AN69133 PSoC 3 / PSoC 5LP Easy Waveform Generation with the WaveDAC8 Component.pdf

AN69133
PSoC® 3 / PSoC 5LP Easy Waveform Generation with the WaveDAC8 Component
Author: Mark Hastings
Associated Project: Yes
Associated Part Family: All PSoC 3 and PSoC 5LP parts
Software Version: PSoC Creator™ 3.2
Related Application Notes: None
AN69133 describes how the WaveDAC8 component works and how to use it to generate either predefined or custom
waveforms. The WaveDAC8 uses DMA to generate continuous waveforms that require no CPU overhead. Several
example projects are included to show simple waveform generation, frequency shift keying (FSK) modulation, and
DTMF tone generation with minimal hardware and user code.
Contents
1
2
3
4
1
Introduction ...............................................................1
Configuring WaveDAC8 ...........................................2
The WaveDAC8 Core ...............................................3
Example Projects......................................................5
4.1
1_WaveDAC8_SimpleSine Project ..................5
4.2
2_WaveDAC8_TwoWaves Project ..................7
4.3
3_WaveDAC8_UART_FSK Project .................8
4.4
4_WaveDAC8_DTMF_EN Project ................. 10
Over Sampling and Filters ...................................... 13
Summary ................................................................ 17
Appendix A: Code Listing for
4_WaveDAC8_DTMF_EN Project.......................... 18
Document History............................................................ 22
Worldwide Sales and Design Support ............................. 23
5
6
7
Introduction
The WaveDAC8 component provides a simple and fast solution for periodic waveform generation. WaveDAC8 has
the following features:








Standard and arbitrary waveform generation
Output may be voltage, current sink, or current source
Hardware selection between two waveforms
External clock input may be used to change the output waveform frequency
Waveform tables can be up to 4000 points
Predefined sine, triangle, square, and sawtooth waveforms are included
Allows you to change waveform arrays during run time
Single line of C code required to initiate waveform output
Figure 1 shows two WaveDAC8s in a schematic, the one on the right shows options enabled for and external clock
and an optional internal voltage buffer. An internal clock is usually selected when a fixed digital-to-analog converter
(DAC) sample rate is used. An external clock provides an additional way to modulate the output or to synchronize the
WaveDAC8 with additional logic. The internal buffer uses one of the on board opamps in the follower mode to drive
up to a 25 mA load.
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PSoC® 3 / PSoC 5LP Easy Waveform Generation with the WaveDAC8 Component
Figure 1. Internal and External Clock Configurations
Internally, the WaveDAC8 contains a voltage or current DAC, two direct memory access (DMA) channels, optional
opamp follower and a clock when the internal clock option is selected. The user interface and the API handle
configuration of the DAC, DMA, and wave table generation. No knowledge of the DAC or DMA is required to take full
advantage of the WaveDAC8 component. For more detailed documentation of all the WaveDAC8 parameters and
terminal definitions, see the datasheet included with the component.
2
Configuring WaveDAC8
The graphical user interface (GUI) of the WaveDAC8 configuration dialog makes it easy to configure the waveform
shape and frequency. To invoke the WaveDAC8 user interface, simply double-click on the component after it is
placed in the schematic. The other option is to right-click on the symbol and select Configure from the menu window.
See Figure 2 for an example of the configuration dialog.
The configuration dialog is divided into three sections. The top section determines the DAC range and mode, the
sampling rate, and whether the clock is internal or external. These settings affect both waveforms configured in the
other two sections, Waveform 1 and Waveform 2. You can select from ten output range options, four voltage ranges
(2 buffered and 2 unbuffered), and six current ranges with the Range Selection parameter in the upper-left corner of
the dialog box.
In the upper-right corner of the dialog, you can select either an internal or an external clock. If you select an internal
clock, you can also configure the DAC sample rate. When you select an internal clock source, the top of the
waveform areas display the waveform frequency and period. If you select an external clock, note the addition of a
clock terminal on the WaveDAC8 schematic symbol.
Figure 2. WaveDAC8 Configuration Dialog
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PSoC® 3 / PSoC 5LP Easy Waveform Generation with the WaveDAC8 Component
The Waveform 1 and Waveform 2 sections allow you to define two separate waveforms that share sample rate and
DAC output range. The state of the “ws” input terminal determines which waveform is generated. A logic low on this
terminal selects Waveform 1 and a logic high selects Waveform 2.
One of the four predefined waveforms may be selected: Sine, Square, Triangle, or Sawtooth. There are two options
to generate a custom or arbitrary waveform. The user can either draw a waveform right on the screen by selecting
the “Arbitrary (draw)” option or select “Arbitrary (from file)” to import a file. To draw a waveform, move the cursor over
the displayed waveform area and press the left mouse button to draw the waveform.
If a waveform must be generated during runtime, there are two provided functions that allow the programmer to use
any memory array to generate the waveform. See the WaveDAC8_Wave1Setup() or WaveDAC8_Wave2Setup()
function descriptions in the WaveDAC8 component datasheet.
The output signal frequency is a function of the Sample Rate (SPS) and the Samples parameter for each of the
waveforms, as shown in Equation 1. The Samples parameter defines how many values in the lookup table will be
used to create each waveform period.
frequency 
SampleRate
Samples
Equation 1
Because each waveform has its own Samples parameter, two waveforms with the same sample rate may have a
different output frequency. For example, if the sample rate is set to 100 kHz, the Samples parameter for Waveform 1
is set to 100 and 50 for Waveform 2, and then each waveform will have a different output frequency. Waveform 1 will
have an output frequency of 1 kHz and Waveform 2 will have a frequency of 2 kHz, as shown in Equations 2 and 3.
Using this configuration, frequency shift keying (FSK) can be achieved by changing the state of the “ws” input.
freqWaveform1 
100,000
 1000 Hz
100
Equation 2
The output frequency for Waveform 2 will be:
freqWaveform2 
100,000
 2000 Hz
50
Equation 3
Refer to the Over Sampling and Filters section when determining how many samples per waveform is right for your
application. It provides an overview of using a sampled system and options to minimize output waveform distortion.
The Amplitude and Offset parameters allow you to scale and add a voltage (or current) offset to the waveform.
The user interface provides a simple interface to quickly select the desired waveform. Other than that to start the
component in the user’s code, there is no other configuration or functions required to generate a waveform.
3
The WaveDAC8 Core
The core of WaveDAC8 is the DAC. It will either be a standard VDAC8 (voltage DAC) or IDAC8 (current DAC)
depending on the range selected. Therefore, the WaveDAC8 analog electrical specifications are identical to those of
the standard PSoC Creator VDAC8 and IDAC8 components. This is a good example of how PSoC Creator allows the
use of standard components to build more complex ones, without reinventing the functionality and APIs of the original
components.
The two DMA channels, Wave1_DMA and Wave2_DMA, are used to transfer the waveform array data in memory to
either the IDAC or VDAC. When you configure a waveform with the user interface, the component automatically
configures each of the DMA channels to transfer the data. Both of these DMA channels transfer data to the DAC, but
only one can operate at a time. The wave select “ws” input selects which of these DMA channels is triggered by the
clock, using the demultiplexer “DMA Select”, to route the signal to the corresponding DMA channel. The two wave
complete outputs “wc1” and “wc2” can be used to signal that the DMA channel transferred the last value in the
waveform table, or that one full waveform is complete. Figure 3 shows the internal WaveDAC8 component
schematic.
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PSoC® 3 / PSoC 5LP Easy Waveform Generation with the WaveDAC8 Component
Figure 3. Internal Schematic of WaveDAC8
The multiplexer labeled “Clock Select” is a virtual mux that is set during configuration and removed automatically
during the build process. It is used to select whether an internal or external clock is used. For example, if the external
clock option is selected, the internal clock “DacClk” will be removed from the project and the virtual mux will connect
the clock input terminal “clk” to the VDAC8 and IDAC8 strobe inputs and the DMA Select demultiplexer. If the internal
clock option is selected, the “clk” terminal will be removed and the “DacClk” clock output will be connected to the
IDAC8, VDAC8, and DMA Select multiplexer instead.
The two analog demultiplexers labeled “Voltage Out” and “Current Out” is also virtual. They are configured such that
one DAC output will be connected to the output terminal “Wave” and the other will not be connected. The DAC that is
not used, based on the current or voltage range, will be removed. The BuffBypass virtual multiplexer is used enable
or disable the BuffAmp (opamp).
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PSoC® 3 / PSoC 5LP Easy Waveform Generation with the WaveDAC8 Component
Figure 4. WaveDAC8 Data Flow
System
Memory
WaveDAC8
Data Flow Diagram
Output DAC
Waveform 1
viDAC8
Wave
Out
DAC
Waveform 2
Wave
Select
Two important parts of the WaveDAC8 component are the waveform arrays and the firmware that forms the API. The
two arrays or tables contain the data that is continuously copied to the DAC with DMA to create the waveforms.
Figure 4 show the internal data flow from the arrays to the DAC. The firmware takes care of all the initialization of the
IDAC8 or VDAC8 and the DMA channels. This is what makes this component so easy to use.
4
Example Projects
The following four example projects show how easily you can configure and use WaveDAC8. The examples include
generation of a simple waveform, FSK modulation, and a very simple DTMF phone dialer. A workspace with all four
projects and the WaveDAC8 library are included with this application note. Although the projects are configured for
the PSoC 3, they will work equally well with the PSoC 5LP by just choosing the PSoC 5LP used in your design with
the Device Selector in the Project menu.
4.1
1_WaveDAC8_SimpleSine Project
This very basic example generates a 1-kHz sine wave. Figure 5 shows the PSoC Creator schematic for this example.
Figure 5. WaveDAC8_SimpleSine Project Schematic
Notice that the schematic contains only three components, WaveDAC8, analog output pin (P00_Vout), and a logic “0”
to select Waveform 1. The configuration of the WaveDAC8 is shown in Figure 6. Only the settings for “Range
Selection”, “Clocking – Sample Rate” and parameters in the “Waveform 1” area matter because Waveform 2 is not
selected in this project.
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PSoC® 3 / PSoC 5LP Easy Waveform Generation with the WaveDAC8 Component
Figure 6. WaveDAC8 Configuration
The code for this example is as simple as the schematic:
#include <device.h>
void main()
{
/* Start WaveDAC8 */
WaveDAC8_1_Start();
for(;;);
/* Loop forever */
}
Notice that the only code required is the “WaveDAC8_1_Start();” function. Setting up the DAC, Internal Clock, and
DMA is handled by the Start() function. After the project is built and the device is programmed, an oscilloscope can
be connected to port pin P0[0] to see the waveform shown in Figure 7.
Figure 7. Output of Project 1_WaveDAC8_SimpleSine
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PSoC® 3 / PSoC 5LP Easy Waveform Generation with the WaveDAC8 Component
This is all that is required. This sine wave will be generated continually with zero CPU overhead until the device is
powered down or the WaveDAC8_1_Stop() function is called. This example can be easily modified to switch between
two waveforms. The easiest method is to connect the “ws” terminal to an I/O pin and control the waveform with an
external switch.
4.2
2_WaveDAC8_TwoWaves Project
This next example adds a little more hardware to alternate between Waveform 1 and Waveform 2. It uses the two
waveform-complete signals, “wc1” and “wc2”, so that it does not switch from one waveform to the other until the
entire waveform is completed. The wc1 and wc2 signals go high after the last value in the waveform table for each
waveform is written to the DAC, therefore, signaling the end of waveform period. Figure 8 is the schematic for this
example project.
Figure 8. 2_WaveDAC8_TwoWaves Project Schematic
The configuration of the WaveDAC8 is shown in Figure 9. Again, the component is configured to use a buffered
voltage DAC output, internal clock, sine wave, and a sample rate of 100 kHz. This time the Samples parameter for
Waveform 1 is set to 200, which will generate an output frequency of 500 Hz. Waveform 2 will also generate a sine
wave, but the Samples parameter is set to 50, which will output a frequency of 2000 Hz.
Figure 9. WaveDAC8 Configuration
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PSoC® 3 / PSoC 5LP Easy Waveform Generation with the WaveDAC8 Component
When the wave select signal “ws” is low, Waveform 1 will be output, which is the slowest of the two waveforms at
500 Hz. When “ws” is high, Waveform 2 will be output with a frequency of 2000 Hz. If you place a scope probe on
pins P0[0] and P0[4], you can see both the waveform output and the waveform selection signal shown in Figure 10.
Figure 10. Output of Project 2_WaveDAC8_TwoWaves
This output is a very simple form of FSK. As before, the code required to generate this modulated waveform is
minimal:
#include <device.h>
void main()
{
/* Initialize WaveDAC8 */
WaveDAC8_1_Start();
for(;;);
/* Loop forever */
}
4.3
3_WaveDAC8_UART_FSK Project
In this third example, another form of FSK is demonstrated. This time, a single waveform is used, but two external
clocks are used to modulate the output signal. By adding the transmit (TX) half of a UART, a fully functional FSK
transmitter will be demonstrated. The schematic for this example is shown in Figure 11.
Figure 11. 3_WaveDAC8 project Schematic
This example emulates a 1200-baud modem in which the output signal switches between 1200 and 2200 Hz. In the
WaveDAC8 configuration shown in Figure 12, only the settings affecting Waveform 1 are important. Notice that the
Samples parameter is set to 100; if you divide the two clock outputs by 100, you will get the 1200 and 2200 Hz sine
waves required.
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PSoC® 3 / PSoC 5LP Easy Waveform Generation with the WaveDAC8 Component
Figure 12. WaveDAC8 Configuration
The code for this example contains a few more lines, but it does more too. It continuously transmits the message
“Hello World” in an FSK-modulated format. The amount of code that is required for the designer is again rather small:
#include <device.h>
void main()
{
/* Initialize WaveDAC8 */
WaveDAC8_1_Start();
UART_1_Start(); /* Initialize UART */
Clock_1_Start(); /* Start both clocks */
Clock_2_Start();
for(;;)
{
/* Send "Hello World" */
UART_1_PutString((uint8 *)"Hello World");
CyDelay(250);
/* Wait 500 mSec */
CyDelay(250);
}
}
The FSK-modulated output can be seen on pin P0[0] and the digital output of the UART transmitter may be observed
on pin P0[4], as shown in Figure 13. Notice that the 2200-Hz tone is present when the TX line is low and the 1200-Hz
tone is present when the TX line is high.
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PSoC® 3 / PSoC 5LP Easy Waveform Generation with the WaveDAC8 Component
Figure 13. Output Waveforms
4.4
4_WaveDAC8_DTMF_EN Project
This fourth and final example implements a simple dual tone multiple frequency (DTMF) phone dialer. It requires two
WaveDAC8s so that it can generate two tones at the same time. See Figure 14 for the schematic.
A single clock in conjunction with two counters generates the proper sampling clock for each WaveDAC8. The period
of the two PWMs, used as frequency dividers, “Col_Divider” and “Row_Divider” is changed for each tone to provide
the proper column and row tones respectively. An opamp “DTMF_Buffer” is added to buffer the output of the
WaveDAC8s so a lower impedance load can be driven.
At first, connecting two voltage DACs in parallel might not sound like good design practice, but it is safe with the
PSoC 3 or PSoC 5LP DACs. The PSoC voltage DACs are actually current DACs with an internal resistor. When you
place two of these DACs in parallel, you are simply combining two current DACs and two parallel resistors. The
output of these two combined DACs is the sum of the two signals divided by two. This is a useful feature when a
design requires combining two signals.
Figure 14. DTMF Encoder Schematic
Table 1 shows the standard DTMF tones for each key. Although the digits 0 through 9, “*”, and “#” are the most
common, the characters A, B, C, and D are also supported. When a single key is pressed, two tones are mixed
together. For example, if the “5” key is pressed, the column tone 1336 Hz and the row tone of 770 Hz are mixed
together. These eight tones were selected to ensure that any harmonics and inter-modulation products do not cause
a signal that can be mistaken for one of the other tones. Notice that no frequency is a sum, difference, or multiple of
one or more of the other frequencies.
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PSoC® 3 / PSoC 5LP Easy Waveform Generation with the WaveDAC8 Component
Table 1. DTMF Keypad Frequencies
Row Tones
Column Tones
1209 Hz
1336 Hz
1477 Hz
1633 Hz
697 Hz
1
2
3
A
770 Hz
4
5
6
B
852 Hz
7
8
9
C
941 Hz
*
0
#
D
To create each of the tone combinations in the DTMF table, the tones are divided into the column tones (1209 Hz,
1336 Hz, 1477 Hz, and 1633 Hz) and the row tones (697 Hz, 770 Hz, 852 Hz, 941 Hz). The row tones are generated
by the “Row_Tone” instance of the WaveDAC8 and the column tones are generated by the “Col_Tone” instance of
the WaveDAC8. Each tone frequency is a function of the clock “ToneClock”, the dividers “Col_Divider” and
“Row_Divider”, and the number of samples per cycle set in the WaveDAC8s configuration.
ToneFreq 
ToneClockFreq
Divider  Samples
Equation 4
For this example, the samples per cycle Samples parameter is set to 50 for the column frequencies and 80 for the
row frequencies. These values are set in the WaveDAC8 Configure dialog box for each waveform with the Samples
parameter. The only thing left to calculate is the divider value, which is the period of each of the counters,
Row_Divider and Col_Divider. If you solve for the divider value, you get:
Divider 
ToneClockFreq
ToneFreq  Samples
Equation 5
When using a counter as a divider, the divide by value is the counter period plus 1. Taking the previous equation and
solving for the counter period:
CounterPer iod 
ToneClockFreq
 1 Equation 6
ToneFreq  Samples
For example, suppose you want to calculate the counter period to generate the row tone if the “5” key is pressed.
Using the previous equation and substituting 770 Hz for the row tone, 80 for the sample size, and 12 MHz for the
clock, you get a period of 194.
CounterPer iod 
12,000,000
 1  194
770  80
Equation 7
Taking the DTMF tone in Table 1 and using the above equation to calculate the counter period for the DTMF
frequencies, the rest of the values can be filled in Table 2.
Table 2. Counter Period Values
Row_Divider
Period
Col_Divider Period
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198 (1209 Hz)
179 (1336 Hz)
161 (1477 Hz)
146 (1633 Hz)
214 (697 Hz)
1
2
3
A
194 (770 Hz)
4
5
6
B
175 (852 Hz)
7
8
9
C
158 (941 Hz)
*
0
#
D
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PSoC® 3 / PSoC 5LP Easy Waveform Generation with the WaveDAC8 Component
The source code for the example is a few more lines but still very easy to follow. You can view the entire source code
listing in Appendix A.
This implementation is not the only possible approach to making a DTMF dialer, but is a good compromise between
hardware resources, code size, and ease-of-use. Here are some alternatives with which you may want to experiment:

Use two clocks and remove the counters. Change the clock dividers to vary the output tones.

Use only one WaveDAC8 but create a larger lookup table for each tone combination. This may require large lookup
tables. To save RAM, just one lookup table can be used but modified each time a key is pressed.

Modify the internal clock dividers to change frequencies.
4.4.1
Examining the output
Figure 15 is an example of the output signal. The two mixed signals are 1336 Hz and 770 Hz simulating the “5” key
pressed on the keypad.
Figure 15. Tones 770 Hz and 1336 Hz Combined
If you enjoy experimenting with some of the more advanced features of your scope such as the FFT module, try using
the same dual tone output as in Figure 15 and enable FFT. Two waveforms appear as shown in Figure 16.
Figure 16. Analyzing DTMF Output of a "5" Key
The input (upper waveform) simulates a ‘5’ on a DTMF keypad being pushed repeatedly. The lower waveform is the
FFT of the input signal. Just as expected, the 770- and 1336-Hz tones show up clearly in the FFT.
In this next test, a simulated key sequence of 1, 5, 9, and D are output. This tests each of the eight different tones
used for the DTMF keypad. Figure 17 clearly shows each of the row and column tones in the FFT.
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PSoC® 3 / PSoC 5LP Easy Waveform Generation with the WaveDAC8 Component
Figure 17. Simulated "1 5 9 D" Key Sequence
5
Over Sampling and Filters
The WaveDAC8 is an easy component to use, but there are a couple of aspects that need to be considered when
using any type of sampled system. A sampled system is one that uses an ADC to sample an AC waveform or a DAC
to generate an AC waveform. The basic system and filter requirements for these systems are similar. Most engineers
are familiar with the Nyquist sampling theorem, which states that you must sample a signal at more than twice its
highest frequency component to reconstruct it. This exact concept holds true for a DAC. In the ADC system shown in
Figure 18, the anti-alias filter limits the input signal bandwidth to less than half of the ADC’s sample frequency.
Figure 18. ADC Sampling System
Input
Signal
Anti-Alias
Filter
LPF
Sampling
ADC
ADC
n
Data
When using a DAC, the system looks identical but in reverse, except for replacing the ADC with a DAC.
Figure 19. DAC Sampling System
Sampling
DAC
Data
n
DAC
Reconstruction
Filter
Output
Signal
LPF
When sampling a continuous signal, the output is turned into a series of discrete points. The output of a DAC before
being filtered may look like a series of stair steps that follow the original signal. The size of these steps, depend on
the DAC resolution and the sample rate of the DAC. The width of these steps is the inverse of the sampling
frequency.
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PSoC® 3 / PSoC 5LP Easy Waveform Generation with the WaveDAC8 Component
Figure 20. Result of Sampling
Sampled Signal
time
DAC Output
1
fs
time
A waveform with large “stair step” edges has a broad frequency spectrum, much higher than the desired output
bandwidth. The actual output spectrum of a DAC looks like the typical sync function.
  f
sin 
f
Amplitude   s
  f 


 fs 



Equation 8
Figure 21 shows the output frequency spectrum of a DAC. Harmonic images appear on each side of the sampling
frequency harmonics that mimic the original bandwidth of the sampled signal. To simplify the diagram, assume the
desired output signal is a sine wave.
Figure 21. DAC Output Frequency Spectrum
Fundamental
Signal
0
Images
Images
Images
|A|
0.5fs
fs
1.5fs
2fs
2.5fs
3fs
3.5fs
4fs
fs is the DAC sample rate
A reconstruction filter is used to attenuate the images to the point where the total harmonic distortion (THD) is low
enough for the application. Ideally, you want to keep this filter small and simple to reduce the cost.
Another way to reduce THD is to increase the over sample rate. When a signal is sampled at more than twice the
maximum bandwidth of that signal, it is said to be over sampled. The over sample rate is defined by:
OverSampleRate 
fs
2B
Equation 9
Where fs is the sample frequency and B is the desired bandwidth of the signal.
Increasing the over sample rate increases the distance between the fundamental frequency and the first image. This,
in turn, also reduces the signal distortion by reducing the amplitude of the harmonic images.
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PSoC® 3 / PSoC 5LP Easy Waveform Generation with the WaveDAC8 Component
To explain this a little better, take the example where a 1-kHz sine wave must be generated and the sample rate is 4
ksps. Using Equation 9, this gives us an over sample rate of 2. Figure 22 shows the representation of a 2x over
sample rate in both the time and frequency domain.
Figure 22. Example of 2x Over Sampling Rate
(Amplitude)
A. Time Domain
(time)
B. Frequency Domain
0
-10
-20
|A|
-30
-40
0.5fs
fs
1.5fs
2fs
2.5fs
3fs
3.5fs
4fs
(freq)
When looking at the DAC output in the time domain in Figure 22(A), it is easy to see that the signal looks more like a
square wave than a sine wave. Looking at the frequency domain plot in Figure 22(B), it is not surprising that the
image harmonics are significant.
Using the following three options, we can reduce or attenuate the effect of the harmonics.



Add a reconstruction filter
Increase the over sample rate
Increase the over sample rate and add a filter
Depending on the application requirements, you may be forced into one of the options. If you need a high frequency
output that is close to the maximum sample rate, your only option may be to use an external reconstruction filter. A
filter will reduce the THD, but with a low over sample rate, it can be difficult to get the results needed with a simple
first-order filter since the first image is close to the fundamental. Figure 23 shows the problem in the frequency
domain between a 2x and 8x over sampled signal. Notice that the filter must be much sharper in the 2x system. Also,
note that the harmonic images in the 8x system are much lower than those in the 2x system and will require a less
complex and cheaper filter to get the same performance.
Figure 23. Reconstruction Filter
0
Fundamental
Signal
2x Over Sampled
0
Fundamental
Signal
8x Over Sampled
Images
Images
|A|
|A|
LP Filter
Bandpass
LP Filter
Bandpass
0.5fs
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fs
1.5fs
2fs
0.5fs
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fs
1.5fs
2fs
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PSoC® 3 / PSoC 5LP Easy Waveform Generation with the WaveDAC8 Component
A first-order filter will reduce the distortion from about 46 percent down to about 12 percent. A second-order filter will
further reduce the THD to less than 4 percent.
If we take the other approach and increase the over sample rate to 16x you can visually see that the generated signal
looks much more like a sine wave in Figure 24 (A). Notice that the first harmonic image is down about 30 dB
compared to only 10 db in the 2x over sampled system in Figure 22.
Figure 24. Example of a 16x Over Sample Rate
(Amplitude)
A. Time Domain
(time)
B. Frequency Domain
0
-10
-20
|A|
-30
-40
0.5fs
fs
1.5fs
2fs
2.5fs
3fs
3.5fs
4fs
(freq)
The THD now is about 5.4 percent, down from 46 percent, without adding a filter. Adding a simple first order filter, the
THD will be less 0.5 percent.
The plot in Figure 25 shows how increasing the over sample rate or adding a simple filter can greatly decrease the
output waveform distortion. The graph uses the harmonic images out to the 5th sampling harmonic to calculate the
distortion with Equation 10.
Figure 25. THD vs Over Sample Rate Guide
25
THD vs Over Sample Rate
THD (%)
20
15
No Filter
1st Order Filter
10
5
0
0
10
20
30
40
50
60
Over Sample Rate
THD 
V22  V32  ....  Vn2
%
V fund
Equation 10
Where Vn is each of the harmonics and Vfund is the fundamental frequency.
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PSoC® 3 / PSoC 5LP Easy Waveform Generation with the WaveDAC8 Component
A good rule of thumb, is that each time the over sample rate is increased by a factor of two; the distortion goes down
by a factor of two. A first-order filter combined with an over sample rate between 4 and 8 can bring the distortion
down very quickly. This may be the best and most economical solution for many applications. An example of firstorder passive RC filter is shown in Figure 26 and Equation 11.
Figure 26. First Order RC Filter
Vin
R
Vout
C
LPF
Fc 
1
2RC
Equation 11
The output resistance of the WaveDAC8 when in the voltage mode is 4 K and 16 K for the 1 V and 4 V ranges
respectively. When connecting a filter directly to the output of the WaveDAC8, consider this resistance. In some
applications, it may be a good idea to buffer the output of the WaveDAC8 with the buffer option as shown in the
following figure.
Figure 27. WaveDAC8 with Buffered Output
This application note has touched on the basic requirements of a reconstruction filter. This information should be
sufficient for most applications, but if your design requires something more complex, there are many filter software
applications and web pages on the internet to assist in your design.
6
Summary
The WaveDAC8 component is a good example of how a highly integrated component can greatly simplify user code.
These examples provide a good starting point for many applications. It is recommended that you refer to the
WaveDAC8 datasheet for detailed descriptions of the API functions. These examples can be run on any of the
PSoC 3 or PSoC 5LP development kits.
About the Author
Name:
Mark Hastings
Title:
Application Engineer MTS
Background:
Mark Hastings graduated from Washington State University in 1984 with a BSEE degree. For most
of the last almost 30 years he has been involved in embedded and mixed signal designs. His free
time is spent hiking in the Washington Cascades taking pictures.
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PSoC® 3 / PSoC 5LP Easy Waveform Generation with the WaveDAC8 Component
7
Appendix A: Code Listing for 4_WaveDAC8_DTMF_EN Project
/*******************************************************************************
* File Name: main.c
*
* Version 1.2
*
* Description:
* This example project demonstrates one way to make a DTMF dialer with
* two WaveDAC8s, two counters, and a clock.
*
* Hardware Dependency:
* Pin Connections:
* P0[0]] --> Dialer output
*
* Code Tested With:
* 1. PSoC Creator 2.2 Service Pack 1
* 2. DP8051 Keil 9.03
* 3. ARM GCC 4.4.1
*
* Theory of operation:
*
* To dial each digit in a phone number, two independent tones must be generated
* at the same time. Below is a table that shows each of the valid 16 keypad
* tones and the row and column tone for each digit.
*
*
DTMF tone matrix
*
Column tone (Hz)
*
1209
1336
1477
1633
* ROW tone (Hz) *------*------*------*------*
*
697 | 1
|
2 | 3
| A
|
*
*------*------*------*------*
*
770 | 4
|
5 | 6
| B
|
*
*------*------*------*------*
*
852 | 7
|
8 | 9
| C
|
*
*------*------*------*------*
*
941 | *
|
0 | #
| D
|
*
*------*------*------*------*
*
*
12.00 MHz
* Tone_Freq = ------------------------*
( Sample_Size * Divider )
*
* Solving for the divider
*
*
12.00 MHz
* Divider =
--------------------------*
( Sample_Size * Tone_Freq )
*
* The Sample size for the column tones is 50 and the sample size for the row
* tones is 80. These values were selected to keep the counters at 8 bits, but
* at the same time keep the tone frequency error less than 1%.
*
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PSoC® 3 / PSoC 5LP Easy Waveform Generation with the WaveDAC8 Component
* Example:
* To generate the row tone for a "5", which is 770 Hz and the sample size for
* row tones is 80;
*
*
12.00 MHz
* Divider =
------------ = 195
*
( 80 * 770 )
*
* Since the period length of a counter is the period register + 1, the value
* loaded into the counter period regiser would be 195 - 1 = 194. ( PWMs
* are used for counters in this design for ease of use. )
*
* The divider values for both the row and column tones are pre-calculated in
* arrays. The "rowDiv" array has the values for each of the four rows and
* "colDiv" contains the divider values for the columns.
*
******************************************************************************/
#include <device.h>
#define TONE_DURATION_MS
#define TONE_SPACE_MS
#define TONE_PAUSE_MS
80
50
100
/* Duration of tone in mSec */
/* Duration of space between tones in mSec */
/* Pause caused by invalid code */
/* Function prototypes */
void DialNumber( char * number );
void PlayTones( char key );
int8 KeyIndex(char * keyString, char key);
char keyCodes[] = "123A456B789C*0#D";
/* Valid character array */
/* The tables below store the dividers loaded into */
/* the counter period register to generate the row */
/* and column tones.
*/
/* Row Tones (Hz) 697 770 852 941
uint8 rowDiv[] = {214, 194, 175, 158};
*/
/* Col Tones (Hz) 1209 1336 1477 1633 */
uint8 colDiv[] = { 198, 179, 161, 146};
void main()
{
/* Initialize all components */
ToneClock_Stop();
Row_Divider_Start();
Col_Divider_Start();
Row_Tone_Start();
Col_Tone_Start();
DTMF_Buffer_Start();
/* Dial a phone number */
DialNumber( "555-123-4567" );
for(;;)
{
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PSoC® 3 / PSoC 5LP Easy Waveform Generation with the WaveDAC8 Component
/* Place user code here */
}
}
/*******************************************************************************
* Function Name: DialNumber( char * number )
********************************************************************************
*
* Summary:
* This function dials the number passed to it in a character string. After
* the number has been dialed the funtions returns.
*
* Parameters:
* char * number: Pointer to phone number string.
*
* Return:
* void
*
*******************************************************************************/
void DialNumber( char * number )
{
uint8 c = 0;
while( number[c] != 0 )
/* Step through the dial string */
{
PlayTones( number[c] ); /* Play tone for each digit
*/
CyDelay(TONE_SPACE_MS); /* Wait minimum space between tones */
c++;
}
}
/*******************************************************************************
* Function Name: PlayTones( )
********************************************************************************
*
* Summary:
* Generates the two tones required to dial the given key code. The valid
* dial characters are "1 2 3 4 5 6 7 8 9 0 A B C D * #". Any invalid
* character will generate a pause.
*
* Parameters:
* char key: Keypad character to be dialed.
*
* Return:
* void
*
*******************************************************************************/
void PlayTones( char key )
{
int8 idx;
uint8 row_div, col_div;
idx = KeyIndex(keyCodes, key);
/* Convert key to "keyCodes" array index */
if (idx >= 0 )
/* Is key valid */
{ /* Valid Key */
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PSoC® 3 / PSoC 5LP Easy Waveform Generation with the WaveDAC8 Component
col_div = colDiv[(uint8)(idx & 0x03)];
/* Get divider for column tone */
row_div = rowDiv[(uint8)((idx >> 2)& 0x03)];/* Get divider for row tone
*/
Row_Divider_WritePeriod(row_div);
Col_Divider_WritePeriod(col_div);
/* Set both dividers
*/
ToneClock_Start();
CyDelay(TONE_DURATION_MS);
ToneClock_Stop();
/* Turn on clock */
/* Wait for the tone duration */
/* Turn off clock */
}
else /* Invalid key, just pause for set period of time. */
{
CyDelay(TONE_PAUSE_MS);
}
}
/*******************************************************************************
* Function Name: KeyIndex( )
********************************************************************************
*
* Summary:
* This function finds the location of a character in a string and returns
* the index. If the character is not found a -1 is returned.
*
* Parameters:
* char * keyString: String to search through. This strings should be null
*
terminated and less than 255 characters in lenth.
* char
key:
Character to find in "String".
*
* Return:
* int8: Index of "key" in "keyString". If not found, return -1.
*
*******************************************************************************/
int8 KeyIndex(char * keyString, char key)
{
int8 i;
/* String index */
int8 charLoc = -1;
/* Location of character in string */
/* Search through string for character match */
for(i=0; (keyString[i] != 0); i++)
{
/* If character is found return index in string */
if (key == keyString[i])
{
charLoc = i;
break;
}
}
return charLoc;
}
/* [] END OF FILE */
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PSoC® 3 / PSoC 5LP Easy Waveform Generation with the WaveDAC8 Component
Document History
Document Title: AN69133 - PSoC® 3 / PSoC 5LP Easy Waveform Generation with the WaveDAC8 Component
Document Number: 001-69133
Revision
ECN
Orig. of
Change
Submission
Date
Description of Change
**
3232074
MEH
09/27/2011
New Spec.
*A
3450410
MEH
11/29/2011
Update project to PSoC Creator 2.0.
Updated template.
*B
3812634
MEH
11/23/2012
Updated Associated Part Family as “All PSoC 3 and PSoC 5LP parts”.
Updated Software Version as “PSoC Creator™ 2.1 SP1 or later”.
Updated Using these DACs in your Project (Updated figures 28 thru 32).
Replaced PSoC 5 with PSoC 5LP in all instances across the document.
*C
4082747
MEH
07/31/2013
Replaced the counters in the “4_WaveDAC8_DTMF_EN” project with PWMs.
*D
4815750
MEH
06/29/2015
Updated projects and application note to reflect version 2.0 of the WaveDAC8,
which is now part of the PSoC Creator installation.
Updated template
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Document No. 001-69133 Rev. *D
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PSoC® 3 / PSoC 5LP Easy Waveform Generation with the WaveDAC8 Component
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