ATMEGA640 datasheet(205/436 Pages) MICROCHIP

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ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]

DS40002211A

The frame format used by the USART is set by the UCSZn2:0, UPMn1:0 and USBSn bits in UCSRnB and UCS-

RnC. 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.

22.4.1

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 exclu-

sive or is inverted. The parity bit is located between the last data bit and first stop bit of a serial frame. The relation

between the parity bit and data bits is as follows:

Peven

Parity bit using even parity.

Podd

Parity bit using odd parity.

Data bit n of the character.

22.5 USART Initialization

The USART has to be initialized before any communication can take place. The initialization process normally con-

sists 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 transmis-

sions during the period the registers are changed. The TXCn 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 TXCn Flag must be cleared before each transmission (before UDRn is written) if it is used for this

purpose.

The following simple USART initialization code examples show one assembly and one C function 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 Registers.

Peven

dn 1

–



d3 d2 d1 d0 0

Podd





dn 1

–



d3 d2 d1 d0 1





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