Datasheet

ATmega48PA/88PA/168PA [DATASHEET]
9223F–AVR–04/14
180
• Bit 7 – RXCn: USART Receive Complete
This flag bit is set when there are unread data in the receive buffer and cleared when the receive buffer is empty (i.e., does
not contain any unread data). If the Receiver is disabled, the receive buffer will be flushed and consequently the RXCn bit
will become zero. The RXCn flag can be used to generate a receive complete interrupt (see description of the RXCIEn bit).
• Bit 6 – TXCn: USART Transmit Complete
This flag bit is set when the entire frame in the transmit shift register has been shifted out and there are no new data currently
present in the transmit buffer (UDRn). The TXCn flag bit is automatically cleared when a transmit complete interrupt is
executed, or it can be cleared by writing a one to its bit location. The TXCn flag can generate a transmit complete interrupt
(see description of the TXCIEn bit).
• Bit 5 – UDREn: USART Data Register Empty
The UDREn flag indicates if the transmit buffer (UDRn) is ready to receive new data. If UDREn is one, the buffer is empty,
and therefore ready to be written. The UDREn flag can generate a data register empty interrupt (see description of the
UDRIE bit). UDREn is set after a reset to indicate that the transmitter is ready.
• Bit 4:0 – Reserved Bits in MSPI mode
When in MSPI mode, these bits are reserved for future use. For compatibility with future devices, these bits must be written
to zero when UCSRnA is written.
21.8.3 UCSRnB – USART MSPIM Control and Status Register n B
• Bit 7 – RXCIEn: RX Complete Interrupt Enable
Writing this bit to one enables interrupt on the RXCn flag. A USART receive complete interrupt will be generated only if the
RXCIEn bit is written to one, the global interrupt flag in SREG is written to one and the RXCn bit in UCSRnA is set.
• Bit 6 – TXCIEn: TX Complete Interrupt Enable
Writing this bit to one enables interrupt on the TXCn flag. A USART transmit complete interrupt will be generated only if the
TXCIEn bit is written to one, the global interrupt flag in SREG is written to one and the TXCn bit in UCSRnA is set.
• Bit 5 – UDRIE: USART Data Register Empty Interrupt Enable
Writing this bit to one enables interrupt on the UDREn Flag. A data register empty interrupt will be generated only if the
UDRIE bit is written to one, the global interrupt flag in SREG is written to one and the UDREn bit in UCSRnA is set.
• Bit 4 – RXENn: Receiver Enable
Writing this bit to one enables the USART receiver in MSPIM mode. The receiver will override normal port operation for the
RxDn pin when enabled. Disabling the receiver will flush the receive buffer. Only enabling the receiver in MSPI mode (i.e.
setting RXENn=1 and TXENn=0) has no meaning since it is the transmitter that controls the transfer clock and since only
master mode is supported.
• Bit 3 – TXENn: Transmitter Enable
Writing this bit to one enables the USART transmitter. The Transmitter will override normal port operation for the TxDn pin
when enabled. The disabling of the transmitter (writing TXENn to zero) will not become effective until ongoing and pending
transmissions are completed, i.e., when the transmit shift register and transmit buffer register do not contain data to be
transmitted. When disabled, the transmitter will no longer override the TxDn port.
• Bit 2:0 – Reserved Bits in MSPI mode
When in MSPI mode, these bits are reserved for future use. For compatibility with future devices, these bits must be written
to zero when UCSRnB is written.
Bit 7 6 5 4 3 2 1 0
RXCIEn TXCIEn UDRIE RXENn TXENn – - - UCSRnB
Read/Write R/W R/W R/W R/W R/W R R R
Initial Value 0 0 0 0 0 1 1 0