barcode reader asp.net web application tris xorwf PORTB PORTB, f ; ; Save WREG in TRISB XOR PORTB with the contents of WREG in Software

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tris xorwf PORTB PORTB, f ; ; Save WREG in TRISB XOR PORTB with the contents of WREG
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Vcc 16F84
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0.1 uF Tantalum Gnd
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Figure 20.14 Circuit to demonstrate the shortest possible application.
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I m not going to reveal the entire source code just yet because there are some aspects of it I want to discuss later in this write-up. In any case, the unique application code is only these two instructions long, but those instructions take advantage of two features of the PIC microcontroller and its processor. I realize that elsewhere in this book I discuss how I feel that the tris instruction never should be used in the mid-range PIC microcontroller. I use the tris instruction in this application simply because it is the method of changing PORTB s TRIS register
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+5 Volt P/S
10 K LED 220
Vcc Gnd
Figure 20.15 application.
Breadboard wiring for the shortest possible
0.1 uF
PIC16F84
4 MHz
PIC MICROCONTROLLER APPLICATION BASICS
TABLE 20.8 PART
BILL OF MATERIALS FOR THE SHORT EXPERIMENT DESCRIPTION
PIC microcontroller 0.1- F 220 4-MHz LED Misc.
PIC16F84 04/P 0.1- F tantalum 220 , 1/4 W
4-MHz ceramic resonator with internal capacitors Visible-light LED, any color Breadboard, wiring, +5-V power supply
in the fewest number of instructions. If I were to update TRISB properly, I would use the code
bsf movwf bcf STATUS, RP0 TRISB ^ 0x080 STATUS, RP0 ; Access Bank 1 ; Store the Contents of WREG in TRISB ; Return to Bank 0
The rst feature that the code takes advantage of is that unprogrammed program memory addresses are all set to 1s. This is a function of EPROM and Flash memory; when it is erased, the bits in the memory are all set to 1s. For mid-range PIC microcontrollers, the instruction addlw 0x0FF has the bit pattern 0x03FFF, which is all the bits set in an instruction word. Thus, as the code executes through the unprogrammed program memory, it is adding 0x0FF (or 1) to the contents of the w register. On power-up, the contents of WREG are unde ned, but as the code executes through the unprogrammed instructions, the value within it will be incremented continually. The other aspect of the PIC microcontroller that this application takes advantage of is that when the program counter reaches the end of program memory, it resets and continues executing from address 0. These two features of the PIC microcontroller mean that the application code, while it is only two instructions long, actually executes as if it were
Loop: call Dlay1019 ; addlw 0x0FF 1022x addlw 2 tris PORTB xorwf PORTB, f goto Loop ; Loop Back Here ; Delay Same Cycles executing ; ; ; Equivalent to addlw 0x0FF 1022x Save WREG in TRISB XOR PORTB with the contents of WREG
The effective adding of two each time through the loop is why I have put the LED on RB7, the most signi cant bit in the PORTB register. As the w register is incremented
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by two, the most signi cant register will be toggled at the slowest rate. The LED could be put on any output from RB1 to RB7, but you will nd that as you use the least signi cant bits, the rate at which the LED ashes increases to the point where it cannot be observed. The actual source code is
title SHORT - Is this the Shortest Possible Application ; ; Look at a Two Instruction Application. Continually Update ; w (with the unprogrammed ADD 0x0FF) and then use the ; value for PORTB and TRISB . ; ; Hardware Notes: ; 16F84 Running at 4 MHz ; Reset is tied directly to Vcc and PWRT is Enabled. ; PortB is used for Output ; ; ; Myke Predko ; ; 99.10.26 - Short Created ; list R=DEC include p16f84.inc __CONFIG _CP_OFF & _XT_OSC & _PWRTE_ON org tris xorwf end 0x03FE PORTB PORTB, f ; Save WREG in TRISB ; XOR PORTB with the contents of WREG & _WDT_OFF
This code runs through the PIC microcontroller s program memory repeatedly, adding 1,022 to the contents of WREG (which is the same as adding +2 when the 8-bit register is taken into account). Next, the new value for WREG is used for the TRISB register and then XORed with the contents of PORTB to change PORTB s output value. The operation of the application can be observed by enabling the Program Memory window (from the Window pull-down menu on MPLAB s top pulldowns), as shown in Fig. 20.16. Once you have brought this up, you can single-step through the application to see the execution of the two instructions explicitly programmed into the otherwise erased program memory. I put the two instructions at the end of program memory because you can see that the contents of WREG have been effectively incremented by 2 by the time you get to the end of the program memory.
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