Datasheet

151
ATmega48/88/168 Automotive [DATASHEET]
7530K–AVR–07/14
17.3.1 Parity Bit Calculation
The parity bit is calculated by doing an exclusive-or of all the data bits. If odd parity is used, the result of the exclusive or is
inverted. The relation between the parity bit and data bits is as follows:
P
even
Parity bit using even parity
P
odd
Parity bit using odd parity
d
n
Data bit n of the character
If used, the parity bit is located between the last data bit and first stop bit of a serial frame.
17.4 USART Initialization
The USART has to be initialized before any communication can take place. The initialization process normally consists of
setting the baud rate, setting frame format and enabling the transmitter or the receiver depending on the usage. For interrupt
driven USART operation, the global interrupt flag should be cleared (and interrupts globally disabled) when doing the
initialization.
Before doing a re-initialization with changed baud rate or frame format, be sure that there are no ongoing transmissions
during the period the registers are changed. The TXCn flag can be used to check that the transmitter has completed all
transfers, and the RXC flag can be used to check that there are no unread data in the receive buffer. Note that the TXCn flag
must be cleared before each transmission (before UDRn is written) if it is used for this purpose.
The following simple USART initialization code examples show one assembly and one C function that are equal in
functionality. The examples assume asynchronous operation using polling (no interrupts enabled) and a fixed frame format.
The baud rate is given as a function parameter. For the assembly code, the baud rate parameter is assumed to be stored in
the r17:r16 registers.
Note: 1. 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”.
P
even
d
n 1–
… d
3
d
2
d
1
d
0
0
P
odd
⊕⊕⊕⊕⊕⊕
d
n 1–
… d
3
d
2
d
1
d
0
1⊕⊕⊕⊕⊕⊕
=
=
Assembly Code Example
(1)
USART_Init:
; Set baud rate
out UBRRnH, r17
out UBRRnL, r16
; Enable receiver and transmitter
ldi r16, (1<<RXENn)|(1<<TXENn)
out UCSRnB,r16
; Set frame format: 8data, 2stop bit
ldi r16, (1<<USBSn)|(3<<UCSZn0)
out UCSRnC,r16
ret
C Code Example
(1)
void USART_Init( unsigned int baud )
{
/* Set baud rate */
UBRRnH = (unsigned char)(baud>>8);
UBRRnL = (unsigned char)baud;
/* Enable receiver and transmitter */
UCSRnB = (1<<RXENn)|(1<<TXENn);
/* Set frame format: 8data, 2stop bit */
UCSRnC = (1<<USBSn)|(3<<UCSZn0);
}