Registers of PIC18F

Registers of PIC18F

• The memory of the PIC is divided into a series of registers. Each of the registers has its own address and memory locations.

• According to the type of working and usage, the registers in PIC are classified as :

■ Special Function Registers (SFRS): Used for control and status of the controller and peripheral functions

■ General Purpose Registers (GPRS): Used for data storage and scratchpad operations in the user's application.

■ WREG working register (acts as an Accumulator) - Used to perform arithmetic or logical functions.

■ Status register that stores flags - Indicates the status of the operation done by ALU.

■ Registers - hold memory address

• Bank Select Register (BSR): 4-bit register used in direct addressing the data memory.

• File Select Registers (FSRs).: 16-bit registers used as memory pointers in indirect addressing data memory.

• Program Counter (PC): 21-bit register that holds the program memory address while executing programs. This means that the PIC18 family can access program addresses 000000 to 1FFFFFH, a total of 2M bytes of code.

• Stack Pointer (SP): PIC18 has a 5-bit stack pointer. It is used to access the stack.

1. Working Register (WREG)

• It is 8-bits wide.

• It is used for ALU operations.

• The WREG register is the same as the accumulator in other microprocessors.

• The contents of WREG register are used for all arithmetic and logic one / two- operand instructions.

• It is not an addressable register.

2. PIC18 Status Register

• The status register is an 8-bit register. It is also referred to as flag register.

• The status register contains the arithmetic status of the ALU. It contains five flag bits Z, DC, C, OV and. These bits are set or cleared according to the result of arithmetic or logical instructions.

• Remaining three bits: 5, 6, and 7 are unimplemented and read as 0.

• CLRF STATUS instruction clears the upper three bits and set the Z bit. This leaves the Status register as 000u uluu (where u = Unchanged).

• Only BCF, BSF, SWAPF, MOVFF and MOVWF instructions are used to alter the status register, because these instructions do not affect the Z, C, DC, OV or N bits from the status register.

• Carry (C) flag: This flag is set if there is an overflow out of bit 7. The carry flag also serves as a borrow flag for subtraction.

• Digital Carry (DC) flag: This flag is set if there is an overflow out of bit 3 i.e. carry from lower nibble to higher nibble (bit 3 to bit 4). This flag is used for BCD operations and it is not, available for the programmer. It is also known as Auxiliary Carry flag.

• Zero (Z) flag: The zero flag sets if the result of operation in ALU is zero and flag resets if result is non zero. The zero flag is also set if a certain register content becomes zero following an increment or decrement operation of that register.

• Over (OV) flag: This flag is set whenever the result of a signed number operation is too large, causing the high-order bit to overflow into the sign bit.

• Negative (N) flag: After the execution of arithmetic or logical operations, if bit7 of the result is 1, the N flag is set indicating result is negative. If bit7 is 0, the number will be considered as positive number.

3. Special Function Registers

• The Special Function Registers (SFRs) are registers used by the CPU and peripheral modules for controlling the desired operation of the device. These registers are implemented as static RAM.

• Table 18.5.1 shows the list of SFRS. The SFRS are classified into two sets:

■ Registers associated with the core function and

■ Registers associated with peripheral functions.

• Here, we will discuss the registers related to the core functions, while those related to the peripheral functions are discussed in the respective sections.

1. This is not a physical register.

2. These registers are not implemented on the PIC18F248 and PIC18F258. Table 18.5.1 Special Function Register map for PIC18FXX8 family

Review Questions

1. Write a note on PIC18 registers.

2. What is the function of WREG register in PIC microcontroller?

3. Explain the status register of PIC18.

4. What are SFRS ?