Datasheet

130
ATmega8A [DATASHEET]
8159E–AVR–02/2013
Figure 20-4. Frame Formats
St Start bit, always low.
(n) Data bits (0 to 8).
P Parity bit. Can be odd or even.
Sp Stop bit, always high.
IDLE No transfers on the communication line (RxD or TxD). An IDLE line must
be high.
The frame format used by the USART is set by the UCSZ2:0, UPM1:0 and USBS bits in UCSRB and UCSRC. The
Receiver and Transmitter use the same setting. Note that changing the setting of any of these bits will corrupt all
ongoing communication for both the Receiver and Transmitter.
The USART Character SiZe (UCSZ2:0) bits select the number of data bits in the frame. The USART Parity mode
(UPM1:0) bits enable and set the type of parity bit. The selection between one or two stop bits is done by the
USART Stop Bit Select (USBS) bit. The Receiver ignores the second stop bit. An FE (Frame Error) will therefore
only be detected in the cases where the first stop bit is zero.
20.4.1 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 exclu-
sive 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.
20.5 USART Initialization
The USART has to be initialized before any communication can take place. The initialization process normally con-
sists 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 transmis-
sions during the period the registers are changed. The TXC 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 TXC Flag must be cleared before each transmission (before UDR 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
10 2 3 4 [5] [6] [7] [8] [P]St Sp1 [Sp2] (St / IDLE)(IDLE)
FRAME
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
=
=