ATMEGA323L_14 Datasheet, PDF (94/247 Pages) ATMEL Corporation – Non-volatile Program and Data Memories

USART Register
Description
USART I/O Data Register –
UDR
USART Control and Status
Register A – UCSRA
Bit
7
6
5
4
3
2
1
0
$0C ($2C) Read
RXB[7:0]
UDR (Read)
$0C ($2C) Write
TXB[7:0]
UDR (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
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 UDR. The Transmit
Data Buffer Register (TXB) will be the destination for data written to the UDR Register
location. Reading the UDR 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.
The transmit buffer can only be written when the UDRE Flag in the UCSRA Register is
set. Data written to UDR when the UDRE 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 TxD pin.
The Receive Buffer consists of a two level FIFO. The FIFO will change its state when-
ever 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.
Bit
7
6
5
4
3
2
1
0
$0B ($2B)
RXC
TXC
UDRE
FE
DOR
PE
U2X
MPCM UCSRA
Read/Write
R
R/W
R
R
R
R
R/W
R/W
Initial Value
0
0
1
0
0
0
0
0
• Bit 7 – RXC: USART Receive Complete
This flag bit is one when there are unread data in the receive buffer and zero when the
receive buffer is empty (i.e., does not contain any unread data). If the Receiver is dis-
abled, the receive buffer will be flushed and consequently the RXC bit will become zero.
The RXC Flag can be used to generate a Receive Complete interrupt (see description of
the RXCIE bit).
• Bit 6 – TXC: USART Transmit Complete
This flag bit is set one 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 (UDR). The
TXC 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 TXC Flag can generate a Trans-
mit Complete interrupt (see description of the TXCIE bit).
94 ATmega323(L)
1457G–AVR–09/03