Embedded SystemSpring, 2011Lecture 4: The PIC MicrocontrollersEng. Wazen M. Shbair

2IUG- Embedded System

Today’s Lecture

The WREG Register The PIC File Register

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The WREG Register

Many registers for arithmetic and logic operation.

The WREG (WORking Register) Register is one of the most widely used registers of the PIC 8-bit register any data larger than 8 bits must be

broken into 8-bits chunks before it is processed. There is only one .

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D7 D6 D5 D2D4 D3 D1 D0

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MOVLW

Moves 8-bit data into WREG MOVLW k; move literal value k into WREG

Example MOVLW 25H MOVLW A5H

Is the following code correct? MOVLW 9H MOVLW A23H

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ADDLW

ADDLW k; Add literal value k to WREG (k +WREG)

Example: MOVLW 12H ADDLW 16H

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0 0 0 01 0 1 0

0 0 1 00 1 0 0

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PIC WREG and ALU Using Literal Value

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The WREG Register

When programming the WREG of PIC , the following points should be noted: Values can be loaded directly into the WREG. If values 0 to F are moved into an 8-bit register

such as WREG, the rest of the bits are assumed to be all zeros.

Moving a value larger than 255 (FF in hex) into the WREG register will truncate the upper byte and cause a warning in the .err file.

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The WREG Register

MOVLW 7F2H; Illegal , becomes F2H MOVLW 456H ; Illegal, becomes 56H MOVLW 60A5H; Illegal, becomes A5H

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The PIC File Register

It is the data memory. Read/Write Static RAM Used for data storage, scratch pad and

registers for internal use and function 8-bit width

Register File Concept

Data

Bus

Data

Bus

d

Decoded Instruction from Program Memory:

Arithmetic/Logic Function to be Performed Result

Destination

Address of Second Source Operand

Register File Concept: All of data memory is part of the register file, so any location in data memory may be operated on directly

All peripherals are mapped into data memory as a series of registers

Orthogonal Instruction Set: ALL instructions can operate on ANY data memory location

w f

w f

ALU

WREGWREG

Data Memory(Register File)

07h

08h

09h

0Ah

0Bh

0Ch

0Dh

0Eh

0Fh

10h

OpcodeOpcode dd aa AddressAddress

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Special Function Registers

Dedicated to specific functions such as ALU status, timers, serial communication, I/O ports, ADC,…

The function of each SFR is fixed by the CPU designer at the time of design it is used for control of the microcontroller or peripheral

8-bit registers Their numbers varies from one chip to another.

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General Purpose RAM

Group of RAM locations 8-bit registers Larger than SFR

Difficult to manage them by using Assembly language Easier to handle them by C Compiler.

The microchip website provides the data RAM size, which is the same as GPR size.

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Figure 2-2. File Registers of PIC12, PIC16, and PIC18

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GP RAM vs. EEPROM in PIC chips

GPRs are used by the CPU for internal data storage.

EEPROM are considered as add-on memory that one can add externally to the ship.

So PIC chip may have zero byte of EEPROM data memory, but impossible for a PIC have zero size for the file register.

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File Register and access bank in the PIC18

The PIC18 Family can have a max. of 4096 Bytes.

The File Register has addresses of 000- FFFH divided into 256-byte banks Max. 16 banks (How?)

At least there is one bank Known as default access bank.

Bank switching is a method used to access all the banks.

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Access bank in the PIC18

It is 256-Byte bank. Divided into equal two discontinuous

sections (each 128 B). GP RAM, from 0 to 7FH SFR, from F80H to FFFH

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SFRs of the PIC18 Family.

19IUG- Embedded System 19

References

Jie Hu , ECE692 Embedded Computing Systems , Fall 2010.

PIC Microcontroller And Embedded Systems: using Assembly and C for PIC 18, M. Mazidi, R. McKinlay and D. Causey, Prentice Fall, 2008.

Eng. Husam Alzaq, Embedded System Course, IUG, 2010