barcode reader web application I/O with Interrupts in Software

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I/O with Interrupts
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So far in these experiments I have focused on the basic hardware that is built into both low-end and mid-range PIC microcontrollers. These experiments are very important because they will help you to understand how the PIC microcontroller works in a variety of situations or how the PIC microcontroller responds to different hardware inputs. In the next series of experiments, I want to look at using the mid-range PIC microcontroller s interrupt capability to enhance applications and, in many cases, simplify the code and operation of the application. This probably seems surprising to you, but interrupts can make applications much easier to work with. In Chap. 21 I will show how a serial LCD interface board can be built without interrupts or serial interface hardware, but the code is quite complex and very accurately timed. Also in Chap. 21 I will go through a lot more of the various interrupt sources and operations than I will in this chapter. In this chapter I want to introduce how interrupts work in mid-range PIC microcontrollers and how handlers are written for them.
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Throughout this book I have pointed out the advantages of using the TMR0 interrupt source for carrying out background tasks. Later in this chapter I will show a TMR0 interrupt button debounce application that debounces button input without affecting the
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operation of the mainline code. To help introduce interrupts and the TMR0 interrupt, I have created a little interrupt handler that moves to lit LEDs back and forth in kind of a Cylon eye after a 1/8-s delay. The name Cylon eye comes from eye movement of the the metal warriors of Battlestar Gallactica. To be honest, the name comes from the original TV show (done in the 1970s), but it has been used for years in different applications to show that the circuit is working properly. I wanted to create the code so that it could be ported into a PIC microcontroller application that supervises the operation of a larger circuit. The Cylon eye application uses PORTB connected to a series of LEDs, with TMR0 providing an interrupt request once every sixteenth of a second. The sixteenth of a second is the maximum delay for a PIC microcontroller running at 4 MHz. This leaves the mainline or foreground execution open to application monitoring code that can stop the eye if any problems are discovered. The circuit itself simply consists of eight LEDs connected to PORTB of the core circuit, as shown in Fig. 20.25 (the bill of materials is listed in Table 20.11). For the breadboard application, I used a bargraph LED to allow simple continuity of the display, as shown in Fig. 20.26. The application code is Cylon.asm and can be found in the code\Cylon folder. Along with the context-saving registers, two variables are required for the programmable delay and to keep track of the direction in which the eye is moving.
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title Cylon - Output a Cylon Eye in the Background #de ne nDebug ; ; This Application uses TMR0 to Move a Cylon Eye back and forth ; Across eight LEDs connected to PORTB. ;
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Vcc 16F84
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0.1 uF Tantalum Gnd
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Vcc 10 K _MCLR
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Osc 1 Osc 2 Vss
4 MHz
Figure 20.25 Circuit to move a lighted LED like a Cylon warrior s scanner.
PIC 0.1- F 10k 4-MHz LED 220
PIC16F84 04/P 0.1- F tantalum 10 k , 1/4 W 4-MHz with built-in capacitors 10-LED bargraph 220- , 1/4 W
; ; ; ; ; ; ; ;
Hardware Notes: This application runs on a 16F84 executing at 4 MHz _MCLR is tied through a 10K Resistor to Vcc and PWRT is Enabled A 220 Ohm Resistor and LED is connected between PORTB.0 and Vcc Myke Predko 99.12.28 LIST R=DEC INCLUDE
; Registers CBLOCK 0x020 _w, _status
Context Register Save Values
+5 Volt P/S
10 K
Vcc Gnd
Breadboard wiring for the Cylon eye.
0.1 uF
4 MHz
Direction Count ENDC
0 for Up, !0 for Down Count the Number of Times Through
__CONFIG _CP_OFF & _WDT_OFF & _XT_OSC & _PWRTE_ON PAGE Mainline of cylon 0
org nop movlw movwf goto org Int: movwf movf movwf bcf
2 Count Mainline 4 _w STATUS, w _status
Setup the Count
; ; ;
Interrupt Handler Save Context Registers - Assume TMR0 is the only enabled Interrupt
Reset the Interrupt Flag
decfsz Count, f goto IntEnd movlw movwf btfss bsf btfss bcf btfsc goto Up: bsf rlf goto STATUS, C PORTB, f IntEnd 2 Count
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