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Lecture 16: Digital to Analog Converter - PWM Implementation

Lecture 16: Digital to Analog Converter - PWM Implementation. Lecturers: Professor John Devlin Mr Robert Ross. Overview. Implementation of a DAC using PWM Class Demonstration Worked example – Software Loops Worked example – Timer Based. DAC PWM Implementation.

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Lecture 16: Digital to Analog Converter - PWM Implementation

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  1. Lecture 16: Digital to Analog Converter - PWM Implementation Lecturers: Professor John Devlin Mr Robert Ross

  2. Overview • Implementation of a DAC using PWM • Class Demonstration • Worked example – Software Loops • Worked example – Timer Based

  3. DAC PWM Implementation • Choose an operating frequency (this specifies the period of the DAC) • Create a filter circuit to operate at the desired frequency • Write software to vary on/off time • When on – drive output high • When off – drive output low

  4. PWM software loop implementation • PWM can be generated in software simply by using loops • This is generally bad design (timers use less CPU resources and are more accurate) • A useful stepping stone to understanding DAC’s

  5. PWM software loop implementation MOV #100, R5 ; Period MOV #50, R6 ; ON Time setup MOV #0, R7 ; Comparison register BIS.b #000000001b, &P1OUT ; bit 1 = high on_time INC R7 CMP R6,R7 ; has on time expired JNE on_time BIC.b #00000001b, &P1OUT ;on time expired, bit 1 = low off_time INC R7 CMP R5, R7 ; has off time expired JNE off_time JMP setup ; off time has expired

  6. Timer based PWM implementation • Timers can be used to perform PWM with less CPU overhead – allowing processing for other tasks • Design the following PWM timer: • Period = 16.38ms • F = 61kHz • OFF Time climbs from 0 to 16.38ms and repeats from 0 • Timers will be covered in detail in lecture 19

  7. Timer based PWM implementation SetupP1 BIS.B #0x0f,&P1DIR ; Set P1.0-P1.7 as outputs BIS.B #00010100b,&P1SEL ; P1.2 and P1.4 TA/SMCLK options SetupP2 BIS.B #11000000b,&P2DIR ; Set P2.6 and P2.7 as outputs Set_clock ; Set to calibrated 1MHz Clock MOV.B &CALBC1_1MHZ,&BCSCTL1 ; Set range; DCO = 1 MHz MOV.B &CALDCO_1MHZ,&DCOCTL ; Set DCO step + modulation setup_timer MOV.W #02000h,&TACCR0 ; CCR0 = PWM Period/2 MOV.W #0,&TACCR1 ; CCR1 = PWM_OFF_Time/2 MOV.W #00C0h,&TACCTL1 ; Output=Toggle/Set MOV.W #0230h, &TACTL ; CLK = SMCLK(1MHz), MODE = ; UP/DOWN

  8. Timer based PWM implementation MOV.W #0, R5 ; Initialise OFF_Time reset_countdown MOV.W #0FFh, R4 ; Initialise countdown main DEC R4 ; Decrement countdown JNZ main ; If countdown != 0, loop INC R5 ; Increment the OFF_time CMP &TACCR0, R5 ; Does OFF_Time = Period? JEQ reset_r5 ; If OFF_Time = Period, jump MOV.W R5,&TACCR1 ; Load OFF_Time into register JMP reset_countdown ; Loop again reset_r5 MOV.W #0, R5 ; Reset OFF time JMP reset_countdown ; Loop again

  9. Circuit Diagram • Add a low pass filter to reduce switching noise and return an analog signal • C should be small – otherwise DAC response will be low MSP430 R = 330k Analog Signal P1.2 C=100nF

  10. Without filtering circuitry With filtering circuitry: Class Demonstration – Timer PWM

  11. Summary • Digital to Analog converters allow digital electronics to output signals which are similar to real world continuous signals • Pulse Width Modulation is a simple and widely used form of DAC

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