ATmega88PA Automotive Atmel Corporation, ATmega88PA Automotive Datasheet - Page 181

• Manufacturer Part Number: ATmega88PA Automotive
• Manufacturer: Atmel Corporation

20.4.1
20.5
9223B–AVR–09/11
USART Initialization
Parity Bit Calculation
St
(n)
P
Sp
IDLE
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 chang-
ing 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.
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
P
d
If used, the parity bit is located between the last data bit and first stop bit of a serial frame.
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 func-
tion 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.
P
P
odd
n
even
even
odd
Atmel ATmega48PA/88PA/168PA [Preliminary]
=
=
d
d
n 1
n 1
–
–
Start bit, always low.
Data bits (0 to 8).
Parity bit. Can be odd or even.
Stop bit, always high.
No transfers on the communication line (RxDn or TxDn). An IDLE line must be
high.
Parity bit using even parity
Parity bit using odd parity
Data bit n of the character
d
d
3
3
d
d
2
2
d
d
1
1
d
d
0
0
0
1
181