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, R ; Comparison register
BIS.b # b, &P1OUT ; bit 1 = high
on_time
INC R7
CMP R6,R7 ; has on time expired
JNE on_time
BIC.b # b, &P1OUT ;on time expired, bit 1 = low
off_time
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 # b,&P1SEL ; P1.2 and P1.4 TA/SMCLK options
SetupP2
BIS.B # b,&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,&TACCR ; CCR0 = PWM Period/2
MOV.W #0,&TACCR ; CCR1 = PWM_OFF_Time/2
MOV.W #00C0h,&TACCTL ; Output=Toggle/Set
MOV.W #0230h, &TACTL ; CLK = SMCLK(1MHz), MODE = ; UP/DOWN

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

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

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