ATmega169P datasheet(174/393 Pages) ATMEL

174 / 393 page

174

8018N–AVR–08/09

ATmega169P

Figure 19-4. 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 (RxD or TxD). An IDLE line must

high.

The frame format used by the USART is set by the UCSZn2:0, UPM1n: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 (UPM1n: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 FEn) will therefore only be detected in the cases

where the first stop bit is zero.

19.4.1

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 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.

[5]

[6]

[7]

[8]

[P]

Sp1 [Sp2]

(St / IDLE)

(IDLE)

FRAME

Peven

dn 1

–

…

Podd

⊕

⊕⊕⊕⊕⊕

dn 1

–

…

⊕

⊕⊕⊕⊕⊕

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