Lesson 10 - Dual Output 8-Bit PWM DAC

Motivation

Many applications such function generators, musical tuners and toys require analog signals. Rather than use a separate DAC, these signals can be achieved with the MSP430 and a lowpass filter (LPF). The latter will be built from external components.

This project conveys how to generate these signals. An 8-bit DAC will generate a 250Hz sine wave, oversampled at 16x.

The sine wave is constructed by using a lookup table and updating the duty cycle of a PWM signal.

Background

The MS430 will be used to generate analog waveforms, either ramp or sine by filtering the PWM outputs through a LPF.

The GUI will be used to:

Configure each PWM output including duty cycle, signal type and frequency
Display the waveforms
Observe the ADC value which corresponds to the outputted waveform

Operation

Looking at Figure 1 below, the duty cycle of the sine-PWM signal varies from one PWM period to the next, whereas the duty cycle of the DC-PWM signal remains fixed.

Figure 1 - Oscilloscope captures of relevant waveforms — Figure 1 – Oscilloscope captures of relevant waveforms

The formula for the sampling rate/PWM frequency is:

250Hz * 32 samples = 8kHz.

The capacitor is basically averaging the output samples over time.

The CCR0 interrupt is enabled and the ISR updates the PWM duty cycle which is stored in CCR1.

CCR2 is loaded with a fixed PWM duty cycle to generate the DC signal.

CCR1 is connected to Pins P1.1/TA0.1

CCR2 is connected to P1.2/TA0.2

Figure 2 - Wiring diagram — Figure 2 – Wiring diagram

The 2-pole RC filter removes the higher-order harmonics, leaving a nice analog waveform.

The filtered analog waveform is depicted in figure 4 below:

Figure 3 - Filtered output signals — Figure 3 – Filtered output signals

Figure 4 - 250Hz output waveform — Figure 4 – 250Hz output waveform

If R_1*C_1 = R_2 * C_2 = RC, then

f_c = \frac{1}{2 \pi R C}

The cutoff frequency is set to be 795Hz. A higher order filter can be built.

The DC level signal can be altered by varying the duty cycle of the DC PWM signal.

V_DC = D * VCC

Where D = duty cycle of the PWM signal.

Figure 5 - Code flowchart — Figure 5 – Code flowchart

The GUI

The GUI provides an interface to adjust the type of signal the user wishes to generate as well as the duty cycle.