Atmel ATmega8535, User Manual

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152
ATmega8535(L)
2502K–AVR–10/06
Data Transmission – The
USART Transmitter
The USART Transmitter is enabled by setting the Transmit Enable (TXEN) bit in the
UCSRB Register. When the Transmitter is enabled, the normal port operation of the
TxD 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 XCK pin will
be overridden and used as transmission clock.
Sending Frames with 5 to 8
Data Bits A data transmission is initiated by loading the transmit buffer with the data to be trans-
mitted. 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 previ-
ous 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 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. When using frames with less than eight bits,
the most significant 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.
Note: 1. See “About Code Examples” on page 7.
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 inter-
rupt is utilized, the interrupt routine writes the data into the buffer.
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;
}