ATMEGA8_14 Datasheet, PDF (133/331 Pages) ATMEL Corporation – High-performance, Low-power Atmel

ATmega8(L)
Frame Formats
Figure 63. Synchronous Mode XCK Timing
UCPOL = 1 XCK
RxD / TxD
UCPOL = 0 XCK
Sample
RxD / TxD
Sample
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.
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
Start bit, always low
(n) Data bits (0 to 8)
P
Parity bit. Can be odd or even
Sp 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.
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