read barcode from image c#.net STACK OPERATION IN AVR PROSESSORS 101 in Software

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STACK OPERATION IN AVR PROSESSORS 101
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Signal bounce on the release of a mechanical switch
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Figure 6.3 Signal bounce on a mechanical switch when it is released.
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time for which the signal bounce of the switch remains). After this period, the logic on the switch has stabilized to 0 . The processor then reads the switch input again to ensure that it is still pressed. The processor then enters a software loop and monitors the switch input pin till the switch is released again. The release of the switch is characterized by the logic at the pin changing from logic 0 to logic 1 . After detecting this logic change, the processor again calls a delay routine to timeout the signal bounce on the switch and again checks if the switch has stabilized to a logic value 1 . If so, the program concludes that the particular switch was pressed and released and then can take any action as necessary. Now, before we write a piece of code to read a switch in the way just described, we need to understand how subroutines are written and called in AVR processors.
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6.3 Stack Operation in AVR Processors
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Subroutine calls are interruptions in the normal sequential flow of the program. To call a subroutine, the address of the subroutine is loaded into the program counter. The processor then starts executing the code resident at this address and onwards. After the subroutine has finished, the program execution must resume from the point where it was suspended in the calling program. To do that, the processor must remember the address of the program memory from where the execution has to resume. This address is stored in a stack. A stack is a special storage area that is used to store return addresses. However, the stack is also used for passing parameters to a subroutine, if required, and to return results to the calling program. Typically, the stack is implemented in RAM and is accessed with a special register called Stack Pointer. Stack Pointer is an address register, and it indicates the address of the RAM memory location of the stack.
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102 HARDWARE AND SOFTWARE INTERFACING WITH THE AVR
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In the AVR processors, the stack is implemented in two different ways. For those processors that do not have any SRAM, such as the AT90S1200, the processor has a hardware stack. The hardware stack is three levels deep, meaning that it can store three return addresses. Thus, at any time, only three nested subroutine calls can be made. The hardware stack is only used by the processor to store return addresses. It cannot be used by the program to pass any parameters to the subroutine, as there is no push or pop instruction to access data on the stack. This may seem like a problem, especially for storing the processor state during an interrupt execution. Since the interrupt occurs asynchronously, the state of the SREG register, which has all the flags, can get changed due to instruction execution within the interrupt subroutine. One way out of this is to store the value of the SREG register into another designated register at the beginning of the interrupt subroutine, and while returning from the subroutine, to restore the value from the designated register back into the SREG register as illustrated below.
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ISR: mov R0, SREG ;Interrupt subroutine code ; .... ; .... mov SREG, R0 reti ;start of interrupt subroutine ;copy SREG value into R0
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;restore R0 value back into SREG ;return from subroutine
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On the other hand, for those processors that have on-chip SRAM, the processor implements a stack in the SRAM. The stack can be initialized anywhere in this SRAM area. To initialize the stack, the stack pointer is loaded with the address of the SRAM memory, and after this is done, the stack can be accessed by the push and pop instruction. The stack gets used when a subroutine is called or when an interrupt occurs. The stack grows from a larger memory address into the lower address. Thus, when some data is pushed, data is stored at the current stack pointer address, and then the stack pointer is decremented. Similarly, when the data is popped from the stack, the stack pointer is first incremented and then the data is copied from the stack to the destination register. Let s now use this information about calling subroutines and improve our lights-andswitches system so that it will now wait for a switch to be pressed, and after a switch is pressed, it will light up the corresponding LED and wait for another switch. If two switches are pressed, then for the one which is pressed earlier, the LED corresponding to that switch will be lit. The following program is also available on the CD in the code directory as file newswich.asm.
;newswich.asm ;4 LEDs on PORTB, 4 switches on PORTD ;PORTD0 SWITCH - > PORTB0 LED ;PORTD1 SWITCH - > PORTB1 LED ;PORTD2 SWITCH - > PORTB2 LED ;PORTD3 SWITCH - > PORTB3 LED ;Press a switch and corresponding to the LED will light up ;press another switch and the first LED will go off and ;the LED corresponding to the new switch will light up
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