barcode reader asp.net web application TimeEnd: TMR0 DELAY THAT NEVER ENDS in Software

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TimeEnd: TMR0 DELAY THAT NEVER ENDS
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The TimeEnd application (which uses the same circuit as the preceding experiments) is designed to use TMR0 to delay 1 ms after reset and turn on a LED on RB0. Unfortunately, it doesn t work out that way, as you will discover when you program a PIC16F84 with the TimeEnd application (found in the code\TimeEnd folder).
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title TimeEnd - Loop While Waiting for TMR0 to End ; ; This Uses TMR0 with a 4x Prescaler to Provide a 1 msec ; Delay. The Access instruction will cause the Timer to
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Hardware Notes: PIC16F84 Running at 4 MHz _MCLR is Pulled Up PortB0 Pulled up and Connected to LED Myke Predko 99.06.22 LIST R=DEC INCLUDE p16f84.inc
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; Register Usage CBLOCK 0x020 ENDC
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PAGE __CONFIG _CP_OFF & _WDT_OFF & _XT_OSC & _PWRTE_ON ; Mainline of TimeEnd org nop bsf bsf clrf movlw movwf bcf movlw movwf Loop: movf btfss goto bcf goto PORTB, 0 ; STATUS, RP0 TRISB ^ 0x080 0x0D1 ; OPTION_REG ^ 0x080 STATUS, RP0 (1024 - 1000) / 4 TMR0 Turn RB0 off 0
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Set TMR0 to Prescaler and 4x
Reset TMR0 to Wait 1 msec
TMR0, f STATUS, Z Loop PORTB, 0 $
Does TMR0 Equal 00
(1 msec Passed)
Turn RB0 ON Loop Forever When Done
SOME BASIC FUNCTIONS
The code is very simple; it sets up the prescaler to be used with TMR0 to divide by four along with having TMR0 driven by the instruction clock. Once this is done, TMR0 is loaded with 6 and waits for the timer to over ow to 0. The only problem is that when you build the circuit and program a PIC microcontroller with TimeEnd, the application sits there and doesn t turn on the LED. When you look at the source code, you probably won t see anything amiss. The problem becomes apparent only when you simulate the application and look at the value of TMR0 as the PIC microcontroller executes through Loop. Each time Loop executes, TMR0 s value stays the same at 6. This can be observed using the MPLAB IDE simulator with a TMR0 watch register and single-stepping through the code. The problem with the application is with the movf TMR0, f instruction just after the Loop label. This instruction moves the contents of TMR0 through the ALU to test the value against 0 and conditionally sets the zero ag and then stores this value back into TMR0. When the value is stored back in TMR0, the prescaler is reset. If you check the number of cycles used by Loop, i.e.,
Loop movf btfss goto
TMR0, f STATUS, Z Loop
Does TMR0 Equal 00
(1 msec Passed)
you ll come up with four instruction cycles each time Loop executes. This is a problem because the TMR0 prescaler is set to be a divide by four counter. Each time Loop executes, the prescaler is reset, never passing a clock tick to TMR0 to increment it. As a result, TMR0 never changes value, and Loop becomes an endless loop. The easiest way to x the problem is to change the movf TMR0, f instruction to movf TMR0, w, with the value in TMR0 being stored in the w register instead of back into itself. When you make this change and program the PIC microcontroller with the updated code, you ll nd that it works without any problems. There may be a question as to how likely this problem is. If you re going to use TMR0, chances are that you are going to use it with an interrupt, and there would be no need for polling it at all. This is true for mid-range devices, but what about the low end Using TMR0 for setting up delays in low-end PIC microcontrollers is a fairly common practice, and the problem I ve shown here is something that causes becomes apparent to many new application developers. The easiest way to avoid this is never to write to the TMR0 register except when setting it originally. If you have a value in WREG you don t want changed, then it should be saved before polling TMR0.
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