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Preliminary
Application Note
Audio Generation by DragonBallTM MC68EZ328 PWM
Output
INTRODUCTION
This document describes an method of audio generation using DragonBallTM MC68EZ328 PWM Output. The
Pulse Width Modulation (PWM) module of DragonBallTM-EZ will modulate the audio signal into a pulse train of
fixed periods but changing width. The changing width of the pulses corresponds to the voltage level of the sine
wave.
A simple RC circuit, fourth-order low pass filter and audio amplifier are used to convert the PWM signal into
audio signal. This circuit has been tested with audio sample at 16kHz and 32kHz reconstruction rate. The output
sound quality is as good as an AM radio.
Figure 1. Block Diagram
HARDWARE
The audio generation circuit consists of a RC circuit, four-order low pass filter, audio amplifier and speaker. The
RC circuit and Low Pass Filter is used for harmonics and carrier frequency generated by PWM signals. The
audio amplifier will drive the speaker to generate the audio.
RC CIRCUIT
The RC circuit (Figure 2) is for attenuation and filtering harmonics of the PWM signals. It prevents the saturation
of the low pass filter and the audio amplifier. After passing the signal to this RC circuit, the signal will change to
a trangular wave. Figure 3 shows the ouput waveform of the RC circuit. The amplitude of each trangular wave
is proportional to the value of each sample.
This document contains information on a product under development. Motorola reserves the right to change or discontinue this product without notice.
SEMICONDUCTOR PRODUCT INFORMATION
 1996 Motorola, Inc. All Rights Reserved.
.
Figure 2. RC Circuit.
PWM OUTPUT SIGNAL
RC CIRCUIT OUPUT SIGNAL
Figure 3. Output Waveform of the RC circuit.
2
DragonBallTM MC68EZ328 Application Notes
MOTOROLA
FOURTH-ORDER LOW PASS FILTER
Figure 4 shows the schematic of the low pass filter. This four-order low pass filter is consisted of LM385 Low
Power Op-Amp. It removes most of the harmonic generated by the PWM output waveform. The cut-off
frequency of the circuit is about 21kHz.
Figure 4. Fourth-order Butterworth Low Pass Filter.
AUDIO AMPLIFIER
The MC34119 Low Power Audio Amplifier is chosen for this application. This amplifier has the Chip-Disable
(CD) pin, which can be set high to shut down the amplifier when it is not in use, thus minimizing the power
consumption of the system.
Figure 5. Audio Amplifier.
The gain of the amplifier is calculated by the following equation.
R32
Gain = 2 × ---------R33
MOTOROLA
DragonBallTM MC68328 Application Notes
3
SOFTWARE
In order to minimize the size of data storing the sound samples, the sound samples is at 8kHz sampling rate.
It is because the human can heard the 8kHz carrier frequency. So, we need to set the reconstruction rate
of PWM output to 32kHz and repeat the same sample to PWM output three times. Figure 6 shows the
diference between the PWM output waveforms with reconstruction rate at 8kHz, 16kHz and 32kHz. The
number of repetitions of each sample and reconstruction rate can be selected by changing the PWM Control
Register of MC68EZ328. The DragonBallTM-EZ User’s Manual will describe the details of programming the
PWM module. A example program will be attached with this document.
Figure 6. Waveform of PWM Output.
4
DragonBallTM MC68EZ328 Application Notes
MOTOROLA
APPENDIX: EXAMPLE AUDIO GENERATION CIRCUIT
MOTOROLA
DragonBallTM MC68328 Application Notes
5
A
B
C
D
1
~SOUND_EN
PWMO
VR2
5K
2
R34
10K
R35
10K
3
R30
100
C40
4700p
3
VCC
+
-
Q4
BC857
U10A
1
R36
10K
4
R37
10K
C41
4700p
5
6
+
-
R41
2
R40
10K
C46
4700p
R39
39K
LM358
R38
1.2K
3
39K
C45
4700p
LOW PASS FILTER
POWER ENABLE
C47
4700p
2
4
5
C39
0.1u
U10B
7
LM358
5
6
R33
3K
6
3
2
1
4
7
7
6
7
5
8
R31
VOUT1
VOUT2
100
CD
VIN
VCC
GND
MC34119
FC1
FC2
U9
R32 75K
C48
4.7u
LOW POWER AUDIO AMPLIFIER
0.1u
C43
C42
0.1u
Audio Generation Circuit with Enable Circuit
~SOUND_EN
PWMO
1
8
4
8
4
VCC
BC857
Q5
C44
0.1u
LS1
8
SPEAKER
8
A
B
C
D
10
A
B
C
D
U10A
10K
R36
E
10K
R37
C41
4700p
6
VCC
+
-
F
7
U10B
G
G
R33
3K
H
R32 75K
1
4
3
2
CD
VIN
VCC
GND
VOUT1
VOUT2
U9
FC1
FC2
MC34119
H
LOW POWER AUDIO AMPLIFIER
0.1u
C43
C42
0.1u
Audio Generation Circuit without Enable Circuit
VCC
+
-
1
5
8
6
7
VCC
I
C44
0.1u
LS1
SPEAKER
J
10
1
2
3
4
5
6
9
C40
4700p
2
LM358
F
C48
4.7u
9
R35
10K
5
J
8
R34
10K
R41
C46
4700p
R40
10K
LM358
R39
39K
C45
4700p
3
I
8
PWMO
C47
4700p
R38
1.2K
D
39K
LOW PASS FILTER
C
E
7
PWMO
VR2
5K
B
7
6
5
4
3
2
1
A
8
4
8
4