Datasheet

144
ATmega32A [DATASHEET]
Atmel-8155D-AVR-ATmega32A-Datasheet_02/2014
Note: 1. These transmit functions are written to be general functions. They can be optimized if the contents of the UCSRB is
static (that is, only the TXB8 bit of the UCSRB Register is used after initialization).
The ninth bit can be used for indicating an address frame when using multi processor communication mode or for
other protocol handling as for example synchronization.
20.6.3 Transmitter Flags and Interrupts
The USART transmitter has two flags that indicate its state: USART Data Register Empty (UDRE) and Transmit
Complete (TXC). Both flags can be used for generating interrupts.
The Data Register Empty (UDRE) Flag indicates whether the transmit buffer is ready to receive new data. This bit
is set when the transmit buffer is empty, and cleared when the transmit buffer contains data to be transmitted that
has not yet been moved into the Shift Register. For compatibility with future devices, always write this bit to zero
when writing the UCSRA Register.
When the Data Register empty Interrupt Enable (UDRIE) bit in UCSRB is written to one, the USART Data Register
Empty Interrupt will be executed as long as UDRE is set (provided that global interrupts are enabled). UDRE is
cleared by writing UDR. When interrupt-driven data transmission is used, the Data Register Empty Interrupt routine
must either write new data to UDR in order to clear UDRE or disable the Data Register empty Interrupt, otherwise
a new interrupt will occur once the interrupt routine terminates.
The Transmit Complete (TXC) Flag bit is set one when the entire frame in the transmit Shift Register has been
shifted out and there are no new data currently present in the transmit buffer. The TXC Flag bit is automatically
cleared when a transmit complete interrupt is executed, or it can be cleared by writing a one to its bit location. The
Assembly Code Example
(1)
USART_Transmit:
; Wait for empty transmit buffer
sbis UCSRA,UDRE
rjmp USART_Transmit
; Copy 9th bit from r17 to TXB8
cbi UCSRB,TXB8
sbrc r17,0
sbi UCSRB,TXB8
; Put LSB data (r16) into buffer, sends the data
out UDR,r16
ret
C Code Example
(1)
void USART_Transmit( unsigned int data )
{
/* Wait for empty transmit buffer */
while ( !( UCSRA & (1<<UDRE))) )
;
/* Copy 9th bit to TXB8 */
UCSRB &= ~(1<<TXB8);
if ( data & 0x0100 )
UCSRB |= (1<<TXB8);
/* Put data into buffer, sends the data */
UDR = data;
}