Atmel ATmega328P Xplained Mini MEGA328P-XMINI MEGA328P-XMINI 数据表
产品代码
MEGA328P-XMINI
192
ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
Atmel-8271H-AVR- ATmega-Datasheet_08/2014
20.11 Register Description
20.11.1 UDRn – USART I/O Data Register n
The USART Transmit Data Buffer Register and USART Receive Data Buffer Registers share the same I/O
address referred to as USART Data Register or UDRn. The Transmit Data Buffer Register (TXB) will be the
destination for data written to the UDRn Register location. Reading the UDRn Register location will return the
contents of the Receive Data Buffer Register (RXB).
address referred to as USART Data Register or UDRn. The Transmit Data Buffer Register (TXB) will be the
destination for data written to the UDRn Register location. Reading the UDRn Register location will return the
contents of the Receive Data Buffer Register (RXB).
For 5-, 6-, or 7-bit characters the upper unused bits will be ignored by the Transmitter and set to zero by the
Receiver.
Receiver.
The transmit buffer can only be written when the UDREn Flag in the UCSRnA Register is set. Data written to
UDRn when the UDREn Flag is not set, will be ignored by the USART Transmitter. When data is written to the
transmit buffer, and the Transmitter is enabled, the Transmitter will load the data into the Transmit Shift Register
when the Shift Register is empty. Then the data will be serially transmitted on the TxDn pin.
UDRn when the UDREn Flag is not set, will be ignored by the USART Transmitter. When data is written to the
transmit buffer, and the Transmitter is enabled, the Transmitter will load the data into the Transmit Shift Register
when the Shift Register is empty. Then the data will be serially transmitted on the TxDn pin.
The receive buffer consists of a two level FIFO. The FIFO will change its state whenever the receive buffer is
accessed. Due to this behavior of the receive buffer, do not use Read-Modify-Write instructions (SBI and CBI)
on this location. Be careful when using bit test instructions (SBIC and SBIS), since these also will change the
state of the FIFO.
accessed. Due to this behavior of the receive buffer, do not use Read-Modify-Write instructions (SBI and CBI)
on this location. Be careful when using bit test instructions (SBIC and SBIS), since these also will change the
state of the FIFO.
20.11.2 UCSRnA – USART Control and Status Register n A
• 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).
(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).
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 UDRIEn bit). UDREn is set after a reset to indicate that the Transmitter is ready.
buffer is empty, and therefore ready to be written. The UDREn Flag can generate a Data Register Empty
interrupt (see description of the UDRIEn bit). UDREn is set after a reset to indicate that the Transmitter is ready.
Bit
7
6
5
4
3
2
1
0
RXB[7:0]
UDRn (Read)
TXB[7:0]
UDRn (Write)
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
RXCn
TXCn
UDREn
FEn
DORn
UPEn
U2Xn
MPCMn
UCSRnA
Read/Write
R
R/W
R
R
R
R
R/W
R/W
Initial Value
0
0
1
0
0
0
0
0