ATmega8(L)
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 63. Synchronous Mode XCK Timing
UCPOL = 1
XCK
RxD / TxD
Sample
Sample
UCPOL = 0
XCK
RxD / TxD
The UCPOL bit UCRSC selects which XCK clock edge is used for data sampling and which is
used for data change. As Figure 63 shows, when UCPOL is zero the data will be changed at ris-
ing 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 complete 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 64 illustrates the possible combinations of the frame formats. Bits inside brackets are
optional.
Figure 64. Frame Formats
FRAME
(IDLE)
St
0
1
2
3
4
[5]
[6]
[7]
[8]
[P] Sp1 [Sp2] (St / IDLE)
St
(n)
P
Start bit, always low.
Data bits (0 to 8).
Parity bit. Can be odd or even.
Stop bit, always high.
Sp
137
2486T–AVR–05/08
ATMEL [ ATMEL ]
