📄 jesper's avr pages - minidds.htm
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<P><BR></CENTER><FONT face=Verdana size=+3>Mini DDS</FONT> <I>(Direct Digital
Synthesis)</I> <!-- end of header part --><IMG height=225 alt="" hspace=30
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<P>I often need a simple function generator. Just to generate a certain
frequency. After all the years I've worked with electronics, I still haven't got
me one. Even though I need it now and then, I just couldn't seem to justify the
cost of one.<BR>So, standard solution - build one yourself.<BR>I designed a
simple sinewave generator based on a Analog Devices AD9832 chip. It will
generate a sinewave from 0.005 to 12 MHz in 0.005 Hz steps.<BR>That's pretty
good, and definitely good enough for me ! But while waiting for the AD9832 chip
to arrive, I came up with a very simple version of the DDS synth, using just the
2313 and a resistor network.
<P>It's controlled over RS232 from a small Windows program, and can generate
Sine-, Sawtooth-, Trangle- and Sqare-waves ranging from 0.07 Hz to about 200-300
kHz in 0.07 Hz steps (<I>depending on your crystal</I>).
<P>
<CENTER><IMG height=165 alt=""
src="Jesper's AVR pages - MiniDDS.files/dds_control.gif" width=263
border=0><BR><I>Windows Control program</I>
<P></CENTER><B>I wont go into details about how a DDS synth works. Maybe later.
The code is pretty simple so you should be able to understand how it works by
just reading through it.</B>
<P>
<CENTER><IMG height=252 alt="" src="Jesper's AVR pages - MiniDDS.files/pcb.jpg"
width=300 border=0> <BR><I>Here's the MiniDDS on a PCB that fits in a TEKO
box.</I> </CENTER>
<P>There's not much to say about the schematic. It's as simple as can be. Just 4
major parts. A voltage regulator/switch, a RS232 interface chip, the 2313 and
the R2R resistor network. The R2R network is connected to PORTB on the 2313,
making it a simple D/A converter and makes it possible to output 256 voltage
levels. Neither the resistor network or port drivers of the 2313 is of perfect
linearity, but it works pretty well anyway. But you'll probably need a buffer
stage as the output impedance is rather high (tens of kOhms in my case). <BR>The
MAX603 handles the voltage regulation as well as the powerup/shutdown function,
and is controlled by the DTR signal on the serial interface. So when you shut
down the control program on the PC, the miniDDS will be shutdown, saving
battery.
<P>
<CENTER><A href="http://www.myplace.nu/avr/minidds/minidds_schematic.gif"><IMG
height=260 alt=""
src="Jesper's AVR pages - MiniDDS.files/minidds_schematic_small.gif" width=400
border=0></A> <BR><I>Click image to see a larger schematic.</I> </CENTER>
<P>The software is written in assembler, as it is very short and it need the
speed in the main loop.<BR>This is the heart of the synth. To be precise, it's
in the 7 lines of assembler that makes up the main loop.<BR>The rest of the code
is the wavetables and the communication code.<BR>The phase accumulator uses 24
bits, which determines the resolution of the output frequency. Maximum available
frequency and resolution is also dependent on your crystal frequency :
<P><B>Resolution = fCPU/150994944</B> and
<P><B>fOut = Accumulator * Resolution</B>
<P>In my case with a <B>11.059200 MHz</B> crystal, the resolution is
<B>0.073242188 Hz</B>.<BR>To get an output frequency of 1 kHz, we need to use a
phase accumulator value of <B>0x003556 (13654 decimal)</B>.<BR>This gives an
output frequency of <B>1000.048835 Hz</B>. Good enough for most hobby work !.
<P>The communication is pretty simple and just allows you to set and read the
phase accumulator value, as well as selecting the type of output waveform.<BR>It
uses a 32 bit value for the phase accumulator to be compatible with the larger
DDS circuit using the AD9832 chip.
<P>
<CENTER><IMG height=225 alt=""
src="Jesper's AVR pages - MiniDDS.files/square.jpg" width=300 border=0> <IMG
height=225 alt="" src="Jesper's AVR pages - MiniDDS.files/sawtooth.jpg"
width=300 border=0> <BR><I>The above pictures (as well as the top one with the
sine wave) show the waveforms at 100kHz</I> </CENTER>
<P>
<P><FONT size=+1><A
href="http://www.myplace.nu/avr/minidds/minidds_schematic.gif">View
schematics</A></FONT>
<P><FONT size=+1><A
href="http://www.myplace.nu/avr/minidds/minidds.asm">Download
ASM-code</A></FONT>
<P><FONT size=+1><A
href="http://www.myplace.nu/avr/minidds/ddscontrol.zip">Download Windows Control
Program 1.02</A></FONT> <BR><FONT size=+2> <BR> <BR></FONT>Comments
are always welcome at <A href="mailto:jesperh@telia.com">jesperh@telia.com</A>
<FONT size=+2> <BR> <BR></FONT>
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