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AT90PWM216/316 [DATASHEET]
7710H–AVR–07/2013
principle is that data input (on RxD) is sampled at the opposite XCK clock edge of the edge the data output (TxDn)
is changed.
Figure 17-3. Synchronous Mode XCK Timing.
The UCPOL bit UCRSnC selects which XCK clock edge is used for data sampling and which is used for data
change. As
Figure 17-3 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.
17.4
Serial Frame
A serial frame is defined to be one character of data bits with synchronization bits (start and stop bits), and option-
ally a parity bit for error checking.
17.4.1
Frame Formats
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 complete frame is transmitted, it can be directly followed by a new frame, or the com-
munication line can be set to an idle (high) state.
Figure 17-4 illustrates the possible combinations of the frame
formats. Bits inside brackets are optional.
Figure 17-4. Frame Formats
St
Start bit, always low.
(n)
Data bits (0 to 8).
P
Parity bit. Can be odd or even.
Sp
Stop bit, always high.
RxDn / TxDn
XCKn
RxDn / TxDn
XCKn
UCPOLn = 0
UCPOLn = 1
Sample
1
0
2
3
4
[5]
[6]
[7]
[8]
[P]
St
Sp1 [Sp2]
(St / IDLE)
(IDLE)
FRAME