Datasheet

code can be placed directly in the main routine, or be combined with initialization code for
other I/O modules.
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About Code Examples on page 18
21.7. Data Transmission – The USART Transmitter
The USART Transmitter is enabled by setting the Transmit Enable (TXEN) bit in the UCSRnB Register.
When the Transmitter is enabled, the normal port operation of the TxDn pin is overridden by the USART
and given the function as the Transmitter’s serial output. The baud rate, mode of operation and frame
format must be set up once before doing any transmissions. If synchronous operation is used, the clock
on the XCKn pin will be overridden and used as transmission clock.
21.7.1. Sending Frames with 5 to 8 Data Bits
A data transmission is initiated by loading the transmit buffer with the data to be transmitted. The CPU
can load the transmit buffer by writing to the UDRn I/O location. The buffered data in the transmit buffer
will be moved to the Shift Register when the Shift Register is ready to send a new frame. The Shift
Register is loaded with new data if it is in idle state (no ongoing transmission) or immediately after the last
stop bit of the previous frame is transmitted. When the Shift Register is loaded with new data, it will
transfer one complete frame at the rate given by the Baud Register, U2X bit or by XCKn depending on
mode of operation.
The following code examples show a simple USART transmit function based on polling of
the Data Register Empty (UDRE) Flag. When using frames with less than eight bits, the
most significant bits written to the UDR0 are ignored. The USART 0 has to be initialized
before the function can be used. For the assembly code, the data to be sent is assumed
to be stored in Register R17.
Assembly Code Example
USART_Transmit:
; Wait for empty transmit buffer
in r17, UCSR0A
sbrs r17, UDRE
rjmp USART_Transmit
; Put data (r16) into buffer, sends the data
out UDR0,r16
ret
C Code Example
void USART_Transmit( unsigned char data )
{
/* Wait for empty transmit buffer */
while ( !( UCSR0A & (1<<UDRE)) )
;
/* Put data into buffer, sends the data */
UDR0 = data;
}
The function simply waits for the transmit buffer to be empty by checking the UDRE Flag,
before loading it with new data to be transmitted. If the Data Register Empty interrupt is
utilized, the interrupt routine writes the data into the buffer.
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About Code Examples on page 18
Atmel ATmega644A [DATASHEET]
Atmel-42716C-ATmega644A_Datasheet_Complete-10/2016
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