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sbit

The sbit type defines a bit within a special function register (SFR). It is used in one of the following ways:

sbit name = sfr-name ^ bit-position;
sbit name = sfr-address ^ bit-position;
sbit name = sbit-address;

Where

name is the name of the SFR bit.
sfr-name is the name of a previously-defined SFR.
bit-position is the position of the bit within the SFR.
sfr-address is the address of an SFR.
sbit-address is the address of the SFR bit.

With typical 8051 applications, it is often necessary to access individual bits within an SFR. The sbit type provides access to bit-addressable SFRs and other bit-addressable objects. For example:

sbit EA = 0xAF;

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

Note

  • Storage of objects accessed using sbit is assumed to be little endian (LSB first). This is the storage format of the sfr16 type but it is opposite to the storage of int and long data types. Care must be taken when using sbit to access bits within standard data types.

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 a number 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 a number 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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