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End PWMOutput TMR0 Interrupt Handler
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This code is quite easy to port into PIC microcontroller assembly language. For example, if the PWM period were 1 ms (executing in a 4-MHz PIC microcontroller), a divide by four prescaler value could be used with the timer, and the interrupt handler assembly-language code would be
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org Int: 4 ; Interrupt Handler
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movwf movf movwf btfsc goto nop bsf
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_w STATUS, w _status PWM PWM_ON
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Save Context Registers - Assume TMR0 is the only enabled Interrupt Is PWM O/P Currently High or Low Low Nop to Match Cycles with High Output the Start of the Pulse
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movlw 6 + 6 ; Get the PWM On Period subwf PWMOn, w ; Add to PWM to Get Correct Period for ; Interrupt Handler Delay and Missed cycles ; in maximum 1024 sec Cycles goto PWM_Done PWM_ON: bcf PWM ; PWM is On Turn it Off ; Output the Low of the PWM Cycle
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movf PWMOn, w ; Calculate the Off Period sublw 6 + 6 ; Subtract from the Period for the Interrupt ; Handler Delay and Missed cycles in maximum ; 1024 sec Cycles goto PWM_Done PWM_Done: sublw 0 movwf TMR0 bcf movf movwf swapf swapf ret e ; ; Have Finished Changing the PWM Value Get the Value to Load into the Timer
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INTCON, T0IF _status, w STATUS _w, f _w, w
Reset the Interrupt Handler Restore the Context Registers
In this code, TMR0 is loaded in such a way that the PWM period is always 1 ms (or a count of 250 ticks with the prescaler value of 4). To get the value added and subtracted from the total, I rst took the difference between the number of ticks to get 1 ms (250) and the full timer range (256). Next, I counted the total number of instruction cycles of the interrupt handler (which is 23) and divided it by 4 and added the result to the 1-ms difference. The operation probably seems confusing because I was able to optimize the time for the PWM signal off:
Time Off = Period ON
PIC MCU INPUT AND OUTPUT DEVICE INTERFACING
movf ON, w sublw 6 + 6 sublw 0 movwf TMR0
I realize that this doesn t make any sense the rst time you look at it, and I will go through it by showing how it works. Using the original equation, you should note that this calculates the number of cycles to be delayed by TMR0, but the actual value to be loaded into TMR0 is calculated as
TMR0 Delay Value = = = = = = = 0x100 - (Time Off) 0x100 - (Period ON) 0x100 - (256 250 + Interrupt Execution ON) 0x100 - (6 + 6 ON) 0x100 - (12 ON) 0x100 - 12 + ON 0xF4 + ON
Going back to the three instructions that load TMR0, you can show that they execute as
movf sublw ON, w 6 + 6 ; ; ; ; ; ; ; ; ; ; ; w = ON w = 6 + 6 - w = 12 - ON = 12 + 0x0FF ^ ON + 1 = 13 + 0x0FF ^ ON w = 0 - w = 0 - (13 + 0x0FF ^ ON) = 0 + 0x0FF ^ (13 + 0x0FF ^ ON) + 1 = 0x0FF ^ 13 + 0x0FF ^ 0x0FF ^ ON + 1 = 0x0FF ^ 13 + ON + 1 = 0x0F4 + ON
sublw
which is (surprisingly enough) the same result that was found with the TMR0 delay value equation earlier. The formula in itself is not that impressive except that it dovetails very well with the PWM on half of the code. This optimization probably belongs in another chapter on optimization, but to be honest with you, I came up with it using nothing but trial and error along with the feeling that this kind of optimization were possible. This is an example of what I mean when I say that you should look for opportunities when processing data in the PIC microcontroller. More often than not, you will come up with something like these few instructions that are very ef cient and integrate different cases. Note that in this code, the PWM signal will never fully be on (a high dc voltage) or fully off (a ground level dc voltage). This is so because when the routine enters the subroutine handler, it changes the output regardless of whether or not it is required for the length of the interrupt handler. In actuality, if you time it out, you will see that the 23 instruction cycles that the interrupt handler takes between changing the value works out to a 2.4 percent loss of full-on and full-off. This should not be signi cant in most
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