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ATmega128
Data Transmission The USART Transmitter is enabled by setting the Transmit Enable (TXEN) bit in the UCSRB
– The USART
Transmitter
Sending Frames with
5 to 8 Data Bit
Register. W hen the Transmitter is enabled, the normal port operation of the TxD pin is overrid-
den 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 synchro-
nous operation is used, the clock on the XCK pin will be overridden and used as transmission
clock.
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 UDR 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. W hen 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 XCK 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. W hen using frames with less than eight bits, the most signifi-
cant bits written to the UDR are ignored. The USART 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 R16
Assembly Code Example (1)
USART_Transmit:
; Wait for empty transmit buffer
sbis UCSRA,UDRE
rjmp USART_Transmit
; Put data (r16) into buffer, sends the data
out
UDR,r16
ret
C Code Example (1)
void USART_Transmit( unsigned char data )
{
/* Wait for empty transmit buffer */
while ( !( UCSRA & (1<<UDRE)) )
;
/* Put data into buffer, sends the data */
UDR = data;
}
Note:
1. See “About Code Examples” on page 8..
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 inter-
rupt routine writes the data into the buffer.
177
2467X–AVR–06/11
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