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LedPWM: TMR0 INTERRUPT USED FOR LED PWM DIMMING
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To demonstrate how TMR0 and interrupts can be used to control the brightness of a LED by producing a PWM signal, I will use the simplest application circuit of this book. This circuit, shown in Fig. 20.29 (with the bill of materials listed in Table 20.13), interfaces with only one LED, which will get brighter and then turn off and repeat using the LedPWM code that can be found in the code\ledpwm folder.
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Figure 20.29 to dim an LED.
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Circuit to demonstrate how a PWM can be used
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PIC MICROCONTROLLER APPLICATION BASICS
TABLE 20.13 MATERIALS PART
LedPWM CIRCUIT BILL OF
DESCRIPTION
PIC 0.1- F 10k 4-MHz LED 220 Misc.
PIC16F84 04/P 0.1- F tantalum 10 k , 1/4 W 4 MHz with built-in capacitors Red LED 220- , 1/4 W Breadboad, wiring, +5-V power supply
On reset, the PIC microcontroller s RB0 pin is enabled for output, and the TMR0 interrupt is enabled. Once this is done, an incrementing LED-power PWM signal is set and is passed to an interrupt handler that outputs a pulse of varying timing widths every 1 ms.
title LedPWM - Show an LED Changing Brightness ; ; This Application simply waits for the TMR0 interrupt handler to ; occur and after the LED PWM ON is complete, decrement the ON ; value. This Application repeats endlessly with the LED getting ; lighter before turning off and starting over. ; ; Hardware Notes: ; This application runs on a 16F84 executing at 4 MHz ; _MCLR is tied through a 4.7K Resistor to Vcc and PWRT is Enabled ; A 220 Ohm Resistor and LED is attached to PORTB.0 and Vcc ; A 10K pull up is connected to RA0 and its state is passed to ; RB0 ; ; Myke Predko ; 99.12.14 ; LIST R=DEC INCLUDE p16f84.inc ; Registers CBLOCK 0x020 _w, _status PWMOn:2 PWMDouble: 2 ENDC #de ne PWM PORTB, 0
; Context Register Save Values ; PWM On Value ; Divide PWM down for Slowing Down ; LED on PORTB.0
I/O WITH INTERRUPTS
__CONFIG _CP_OFF & _WDT_OFF & _XT_OSC & _PWRTE_ON PAGE Mainline of ledpwm 0
org nop clrf clrf goto org Int: movwf movf movwf btfsc goto nop
PWMDouble PWMDouble + 1 Mainline 4 ; _w STATUS, w _status PWM PWM_ON ; ; ; ; Interrupt Handler 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
bsf movlw subwf
PWM 6 + 6 PWMOn, w
; Get the PWM On Period ; Add to PWM to Get Correct Period ; for ; Interrupt Handler Delay and ; Missed ; cycles in maximum 1024 sec ; Cycles
goto PWM_ON: bcf movf sublw
PWM_Done ; PWM PWMOn, w 6 + 6 PWM is On - Turn it Off
; Output the Low of the PWM Cycle ; Calculate the Off Period ; Subtract from the Period for the ; Interrupt Handler Delay and ; Missed ; cycles in maximum 1024 sec ; Cycles
PIC MICROCONTROLLER APPLICATION BASICS
goto
PWM_Done ; ; ; ; Have Finished Changing the PWM Value Get the Value to Load into the Timer
PWM_Done:
sublw movwf bcf movf movwf swapf swapf ret e
0 TMR0 INTCON, T0IF _status, w STATUS _w, f _w, w
Reset the Interrupt Handler Restore the Context Registers
Mainline: bsf bsf bcf movlw movwf bcf clrf movlw movwf Loop: btfsc goto PWM $ - 1 PORTB, 0 STATUS, RP0 TRISB ^ 0x080, 0 0x0D1 OPTION_REG ^ 0x080 STATUS, RP0 TMR0
; Setup the PWM And then Monitor it, ; Updating PWMOn ; Make the LED on PORTB.0 off ; Initially ; Goto Bank 1 to set Port Direction ; Set RB0 to Output ; Setup TMR0 with a 4x prescaler ; ; Go back to Bank 0 Start the Timer from Scratch
(1 << GIE) + (1 << T0IE) INTCON ; ; ;
Enable Interrupts Loop Here Wait for PWM to go Low
incfsz TMR0, w goto $ - 1 movf btfsc decf decf rrf movwf rrf movwf PWMDouble, f STATUS, Z PWMDouble + 1, f PWMDouble, f PWMDouble + 1, w PWMOn + 1 PWMDouble, w PWMOn
Wait for TMR0 to Equal 0x0FF
Decrement PWM Double
Divide by 4
I/O WITH INTERRUPTS
rrf rrf addlw addlw btfss movlw
PWMOn + 1, w PWMOn, w 255 - 0x0FE 0x0FE - 0x00E + 1 STATUS, C 0x0FC ; ; ; ; ; ; ; ; Get the High limit Add Lower Limit to Set Carry If Carry Set, then Lower Case Carry NOT Set, Reset the Character Add Lower Limit to restore the Save the New Value Wait for the PWM to Go High
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