mega16串口通信 还是mega16的UCSRC 和 UBRRH的问题，总是同时读写，URSEL位竟然没用？！

suixinsuoyu

这是初始化，保证没有问题
void usart_init()
{
    UBRRH = 0x00;
    UBRRL = 0x08;
    UCSRA = 0x00;
    UCSRB = ((1<<RXCIE)|(1<<RXEN)|(1<<TXEN));
    UCSRC = 0x86;
}
这样走下来，用AVR studio仿真时发现UBRRH低位也被写成  0110  了？？

是不是还和其他的寄存器有关系？

suixinsuoyu

执行UCSRC=0x86这一步前 (原文件名:新建 BMP 图像.JPG)


执行UCSRC=0x86以后 (原文件名:新建 BMP 图像 (2).JPG)

suixinsuoyu

大家帮帮我吧，纠结两天了，UCSRC的SEL位怎么回事呀，怎么不管用呀。竟然用同一个地址真让人蛋疼

machao

器件手册中有详细说明，你应该先仔细看一下，养成习惯，先看手册。
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);
...

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. 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;
}
Note: 1. See “About Code Examples” on page 7.
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.

machao

实时仿真方式下读这个寄存器需要注意时间，否则是读不对的。

为什么要仿真？建议不要依赖仿真器调试。