Файл: Microcontroller Programming. Thi Micro Chip PIC (Julio Sanchez, 2007).pdf

10.1.3 Comparisons in PIC Programming

The power and usefulness of programs is due, in great measure, to their deci- sion-making ability, and decisions are based on comparison. In a comparison code, it is able to make decisions based on the relative values of two operands. For example, compare the values a and b. If a is greater than b execute a certain code routine. If b is greater than a, execute another one. If both operands have the same value then proceed to a third code branch.

CISC and even some RISC microprocessors contain a compare operator in their instruction set. However, the compare can be substituted, with some inconvenience, by a subtraction. Since there is no compare operation in the PIC instruction set, we have to simulate the comparison by subtracting the w register from a literal value or from a file register. The sublw and subwf instructions can be used. After the subtraction takes place, code can make decisions based on the state of the zero and the carry flags. For example, the following code fragment compares the value in the two registers, labeled OP1 and OP2 respectively, and directs execution to three possible routines:

; Declare variables at 2 memory locations

;At this point the zero flag is not set. Therefore the two operands

;are not equal

;Now test the carry flag for OP1 < OP2, in this case C = 1

194 Chapter 10

ops_are_eq:

; Processing for the case OP1 = OP2l nop

nop

goto done

op2big:

; Processing for the case OP1 < OP2 nop

nop

done:

The Infamous PIC Carry Flag

In PIC programming, the effects on the carry flag are different in addition than in subtraction. During addition (addwf and addlw) the carry flag indicates a carry-out of the most significant bit of the result. In this case, C = 1 if there was a carry out, and C = 0 otherwise. However, in subtraction the carry flag is described in the Microchip documentation as behaving as an inverted borrow. This means that when two numbers are subtracted and the result is too big to fit in the destination operand, then the carry flag is clear. What this amounts to is that in PIC subtraction (sublw and subwf operations) the carry bit is set if there is no carry-out of the high-order bit. This unusual behavior is shown in the preceding code fragment.

10.2 Simple Circuits and Programs

In the following sections we describe very simple PIC-based circuits that can be assembled with few components on a breadboard. The corresponding programs exercise the circuit components. The beginner should not skip building these circuits and coding the programs since they demonstrate essential hardware and software elements.

As a learning experience, it is a good idea to reverse engineer the code in these sample programs. With the processor’s instruction set at hand, listed in Appendix C, proceed to follow the code one instruction at a time until you can understand every processing detail.

10.2.1 A Single LED Circuit

One of the simplest circuits consists of a single LED lamp wired to Port-B, line 0, of a 16F84A PIC, as shown in Figure 10-3.

The power source for the circuit in Figure 10-3 is not shown in the diagram. Typically, a battery source or an AC/DC converter and a voltage stabilizer circuit as in the one in Figure 10-2 are used.

A program to turn on the LED on Port-B, line 0, requires a few but essential processing operations. Code must perform the following operations: