ATmega48/88/168
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 19-4 illustrates the possible combinations of the frame formats. Bits inside brackets are
optional.
Figure 19-4. Frame Formats
FRAME
(IDLE)
St
0
1
2
3
4
[5]
[6]
[7]
[8]
[P] Sp1 [Sp2] (St / IDLE)
St
Start bit, always low.
Data bits (0 to 8).
(n)
P
Parity bit. Can be odd or even.
Stop bit, always high.
Sp
IDLE
must be
No transfers on the communication line (RxDn or TxDn). An IDLE line
high.
The frame format used by the USART is set by the UCSZn2:0, UPMn1:0 and USBSn bits in
UCSRnB and UCSRnC. 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.
The USART Character SiZe (UCSZn2:0) bits select the number of data bits in the frame. The
USART Parity mode (UPMn1:0) bits enable and set the type of parity bit. The selection between
one or two stop bits is done by the USART Stop Bit Select (USBSn) bit. The Receiver ignores
the second stop bit. An FE (Frame Error) will therefore only be detected in the cases where the
first stop bit is zero.
19.4.1
Parity Bit Calculation
The parity bit is calculated by doing an exclusive-or of all the data bits. If odd parity is used, the
result of the exclusive or is inverted. The relation between the parity bit and data bits is as
follows:
P
P
= d
= d
⊕ … ⊕ d ⊕ d ⊕ d ⊕ d ⊕ 0
3 2 1 0
even
n – 1
n – 1
⊕ … ⊕ d ⊕ d ⊕ d ⊕ d ⊕ 1
odd
3 2 1 0
Peven
Podd
dn
Parity bit using even parity
Parity bit using odd parity
Data bit n of the character
If used, the parity bit is located between the last data bit and first stop bit of a serial frame.
177
2545M–AVR–09/07
ATMEL [ ATMEL ]
