ATmega165 datasheet(152/336 Pages) ATMEL

152 / 336 page

152

ATmega165/V

2573G–AVR–07/09

Synchronous Clock Operation

When synchronous mode is used (UMSEL = 1), the XCK pin will be used as either clock

input (Slave) or clock output (Master). The dependency between the clock edges and

data sampling or data change is the same. The basic principle is that data input (on

RxD) is sampled at the opposite XCK clock edge of the edge the data output (TxD) is

changed.

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.

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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