Datasheet

ATmega164A/PA/324A/PA/644A/PA/1284/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002070A-page 180
Figure 19-4. Frame formats
StStart bit, always low.
(n)Data bits (0 to 8).
PParity bit. Can be odd or even.
SpStop bit, always high.
IDLENo transfers on the communication line (RxDn or TxDn). 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
UCSRnC. 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 FE (Frame Error) will
therefore only be detected in the cases where the first stop bit is zero.
19.5.1 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.
19.6 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 the
USART interrupts should be disabled.
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
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
=
=