ATmega644PA datasheet(178/468 Pages) ATMEL

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178

8152G–AVR–11/09

ATmega164PA/324PA/644PA/1284P

Figure 18-4. Frame Formats

Start bit, always low.

(n)

Data bits (0 to 8).

Parity bit. Can be odd or even.

Stop bit, always high.

IDLE

No 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.

18.5.1

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::

even

Parity bit using even parity

odd

Parity bit using odd parity

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.

18.6

USART Initialization

The USART has to be initialized before any communication can take place. The initialization pro-

cess 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

[5]

[6]

[7]

[8]

[P]

Sp1 [Sp2]

(St / IDLE)

(IDLE)

FRAME

even

–

…

odd

⊕

⊕⊕⊕⊕⊕

–

…

⊕

⊕⊕⊕⊕⊕

ATmega644PA Datasheet(PDF) 178 Page - ATMEL Corporation

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