Datasheet

173
ATmega169A/PA/329A/PA/649A/P/3290A/PA/6490A/P [DATASHEET]
8284E–AVR–02/2013
munication line can be set to an idle (high) state. Figure 20-4 illustrates the possible combinations of the frame
formats. Bits inside brackets are optional.
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 UCSZn2:0, UPMn1:0 and USBSn bits in UCSRnB and UCS-
RnC. 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 (UCSZn2:0) bits select the number of data bits in the frame. The USART Parity mode
(UPMn1:0) 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 (USBSn) bit. The Receiver ignores the second stop bit. An FEn (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 TXCn Flag can be used to check that the Transmitter has
completed all transfers, and the RXCn Flag can be used to check that there are no unread data in the receive buf-
fer. Note that the TXCn Flag must be cleared before each transmission (before UDRn is written) if it is used for this
purpose.
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
=
=