File Register, Access Bank and Bank Switching in PIC18

File Register, Access Bank and Bank Switching in PIC18

1. PIC File Register

• In PIC18 microcontrollers, the data (Read/Write) memory is implemented as static RAM. It is also known as File Register. Like WREG we can perform arithmetic and logic operations on many locations of the file register data RAM.

• PIC microcontroller's file register size ranges from 32 bytes to several thousands of bytes depending on the chip.

• Fig. 2.7.1 shows the file register organization for the PIC18FXX8 devices. It is 4096 . bytes of data memory with 12-bit address for each register.

• The data memory map is divided into as many as 16 banks that contain 256 bytes each. The lower 4 bits of the Bank Select Register (BSR) select which bank will be accessed.

• When using direct addressing, the Bank Select Register (BSR) is used to select the desired bank.

• BSR<3:0> holds the upper 4 bits of the 12-bit RAM address. These 4 bits are used to select the desired bank. The BSR<7:4> bits will always read '0's and writes will have no effect.

• The file register data memory of PIC is divided into two sections :

■ 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.

• The SFRS start at the last location of Bank 15 (FFFh) and grow downwards. GPRS start at the first location of Bank 0 and grow upwards.

• The entire data memory may be accessed directly or indirectly. Direct addressing may require the use of the BSR register. Indirect addressing requires the use of the File Select Register (FSR). Each FSR holds a 12-bit address value that can be used to access any location in the data memory map without banking. Fig 18.7.2 demonstrates direct and indirect addressing methods.

• File Select Registers: FSRO, FSR1 and FSR2. These registers are composed of two 8-bit registers: FSRH and FSRL

2. Access Bank and Bank Switching

• The Access Bank is comprised of the upper 160 bytes in Bank 15 (SFRS) and the lower 96 bytes in Bank 0. These two sections will be referred to as Access Bank High and Access Bank Low, respectively.

• All the instructions without 'A' letter in instructions assume the access bank as a default bank. Here, A = 0. When 'A' letter is included in the instruction, A is 1, the instruction use bank select register to select the bank.

• Upon reset BSR = 0000H and by default Bank0 is selected. We can load 0001H in- BSR to select Bank1, 0002H in BSR to select Bank2 and so on.

Review Questions

1. What is File Register in PIC18?

2. Explain the File Register data memory of PIC18F458.

3. Explain programmable memory organization of PIC18FXXX microcontroller.

4. What is access bank?

5. Explain the direct and indirect addressing methods for accessing data memory.

6. What is BSR? Explain