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'Sinewave generation'
1998\12\12@075836 by Darren Logan

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Hi Pic'ers,

Anyone know how to generate a sinewave using PWM on say a PIC 12Cxx ?

I want to generate 100Hz (plus or minus 0.1Hz).

Any code will do, i.e. BASIC, C or assembly.

Thanks in advance.

Darren

1998\12\12@103045 by Chris Eddy

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Do you need to have a clean high accuracy sine wave?  You might try
building a simple op amp oscillator, and coupling the PWM signal in as a
sync signal.  Then you get the benefits of a clean wave with a clock
accuracy.  Simply low passing the PWM will surely mash the signal, by the
time you get all of the harmonics out, then you have to adjust it for
height afterwards.

Chris Eddy, PE
Pioneer Microsystems, Inc.

Darren Logan wrote:

{Quote hidden}

1998\12\12@103636 by Quentin

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Read AN 655, It will explain it to you. Best is to use a R-2R ladder with
4 to 8 pins, depending on your aplication. Basically what you do is to
create a lookup table with your sine steps (0 to 255 or 0 to 16)  in it.
Then your programs puts these values in sequence on your output pins. I
recently wrote a small test program that does a 4 bit sine for tone
generation and it worked great. Pity I lost it in a hard drive crash
otherwise I would have posted it to you.

Quentin

Darren Logan wrote:

{Quote hidden}

1998\12\12@191024 by Mike Keitz

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On Sat, 12 Dec 1998 17:35:43 +0200 Quentin <spam_OUTqscTakeThisOuTspamICON.CO.ZA> writes:
>Read AN 655, It will explain it to you. Best is to use a R-2R ladder
>with
>4 to 8 pins, depending on your aplication. Basically what you do is to
>create a lookup table with your sine steps (0 to 255 or 0 to 16)  in
>it.
>Then your programs puts these values in sequence on your output pins.

There are lots of ways to make sine waves, especially at 100 Hz.  A
resistor DAC works OK.  It is better to use a nonlinear resistor network
that is optimized for sine waves.  That trick dates back to very early
days of digital logic.  Five resistors in the ratio 1.5, 3.9, 4.7, 6.8,
and 20 makes a nice 16-sample approximate sine wave.  They are also
standard value resistors.  One end of each rresistor goes to a PIC pin
which is always an output, the other end of all 5 resistors goes to a
common node which is the sine output.  The digital sequence applied tot
he pins is pretty much what a shift register would make (early
implementations did of course use a shift register, but needed a few more
resistors.)

You end up with a "staircase" looking wave that still needs a little
analog filtering before you really can call it a sine.  Often just a
single capacitor to ground will be sufficient.

In the most recent generation of my CTCSS encoders (that is what the guy
who wants a 100 Hz sine wave needs it for, right?), I've used "magic
sine" techniques described by Don Lancaster (http://www.tinaja.com).  Only one
PIC pin is required, it is driven either on or tri-state for each sample
period.  During half of the sine wave, the pin drives high, during the
other half it drives low.  The sampling rate is quite high, hundreds of
samples per output cycle.  The sequence of whether to output a pulse for
each sample is stored in a table.  The table can represent as little as
1/4 of the full wave's samples if you re-use it properly for each
quadrant.  The data in the table is of course one bit for each sample.
The density of "1" bits increases and decreases in a sinusoidal manner.
Lancaster's site has many suitable sequences as well as methods to use to
derive new ones.

An R-C integrator will clean up the pulses to a decent sine wave.  Using
two integrators in series makes a really nice sine wave.  In order to
integrate properly, the integrator neds to be set up so the output
amplitude is much less than VDD (this causes the voltage across the
resistor to be relativley constant during each pulse).  An amplifier may
be needed if a level of more than 0.5Vpp or so is required.  But, the
same sine amplitude can be maintained over a wide range of frequencies
using the same filter if the output pulses are a constant width (i.e.
reduce the frequnecy by increasing a "dead" off-time after each pulse.
Then the energy coupled to the integrator per cycle is the same
regardless of the frequency).

The downside of the "magic sine" is that with 210 or more samples per
cycle it takes a lot more PIC time than a DAC based approach.  I haven't
tried anything with PWM.  I think software PWM would need even more CPU
than magic sine, but on a PIC with PWM hardware it could work well.

Someday I'll put all this on a web page.  For right now I'll just tease
you.


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