AT90CAN128
Sending Frames with 9 Data
Bit
If 9-bit characters are used (UCSZn = 7), the ninth bit must be written to the TXB8n 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)(2)
USART0_Transmit:
; Wait for empty transmit buffer
lds
r18, UCSR0A
sbrs r18, UDRE0
rjmp USART0_Transmit
; Copy 9th bit from r17-bit0 to TXB80 via T-bit of SREG
lds
bst
bld
sts
r18, UCSR0B
r17, 0
r18, TXB80
UCSR0B, r18
; Put LSB data (r16) into buffer, sends the data
sts
ret
UDR0, r16
C Code Example(1)(2)
void USART0_Transmit (unsigned int data )
{
/* Wait for empty transmit buffer */
while ( !( UCSR0A & (1<<UDRE0)))
;
/* Copy 9th bit to TXB8 */
UCSR0B &= ~(1<<TXB80);
if ( data & 0x0100 )
UCSR0B |= (1<<TXB80);
/* Put data into buffer, sends the data */
UDR0 = data;
}
Notes: 1. These transmit functions are written to be general functions. They can be optimized if
the contents of the UCSR0B 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.
sparc.
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.
Transmitter Flags and
Interrupts
The USARTn Transmitter has two flags that indicate its state: USART Data Register
Empty (UDREn) and Transmit Complete (TXCn). Both flags can be used for generating
interrupts.
The Data Register Empty (UDREn) 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 UCSRnA Register.
179
4250E–CAN–12/04
ATMEL [ ATMEL ]
