# PIC Programming With the Basics – Article 1 (Assembly)

This article will help you to develop the knowledge of your Assembly language, which is used to program a pic. Example programs, projects would be covered in the next articles.

PIC (16f) has only 35 instructions.35 means not that much. Every specific instruction would be done a specific task. The whole article is reserved to understand the tasks of the each and every instruction.

Notes!

Some PICs have slightly different instruction sets, but if you know the basics, you'll be able to handle the differences with just a little study.

Here there are some of the instruction were discripped. Take a look at them. We will use these instructions little later when we get in to, tutorials of Assembly language.

MOVLW

Example: MOVLW 07

Side effects: none

The W register is central to most programs. This instruction moves a constant into the W register.

MOVF, MOVWF

Examples:

MOVF temp, W

MOVF temp, F

MOVWF temp

Side effects: Sets or clears Z flag (MOVF); none (MOVWF)

These instructions allow you to move data between registers and the W register. The first example moves the value in register temp into W. The third example moves W into temp. The second example moves the value of temp into temp. At first, that seems pointless, but it sets the Z condition flags without disturbing anything else, so it is useful for conditional jumps. If the number moved is 0, the Z flag is set. Otherwise it is cleared.

While you can use a number for a register is more common & easy to name them using EQU.

For example:

temp equ 0 × 70

movlw 0 × 10

movwf temp

Example:

XORWF x, F

These instructions do the specified operation on the W register and another register of your choice. Add and subtract work as you'd expect. The logical AND, IOR (inclusive OR) and XOR functions are the classic boolean logic primitives.

The result can be placed in the W register or back in the original register as you wish. For example:

MOVLW 1; put 1 in W

; w still equals 1 here

ADDWF x, W; W = x + W, x is unchanged

The subtraction operation always computes FW, so be careful that you do not mean WF. So if W contains 3 and x contains 10, performing a subtraction will result in 7.

To compute the two's compliment of a negative number, write the magnitude as a binary number. So for 7, we have 00000111. Then invert all the bits (11111000) and add 1 (11111001). You can easily reverse the process, so if I tell you the subtraction yields 0xF9, you can write it as binary (11111001), subtract 1 (11111000) and then invert the bits (00000111) to know that it means -7.

All the instructions in this section set the Z flag if their result is zero. Otherwise, the Z flag is clear.

Example:

XORLW 0x80

These instruction act just like the normal add, subtract, and, inclusive or, exclusive or instructions except they operate on a literal.

So:

adds 2 to W (and, of course, leaves the result in W). All the flags are modified just as they are in the register-based versions of these instructions (see above). The subtract instruction computates literal-W which is often confusing. So instead of writing:

SUBLW 2; computes 2-W

You probably mean to write:

ADDLW-2; This is probably what you really want

CLRF, CLRW

Example:

CLRF temp

CLRW

Side effect: Sets the Z flag to 1

It is a very common operation to load a zero into a register and this instruction can do that in one step. The CLRW instruction clears W, which is odd since MOVLW 0 will do this also

INCF, DECF

Example:

INCF temp, W

DECF x, F

Another common operation is to add or subtract 1 from a register. These instructions do that (INCF is +1; DECF is -1).

INCFSZ, DECFSZ

Example:

INCFSZ temp, W

DECFSZ x, F

These instructions are similar to INCF and DECF. They add or subtract one of the listed register and store the result as directed. However, these instructions do not set the Z flag. Instead, if the result is zero, they skip the next instruction. This is useful, of course, for loops. Here's an example:

CLRF y

MOVLW .10;

MOVWF i; i = 10

MOVF x, W; W = X

LOOP: ADDWF y, F; Y = Y + X

DECFSZ i, F

GOTO LOOP

; Leave here with the answer in y

RLF, RRF

Example:

RLF temp, F

These instructions move the bits in the register left by one bit (RLF) or right (RRF).

This is often useful to manipulate bits, but there is another interesting property: shifting left multiplies a number by 2 and shifting right divides by 2! You can often combine this with adding to get easy multiplications. For example, suppose you want to compute y = 10 * x. Well, that is the same as y = 8 * x + 2 * x, right? So try this:

BCF status, C; C = 0

RLF x, F; X = 2 * X

BCF status, C

RLF x, W; W = 2 * X (so 4 * X)

MOVWF temp

BCF status, C

RLF temp, W; W = 2 * X (so 8 * X)

ADDWF x, W; W = 8X + 2X = 10X

MOVWF y

BCF, BSF

Example:

BCF status, Z

These instructions clear (BCF) or set (BSF) the indicated bit in a register. The bit may have a name (like Z) or you can use a number from 0 to 7 (0 is the least significant bit and 7 is the most significant). Sometimes you can use these to save a few instructions. For example, suppose you had written:

MOVLW 0x7F

ANDWF temp, F

You could replace this with a single BCF temp, 7 instruction. Not only is this faster and takes less space, but it does not destroy the W register either!

GOTO

Example:

GOTO main

As you'd expect, the GOTO instruction forces your program to resume execution at the label you indicate. The instruction only holds an 11 bit address, so the top bits of the new program counter come from the PCLATH register.

RETLW

Example:

RETLW .99

RETLW is exactly like RETURN except that it loads a literal value into the W register before it returns.

RETFIE

Example:

RETFIE

This will Sets GIE

RETFIE is exactly like a return, but it also sets the global interrupt enable (GIE). When a hardware interrupt occurs, it clears GIE and executes what amounts to a CALL instruction. Using RETFIE allows you to enable interrupts and return to the main program all in one step. If you do not want interrupts enabled again, just execute a RETURN instead.

BTFSC, BTFSS

Example:

BTFSS temp, 7

When you want to perform a conditional jump, you need one of these instructions. They test a bit and skip the next instruction if the bit is set (BTFSS) or clear (BTFSC). Most of the next instruction is a GOTO or a CALL, but it could be any single instruction. For example, this code tests temp to see if it is zero. If it is, it loads temp with 0 × 80. Otherwise, temp is unchanged:

MOVF temp, F; set Z flag, no data really moved

BTFSC status, Z; skip if not zero

BSF temp, 7; since temp was 0, now it is 0x80!

CLRWDT

Example:

CLRWDT

This instruction informs the watchdog timer that your program is still executing.

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I know that the above description will not be helpful to you to carry on. We have to have the practical experience. So we will do some tutorials, based on Assembly language in future. Stay with us.

If there are any questions, feel free to ask.