ATmega16 datasheet(141/315 Pages) ATMEL

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141

ATmega16(L)

2466E–AVR–10/02

Figure 71. Synchronous Mode XCK Timing.

The UCPOL bit UCRSC selects which XCK clock edge is used for data sampling and

which is used for data change. As Figure 71 shows, when UCPOL is zero the data will

be changed at rising XCK edge and sampled at falling XCK edge. If UCPOL is set, the

data will be changed at falling XCK edge and sampled at rising XCK edge.

Frame Formats

A serial frame is defined to be one character of data bits with synchronization bits (start

and stop bits), and optionally a parity bit for error checking. The USART accepts all 30

combinations of the following as valid frame formats:

•

1 start bit

•

5, 6, 7, 8, or 9 data bits

•

no, even or odd parity bit

•

1 or 2 stop bits

A frame starts with the start bit followed by the least significant data bit. Then the next

data bits, up to a total of nine, are succeeding, ending with the most significant bit. If

enabled, the parity bit is inserted after the data bits, before the stop bits. When a com-

plete frame is transmitted, it can be directly followed by a new frame, or the

communication line can be set to an idle (high) state. Figure 72 illustrates the possible

combinations of the frame formats. Bits inside brackets are optional.

Figure 72. 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 be

high.

The frame format used by the USART is set by the UCSZ2:0, UPM1:0, and USBS bits in

UCSRB and UCSRC. 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.

RxD / TxD

XCK

RxD / TxD

XCK

UCPOL = 0

UCPOL = 1

Sample

[5]

[6]

[7]

[8]

[P]

Sp1 [Sp2]

(St / IDLE)

(IDLE)

FRAME

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