User Manual

179
ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
8271E–AVR–07/2012
20.6 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 func-
tion 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.
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 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. See ”About Code Examples” on page 7.
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.
20.6.2 Sending Frames with 9 Data Bit
If 9-bit characters are used (UCSZn = 7), the ninth bit must be written to the TXB8 bit in UCSRnB before the low
byte of the character is written to UDRn. The following code examples show a transmit function that handles 9-bit
characters. For the assembly code, the data to be sent is assumed to be stored in registers R17:R16.
Assembly Code Example
(1)
USART_Transmit:
; Wait for empty transmit buffer
in r16, UCSRnA
sbrs r16, 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;
}