ATmega16 datasheet(162/357 Pages) ATMEL

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2466S–AVR–05/09

ATmega16(L)

Accessing

UBRRH/ UCSRC

Registers

The UBRRH Register shares the same I/O location as the UCSRC Register. Therefore some

special consideration must be taken when accessing this I/O location.

Write Access

When doing a write access of this I/O location, the high bit of the value written, the USART Reg-

ister Select (URSEL) bit, controls which one of the two registers that will be written. If URSEL is

zero during a write operation, the UBRRH value will be updated. If URSEL is one, the UCSRC

setting will be updated.

The following code examples show how to access the two registers.

Note:

1. See “About Code Examples” on page 7.

As the code examples illustrate, write accesses of the two registers are relatively unaffected of

the sharing of I/O location.

Read Access

Doing a read access to the UBRRH or the UCSRC Register is a more complex operation. How-

ever, in most applications, it is rarely necessary to read any of these registers.

The read access is controlled by a timed sequence. Reading the I/O location once returns the

UBRRH Register contents. If the register location was read in previous system clock cycle, read-

ing the register in the current clock cycle will return the UCSRC contents. Note that the timed

sequence for reading the UCSRC is an atomic operation. Interrupts must therefore be controlled

(for example by disabling interrupts globally) during the read operation.

Assembly Code Example

(1)

...

; Set UBRRH to 2

ldi

r16,0x02

out

UBRRH,r16

...

; Set the USBS and the UCSZ1 bit to one, and

; the remaining bits to zero.

ldi

r16,(1<<URSEL)|(1<<USBS)|(1<<UCSZ1)

out

UCSRC,r16

...

C Code Example

(1)

...

/* Set UBRRH to 2 */

UBRRH = 0x02;

...

/* Set the USBS and the UCSZ1 bit to one, and */

/* the remaining bits to zero. */

UCSRC = (1<<URSEL)|(1<<USBS)|(1<<UCSZ1);

...

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