Datasheet

ATmega48/88/168 Automotive [DATASHEET]
7530K–AVR–07/14
152
More advanced initialization routines can be made that include frame format as parameters, disable interrupts and so on.
However, many applications use a fixed setting of the baud and control registers, and for these types of applications the
initialization code can be placed directly in the main routine, or be combined with initialization code for other I/O modules.
17.5 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.
17.5.1 Sending Frames with 5 to 8 Data Bit
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, U2Xn 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 (UDREn)
flag. When using frames with less than eight bits, the most significant bits written to the UDRn 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. The example code assumes that the part specific header file is included.
For I/O registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI” instructions must
be replaced with instructions that allow access to extended I/O. Typically “LDS” and “STS” combined with
“SBRS”, “SBRC”, “SBR”, and “CBR”.
The function simply waits for the transmit buffer to be empty by checking the UDREn 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.
Assembly Code Example
(1)
USART_Transmit:
; Wait for empty transmit buffer
sbis UCSRnA,UDREn
rjmp USART_Transmit
; Put data (r16) into buffer, sends the data
out UDRn,r16
ret
C Code Example
(1)
void USART_Transmit( unsigned char data )
{
/* Wait for empty transmit buffer */
while ( !( UCSRnA & (1<<UDREn)) )
;
/* Put data into buffer, sends the data */
UDRn = data;
}