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.
The USART Character SiZe (UCSZ2:0) bits select the number of data bits in the frame. The
USART Parity mode (UPM1:0) 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 (USBS) 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.
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
⊕ … ⊕ d ⊕ d ⊕ d ⊕ d ⊕ 1
3 2 1 0
even
n – 1
n – 1
3 2 1 0
odd
Peven Parity bit using even parity.
Podd
dn
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.
USART
Initialization
The USART has to be initialized before any communication can take place. The initialization pro-
cess normally consists of setting the baud rate, setting frame format and enabling the
Transmitter or the Receiver depending on the usage. For interrupt driven USART operation, the
Global Interrupt Flag should be cleared (and interrupts globally disabled) when doing the
initialization.
Before doing a re-initialization with changed baud rate or frame format, be sure that there are no
ongoing transmissions during the period the registers are changed. The TXC Flag can be used
to check that the Transmitter has completed all transfers, and the RXC Flag can be used to
check that there are no unread data in the receive buffer. Note that the TXC Flag must be
cleared before each transmission (before UDR is written) if it is used for this purpose.
The following simple USART initialization code examples show one assembly and one C func-
tion that are equal in functionality. The examples assume asynchronous operation using polling
(no interrupts enabled) and a fixed frame format. The baud rate is given as a function parameter.
For the assembly code, the baud rate parameter is assumed to be stored in the r17:r16 Regis-
ters. When the function writes to the UCSRC Register, the URSEL bit (MSB) must be set due to
the sharing of I/O location by UBRRH and UCSRC.
138
ATmega8(L)
2486T–AVR–05/08
ATMEL [ ATMEL ]
