barcode reader project in asp.net Simulating Logic in Software

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Simulating Logic
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One of Microchips early application notes was on how to simulate logic functions using a PIC microcontroller. This was an interesting example of how logic functions could be processed by the PIC microcontroller in very slow-speed application. Included in the application note were details on how the PIC microcontroller could be made to simulate logic functions. For example, to simulate the 8-bit address, look at the circuit shown in Fig. 8.1, which could be implemented using the hardware shown in Fig. 8.2. This circuit could be simulated within a PIC microcontroller using the model shown in Fig. 8.3, where the comparison to the set addr, which was stored previously in the PIC microcontroller memory, could be implemented in software as
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Addr7-0 Compare Addr7-0 In
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Output
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Eight-bit address comparator.
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Loop movf PORTB, w xorwf SetAddr,w btfsc STATUS, Z goto matchHI nop bcf RAO goto Loop matchHI bsf RAO goto Loop
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; Get Current ; Zero if Match
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; No Match
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This sequence of instructions takes eight instruction cycles under a match or mismatch, and herein lies the problem with using the PIC microcontroller for simulating logic; this sequence is relatively very slow. For the preceding above, if the PIC microcontroller were running at 20 mHz, the worst-case switching time you could expect is 1.6 s. The best case is 1.2 se (if the change is just before the movf instruction). Comparatively, most CMOS and TTL logic would execute this in much less than 100 ns one full order of magnitude speed increase (with two or three orders of magnitude increase being more likely). This improvement becomes even more profound when more complex operations are required, and the PIC microcontroller code becomes lengthier.
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Addr7-0 In Addr7-0 Compare Bit 0 Match
Bit 1 Match
Output
Bit 7 Match
Equivalent circuit to Fig. 8.1.
SIMULATING LOGIC
PIC microcontroller
Addr 7-0 Compare in Software
Software RA0 Compare=
Output
Addr7-0 In PORT B 7-0
Figure 8.3 Eight-bit address comparator using a PIC microcontroller.
Even a simple case such as devoting a PIC microcontroller to ANDing two inputs together is very slow. For example, if you had two inputs and wanted a implement an AND output, you could use the code
Loop btfsc btfss goto nop bsf goto reset bcf goto Bit1 Bit2 reset Output Loop Output Loop
; If Bit1 or Bit2 is Low, Turn Off Output
This requires seven instruction cycles. You might want to simplify the code by assuming the output is high before the comparison:
Loop bsf btfsc btfss bcf goto Output Bit1 Bit2 Output ; If Either Input is Low, Output is Low Loop
which reduces the number of cycles in the loop by 1 but will either have a constant 1 output or an alternating 1 and 0 (three cycles each) if either one of the inputs is low. This is shown in Fig. 8.4. Even at six cycles a bit, this is really not a contender for competing against any kind of logic in terms of speed, and the PIC microcontroller always will be more expensive in terms of cost. In the nal analysis, I never recommend using a PIC microcontroller for a logic replacement. Instead, TTL/CMOS chips should be used. This is not to say that the PIC microcontroller cannot be used for implementing simple logic functions in software that will avoid the need for external gates. Logic functions can be implemented in software and let you avoid adding logic externally to the PIC microcontroller. Providing the logic functions within the PIC microcontroller will
ASSEMBLY-LANGUAGE SOFTWARE TECHNIQUES
btfsc Bit 1 btfss Bit 2 bcf Output Output Signal bsf Output
bsf Output
Goto Loop
Figure 8.4 Two input AND gate software operation analysis.
reduce the number of input-output (I/O) pins required for the PIC microcontroller and will reduce the cost of the application. I realize that there will be cases where you will consider the PIC microcontroller because one gate is required and power, space, and cost requirements do not allow an additional chip to be put into the application. In such cases, I recommend that you look at different logic analogs that are available using simple discrete circuits even a few resistors and diodes will provide meaningful logic functions for CMOS circuitry.
Event-Driven Programming
The PIC microcontroller is particularly well suited to event-driven programming applications owing to how its interrupt handler code works. Many of the experiments and applications presented in this book use event-driven programming to respond to numerous inputs without affecting the processing of other inputs or tasks within the application. The typical high level format for event-driven programming is
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