Datasheet

142
ATmega32A [DATASHEET]
Atmel-8155D-AVR-ATmega32A-Datasheet_02/2014
Note: 1. See “About Code Examples” on page 6.
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.
20.6 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.
20.6.1 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 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. When
the Shift Register is loaded with new data, it will transfer one complete frame at the rate given by the Baud Regis-
ter, 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.
Assembly Code Example
(1)
USART_Init:
; Set baud rate
out UBRRH, r17
out UBRRL, r16
; Enable receiver and transmitter
ldi r16, (1<<RXEN)|(1<<TXEN)
out UCSRB,r16
; Set frame format: 8data, 2stop bit
ldi r16, (1<<URSEL)|(1<<USBS)|(3<<UCSZ0)
out UCSRC,r16
ret
C Code Example
(1)
void USART_Init( unsigned int baud )
{
/* Set baud rate */
UBRRH = (unsigned char)(baud>>8);
UBRRL = (unsigned char)baud;
/* Enable receiver and transmitter */
UCSRB = (1<<RXEN)|(1<<TXEN);
/* Set frame format: 8data, 2stop bit */
UCSRC = (1<<URSEL)|(1<<USBS)|(3<<UCSZ0);
}