ATmega88PA Automotive Atmel Corporation, ATmega88PA Automotive Datasheet - Page 185

• Manufacturer Part Number: ATmega88PA Automotive
• Manufacturer: Atmel Corporation

20.6.3
20.6.4
20.6.5
20.7
9223B–AVR–09/11
Data Reception – The USART Receiver
Transmitter Flags and Interrupts
Parity Generator
Disabling the Transmitter
The USART Transmitter has two flags that indicate its state: USART Data Register Empty
(UDREn) and Transmit Complete (TXCn). Both flags can be used for generating interrupts.
The Data Register Empty (UDREn) Flag indicates whether the transmit buffer is ready to
receive new data. This bit is set when the transmit buffer is empty, and cleared when the
transmit buffer contains data to be transmitted that has not yet been moved into the Shift Reg-
ister. For compatibility with future devices, always write this bit to zero when writing the
UCSRnA Register.
When the Data Register Empty Interrupt Enable (UDRIEn) bit in UCSRnB is written to one, the
USART Data Register Empty Interrupt will be executed as long as UDREn is set (provided that
global interrupts are enabled). UDREn is cleared by writing UDRn. When interrupt-driven data
transmission is used, the Data Register Empty interrupt routine must either write new data to
UDRn in order to clear UDREn or disable the Data Register Empty interrupt, otherwise a new
interrupt will occur once the interrupt routine terminates.
The Transmit Complete (TXCn) 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 buf-
fer. 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 is useful in half-duplex
communication interfaces (like the RS-485 standard), where a transmitting application must
enter receive mode and free the communication bus immediately after completing the
transmission.
When the Transmit Compete Interrupt Enable (TXCIEn) bit in UCSRnB is set, the USART
Transmit Complete Interrupt will be executed when the TXCn Flag becomes set (provided that
global interrupts are enabled). When the transmit complete interrupt is used, the interrupt han-
dling routine does not have to clear the TXCn Flag, this is done automatically when the
interrupt is executed.
The Parity Generator calculates the parity bit for the serial frame data. When parity bit is
enabled (UPMn1 = 1), the transmitter control logic inserts the parity bit between the last data
bit and the first stop bit of the frame that is sent.
The disabling of the Transmitter (setting the TXEN 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 Transmit-
ter will no longer override the TxDn pin.
The USART Receiver is enabled by writing the Receive Enable (RXENn) bit in the
UCSRnB Register to one. When the Receiver is enabled, the normal pin operation of the
RxDn pin is overridden by the USART and given the function as the Receiver’s serial input.
The baud rate, mode of operation and frame format must be set up once before any serial
reception can be done. If synchronous operation is used, the clock on the XCKn pin will be
used as transfer clock.
Atmel ATmega48PA/88PA/168PA [Preliminary]
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