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

The calling routine places in the w register the numeric value whose code is desired, and then calls the table lookup, as follows:

10.4 A Demonstration Board

A demonstration board, also known as a demo board, is a useful tool in mastering PIC programming. Many are available commercially; like programmers, there is a cottage industry of PIC demo boards on the internet. Constructing your own demo boards and circuits is not difficult. The components can be placed on a breadboard, or wire-wrapped onto a special circuit board, or a printed circuit board can be homemade, or ordered through the internet. These options have been previously discussed and Appendix B contains instructions on how to build your own PCBs.

Figure 10-7 shows a simple 16F84A-based demo board with a seven-segment LED, buzzer, pushbutton switch, and a bank of four toggle switches.

10.4.1 PCB Images for Demo Board

Some PCBs contain circuit etchings on both sides. In this case two circuit board images are required. In addition, most boards contain a top-side image of the components, company logos, model numbers, and other information. Commercially, this image is silk-screened onto the board.

The homemade board (see Appendix B) usually contains a single etched image and a top-side image with informational text and graphics. Both images can be created with a conventional drawing program, such as Corel Draw, Adobe Illustrator, or Windows Paint, or with a specialized application, several of which are available free and for purchase on the Web. Figure 10-7 shows the images used for making the PCB for the circuit in Figure 10-8.

Note that the top-side (text) image has been mirrored on the horizontal plane. This is necessary so that the text and graphics coincide with the circuit etchings once the images are transferred to the board. The process for making your own PCBs is described in Appendix B.

10.4.2 TestDemo1 Program

The following program exercises some of the experiments that can be implemented on the demo boards in Figure 10-7:

;File: TestDemo1.asm

;Date: June 2, 2006

;Author: Julio Sanchez

;Processor: 16F84A

;Description:

;Program to exercise the demonstration circuit and board

;number 1

;===========================

;switches ;===========================

;Switches used in __config directive:

;_PWRTE_OFF

;|

;|_____ * indicates setup values

;=======================================================

; P R O G R A M

;=======================================================

;==============================

;Pushbutton switch processing ;============================== pbutton:

;Push button switch on demo board is wired to RA4

;Switch logic is active low

;==============================

;DIP switch processing ;==============================

;Read all bits of Port-A readdip:

;If board uses active low then all switch bits must be negated

;This is done by XORing with 1-bits

;******************************* ; 7-segment table of hex codes ;******************************* segment:

;****************************

;piezo buzzer ON ;****************************

;Routine to turn on piezo buzzer on Port-B bit 7 buzon:

return

;****************************

;piezo buzzer OFF ;****************************

;Routine to turn off piezo buzzer on Port-B bit 7 buzoff: