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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. |
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