192
4317I–AVR–01/08
AT90PWM2/3/2B/3B
Notes:
1. These transmit functions are written to be general functions. They can be optimized if the con-
tents of the UCSRB is static. For example, only the TXB80 bit of the UCSRB0 Register is used
after initialization.
2. 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 ninth bit can be used for indicating an address frame when using multi processor communi-
cation mode or for other protocol handling as for example synchronization.
18.6.3
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 compat-
ibility 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
Assembly Code Example
(1)(2)
USART_Transmit:
;
Wait for empty transmit buffer
sbis
UCSRA,UDRE
rjmp
USART_Transmit
;
Copy 9th bit from r17 to TXB80
cbi
UCSRB,TXB80
sbrc
r17,0
sbi
UCSRB,TXB80
;
Put LSB data (r16) into buffer, sends the data
sts
UDR,r16
ret
C Code Example
(1)(2)
void
USART_Transmit(
unsigned int
data )
{
/*
Wait for empty transmit buffer
*/
while
( !( UCSRA & (1<<UDRE))) )
;
/*
Copy 9th bit to TXB8
*/
UCSRB &= ~(1<<TXB80);
if ( data & 0x0100 )
UCSRB |= (1<<TXB80);
/*
Put data into buffer, sends the data
*/
UDR = data;
}
