||
| Reset0 | 0 | 0 |
USART RX Start Interrupt Enable
Writing this bit to one enables the interrupt on the RXS flag. In sleep modes this bit enables start frame detector that can wake up the MCU when a start condition is detected on the RxD line. The USART RX Start Interrupt is generated only, if the RXSIE bit, the Global Interrupt flag, and RXS are set.
USART RX Start
The RXS flag is set when a start condition is detected on the RxD line. If the RXSIE bit and the Global Interrupt Enable flag are set, an RX Start Interrupt will be generated when the flag is set. The flag can only be cleared by writing a logical one on the RXS bit location.
If the start frame detector is enabled (RXSIE = 1) and the Global Interrupt Enable flag is set, the RX Start Interrupt will wake up the MCU from all sleep modes.
Start Frame Detection Enable
Writing this bit to one enables the USART Start Frame Detection mode. The start frame detector is able to wake up the MCU from sleep mode when a start condition, i.e. a high (IDLE) to low (START) transition, is detected on the RxD line.
| UCSR0D.SFDE | UCSR0D.RXSIE | UCSR0B.RXCIE | Description |
|---|---|---|---|
| 0 | X | X | Start frame detector disabled |
| 1 | 0 | 0 | Reserved |
| 1 | 0 | 1 | Start frame detector enabled. RXC flag wakes up MCU from all sleep modes |
| 1 | 1 | 0 | Start frame detector enabled. RXS flag wakes up MCU from all sleep modes |
| 1 | 1 | 1 | Start frame detector enabled. Both RXC and RXS wake up the MCU from all sleep modes |