User guide

185
ATmega162/V
2513E–AVR–09/03
USART Register
Description
USART I/O Data Register –
UDR
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 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.
USART Control and Status
Register A – UCSRA
• Bit 7 – RXC: 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 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 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 Transmit
Complete interrupt (see description of the TXCIE bit).
• Bit 5 – UDRE: USART Data Register Empty
The UDRE Flag indicates if the transmit buffer (UDR) is ready to receive new data. If
UDRE is one, the buffer is empty, and therefore ready to be written. The UDRE Flag can
generate a Data Register Empty interrupt (see description of the UDRIE bit).
Bit 76543210
RXB[7:0] UDR (Read)
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
Bit 76543210
RXC TXC UDRE FE DOR UPE U2X MPCM UCSRA
Read/Write R R/W R R R R R/W R/W
Initial Value00100000