barcode reader project in asp.net INTERRUPTS in Software

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This method is quite clever and ef cient and avoids having to do something like
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movf _status, w movwf STATUS movlw 0 btfss STATUS, Z movlw 1 movwf _status movf _w, w movf _status, f ret e ; Restore the Status Register
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; Save Value According to Zero ; Set Zero According to the Original ; Value
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This code, which was the method I came up with originally for returning from interrupts, uses the _status variable and makes it zero or nonzero based on the zero ag s original state. Once the w register is restored, the new _status value is compared with zero. This method requires twice the number of instructions as the method using the two swap f instructions to restore the w register from _w without changing the zero ag.
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NO CONTEXT SAVING INTERRUPT HANDLERS
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Sometimes in the PIC microcontroller the interrupt handler just resets an interrupt active (IF) ag along with a program ag to indicate that the interrupt happened. Another simple interrupt implementation could be a timer that only requires an interrupt handler to increment or decrement a counter. In these cases, there is no reason to save the context information, which reduces the number of active variables and allows the interrupt handler to execute in as few as seven instruction cycles. Looking through the mid-range PIC microcontroller instruction set, there are eight instructions that are appropriate in this type of interrupt handler because they don t change the w or STATUS registers (Table 8.6).
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TABLE 8.6 THE EIGHT MID-RANGE PIC MICROCONTROLLER INSTRUCTIONS THAT DO NOT AFFECT W OR STATUS REGISTER BITS
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bcf bsf btfsc btfss decfsz incfsz nop swapf
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I realize that goto and call do not change either the w register or the STATUS register, but they do use PCLATH, which may be incorrect depending on whether or not the application is executing outside the rst page. Using these instructions, an interrupt handler that sets a ag when TMR0 over ows could be
int: bcf bsf INTCON, T0IF T0Flag
ret e
and an interrupt handler that increments a counter on TMR0 s over ow could be
int: bcf INTCON, T0IF incfsz TMR0Count, f nop ret e
The two interrupt handlers can be combined to set a ag when TMR0 has over owed a set number of times:
int: bcf INTCON, T0IF bsf T0Flag decfsz TMR0Count, f bcf T0Flag ret e
In the last example, 9 to 10 instruction cycles are required for the interrupt handler. These examples assume that register bank 0 will always be the active bank. In my basic program format, during hardware initialization, I execute out of the different banks, but when the application is running, I work at staying within bank 0. If you cannot guarantee that execution will always take place in register bank 0, then you probably will not be able to execute the no context save interrupt handlers shown here. One style of writing programs is to execute the application functions from entirely within interrupt handlers. After initializing hardware, the mainline just executes an endless loop (goto $). When an interrupt occurs, the handler not only handles the interrupt request but also provides all the responses to the interrupt. In this case, saving the context registers is not required because the mainline code is only executed intermittently and does not perform any logical functions.
SIMULATING LOGIC
I would like to discourage this style of application coding because it has the very definite possibility that interrupts will be missed if an interrupt request is received while another is being processed. This is especially true if multiple interrupt sources are used with an application.
Reentrant Subroutines
In many applications, subroutines that are used both by interrupt handlers and mainline code are required. In these cases, the subroutines may be interrupted and called again from the interrupt handler. Subroutines that can support being called multiple times, from the mainline and interrupt handlers, are known as reentrant. For most processors, sharing a subroutine between the mainline and interrupt handler code can be carried out quite safely and ef ciently. However, in the PIC microcontroller, there can be problems with doing this, and I would recommend that you duplicate the subroutine and make the two copies speci ed to the interrupt handler and the mainline. The reason for this recommendation is the PIC microcontroller s lack of a data stack that can be used to store temporary or local variables. Some high level programming languages may provide this capability (and you can provide it as well in assemblylanguage programming), but doing this will require more complex subroutines and will take FSR away from mainline use in the code. Often a PIC microcontroller subroutine that supports interrupt handler and mainline access will be larger than two copies of the routine. If nested interrupts are allowed, care should be taken to avoid calling subroutines that may have been called and were executing by the previous handler when the nested interrupt took place. Not properly handling the subroutines can result in data variables that are overwritten or previous interrupts data being lost. To avoid these potential problems, simply do not call subroutines from your interrupt handler code, and avoid allowing nested interrupts from executing.
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