qingfengke 发表于 2007-1-29 17:15:29

atmega16的ucsrc的设置

马潮老师,我是avr单片机新手,发现atmega16的mcsrc无法设置,请问是怎么回事

machao 发表于 2007-3-9 22:07:43

见器件手册:



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 Register 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.



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);

   ...



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. However, 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, reading 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.



The following code example shows how to read the UCSRC Register contents.



Assembly Code Example(1)



USART_ReadUCSRC:

; Read UCSRC

   in r16,UBRRH

   in r16,UCSRC

   ret



C Code Example(1)

unsigned char USART_ReadUCSRC( void )

{

   unsigned char ucsrc;

/* Read UCSRC */

   ucsrc = UBRRH;

   ucsrc = UCSRC;

   return ucsrc;

}



The assembly code example returns the UCSRC value in r16.



Reading the UBRRH contents is not an atomic operation and therefore it can be read as an ordinary register, as long as the previous instruction did not access the register location.
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