c51中sbit/sfr 用法

　　1 首先區分bit 和sbit

　　bit和int char之類的差不多，只不過char=8位， bit=1位而已。都是變量，編譯器在編譯過程中分配地址。除非你指定，否則這個地址是隨機的。這個地址是整個可尋址空間，RAM+FLASH+擴展空間。bit只有0和1兩種值，意義有點像Windows下VC中的BOOL。

　　sbit是對應可位尋址空間的一個位，可位尋址區：20H～2FH。一旦用了sbi xxx = REGE^6這樣的定義，這個sbit量就確定地址了。sbit大部分是用在寄存器中的，方便對寄存器的某位進行操作的。

　　sbit的用法有三種：

　　第一種方法：sbit 位變量名＝地址值

　　第二種方法：sbit 位變量名＝SFR名稱^變量位地址值

　　第三種方法：sbit 位變量名＝SFR地址值^變量位地址值

　　如定義PSW中的OV可以用以下三種方法：

　　sbit OV=0xd2 （1）說明：0xd2是OV的位地址值

　　sbit OV=PSW^2 （2）說明：其中PSW必須先用sfr定義好

　　sbit OV=0xD0^2 （3）說明：0xD0就是PSW的地址值

　　因此這里用sfr P1_0=P1^0;就是定義用符號P1_0來表示P1.0引腳，如果你愿意也可以起P10一類的名字，只要下面程序中也隨之更改就行了。

　　注意：“^”實質代表的是異或運算，可以算一下，恰好符合。

　　2．Sfr用法

　　sfr 似乎不是標準C 語言的關鍵字，而是Keil 為能直接訪問80C51 中的SFR 而提供了一個新的關鍵詞，其用法是：sfrt 變量名=地址值。

　　例：sfr P1 = 0x90;

　　這樣的一行即定義P1 與地址0x90 對應，P1 口的地址就是0x90.

　　SFR的定義在頭文件reg51.h或reg52.h中。

　　［Keil 幫助］

　　sfr Types

　　SFRs are declared in the same fashion as other C variables. The ONLY difference is that the data type specified is sfr rather than CHAR or int. For example：

　　sfr P0 = 0x80; /* Port-0， address 80h */

　　sfr P1 = 0x90; /* Port-1， address 90h */

　　sfr P2 = 0xA0; /* Port-2， address 0A0h */

　　sfr P3 = 0xB0; /* Port-3， address 0B0h */

　　P0， P1， P2， and P3 are the SFR NAME declarations. Names for sfr variables are defined just like other C variable declarations. Any symbolic NAMEmay be used in an sfr declaration.

　　The address specification after the equal sign （‘=’） must be a numeric constant. Expressions with operators are not allowed. Classic 8051 devicesSUPPORT the SFR address range 0x80 to 0xFF. The PHILIPS 80C51MX provides an additional extended SFR space with the address range 0x180 to 0x1FF.

　　Note

　　sfr variables may not be declared inside a function. They must be declared outside of the function body.

　　COPYRIGHT （c） Keil SOFTWARE， Inc. and Keil Elektronik GmbH. All rights reserved.

　　sbit Types

　　With typical 8051 applications， it is often necessary to access individual bits within an SFR. The Cx51 Compiler makes this possible with the sbit data type which provides access to bit-addressable SFRs and other bit-addressable objects. For example：

　　sbit EA = 0xAF;

　　This declaration defines EA to be the SFR bit at address 0xAF. On the 8051， this is the enable all bit in the interrupt enable register.

　　Any symbolic NAME can be used in an sbit declaration. The expression to the right of the equal sign （‘=’） specifies an absolute bit address for the symbolic NAME. There are three variants for specifying the address：

　　Variant 1

　　sbit NAME = sfr-NAME ^ bit-position;

　　The previously declared SFR （sfr-NAME） is the base address for the sbit. It must be evenly divisible by 8. The bit-position （which must be aNUMBER from 0-7） follows the carat symbol （‘^’） and specifies the bit position to access. For example：

　　sfr PSW = 0xD0;

　　sfr IE = 0xA8;

　　sbit OV = PSW^2;

　　sbit CY = PSW^7;

　　sbit EA = IE^7;

　　Variant 2

　　sbit NAME = sfr-address ^ bit-position;

　　A CHARacter constant （sfr-address） specifies the base address for the sbit. It must be evenly divisible by 8. The bit-position （which must be aNUMBER from 0-7） follows the carat symbol （‘^’） and specifies the bit position to access. For example：

　　sbit OV = 0xD0^2;

　　sbit CY = 0xD0^7;

　　sbit EA = 0xA8^7;

　　Variant 3

　　sbit NAME = sbit-address;

　　A CHARacter constant （sbit-address） specifies the address of the sbit. It must be a VALUE from 0x80-0xFF. For example：

　　sbit OV = 0xD2;

　　sbit CY = 0xD7;

　　sbit EA = 0xAF;

　　Note

　　Not all SFRs are bit-addressable. ONLY those SFRs whose address is evenly divisible by 8 are bit-addressable. The lower nibble of the SFR‘s address must be 0 or 8. For example， SFRs at 0xA8 and 0xD0 are bit-addressable， whereas SFRs at 0xC7 and 0xEB are not. To calculate an SFR bit address， add the bit position to the SFR byte address. So， to access bit 6 in the SFR at 0xC8， the SFR bit address would be 0xCE （0xC8 + 6）。

　　Special function bits represent an independent declaration class that may not be interchangeable with other bit declarations or bit fields.

　　The sbit data type declaration may be used to access individual bits of variables declared with the bdata MEMORY type specifier. REFER to Bit-addressable Objects for more information.

　　sbit variables may not be declared inside a function. They must be declared outside of the function body.

　　COPYRIGHT （c） Keil SOFTWARE， Inc. and Keil Elektronik GmbH. All rights reserved.