Simple function generator using microcontroller

Question

Is it possible to make a function generator with a microcontroller (PIC18F4550 OR PICAXE20X2)?

I am thinking that for generating square waves it can be enough to make a pin output high and low on desired time frames. But how about a sinewave? Maybe it is possible with the same idea but putting a inductor or something at the output?

If you generaly think this can work I want to start doing it!

UPDATE First of all I have to thank all the answers, it really gave me some nice start points.

I have a rather good experience with PIC18F4550 as I did lots of small home project around this chip. So it would be my main choice. Here is a link to its datasheet

My main points is now as following: 1- Best way and mathematic approach for square waves 2- same thing for sine waves 3- Amplitude and frequency control

Answer 1

Not a complete answer, but to address one of your points:

I am thinking that for generating square waves it can be enough to make a pin output high and low on desired time frames.

That depends on your application. This is considered "naive" square wave synthesis, and doesn't produce a mathematically correct square wave. (It's equivalent to sampling an ideal mathematical function without putting it through an anti-aliasing filter first.)

This also applies to triangle waves, sawtooth waves, and anything else with harmonics above the Nyquist frequency.

It will often be "good enough" if you have many samples (or time frames) per cycle, but not otherwise. For example, if you generate a 10 kHz square wave with a 44.1 kHz sampling rate, it will look like this:

You can see that every few cycles are different lengths. The transitions can only occur on sample boundaries, but an actual square wave would transition at a time in between them. Practically, this results in lots of aliased harmonics below the square wave frequency, which you probably don't want, depending on your application. In audio applications, this sounds awful.

You can avoid this by generating a correct band-limited square wave in software, or by using a sampling frequency much higher than necessary for your signal.

Here's a comparison of the two methods on a 5 kHz square:

Simplistic:

Mathematically correct (generated with additive synthesis):

Answer 2

Jesper developed a DDS (Direct Digital Synthesis) function generator around an AVR, controllable via EIA-232 and PC.
Schematic and assembly code are simple and available on his website, so you should be able to adapt them for the PIC. Or you can simply use the AVR version.

Answer 3

Yes, a PIC can be used to make a function generator, especially since you didn't specify a accuracy or frequency.

If you want a arbitrary digital pattern, you can put the serial data in a table and use a SPI port to update the output pin one bit at a time from hardware. The firmware only needs to reload the hardware every byte. For a little slower, this can be done in a periodic interrupt without the SPI hardware.

For making analog signals, the easiest is to low pass filter a PWM output. Once again, there is a speed versus resolution tradeoff. With the PIC 18 running at 10 MHz instruction clock, you can get 8 bit resolution at 39 kHz. Even with just a bunch of passive resistors and capacitors, this can make reasonable quality voice audio.

For a higher resolution*frequency tradeoff, you can use a external D/A converter.

Unless this function generator is for a specific application and the PIC 18 is good enough for that, I'd use a PIC 24H instead. Those can handle 16 bits at a time and run at 40 MHz instruction rate. That gives you a 4x better PWM resolution*frequency tradeoff. Some of the dsPICs also have one of the two special high frequency PWM modules. These sortof run off a 16x higher internal clock to get nearly 1ns pulse width resolution. Watch out for jitter and restrictions on the duty cycle though.

Answer 4

For a sine wave the best way is to use what is known as a Wave Table. This is basically a list of discrete values to pass to a DAC to generate a waveform.

It uses more storage memory than generating a sine mathematically, but is far simpler and more efficient from a processing point of view. Plus, it has the advantage that the waveform generated can be changed by changing the wave table data.

There are many ways of interfacing a PIC to a DAC, and there are many ways of making a DAC yourself. It all depends on the resolution you require for the final waveform, and the frequency ranges you are looking to generate, and how accurate you want the output.

Answer 5

To begin to get other waveforms, you first have to work out how to acheive more than the two levels out of pin than digital outputs provide. There is a type of integrated circuit called a Digital to Analogue converter which is specifically designed for this task. A recommended way to use a DAC would be to connect to it over a serial link such as I2C or SPI, which minimises the number of pins you need on the micro compared to a parallel input DAC. Once you have got that working, you can send the sinewave or whatever waveform using either numbers that you calculate on the fly using a sinewave formula or from a look up table. Note that the lookup table for the sinewave only has to contain one quarter cycle of data, the other parts of the waveforms are repeats or inversions. A formula calculation will be slower than a lookup table, so your choice here depends on desired output frequency. However, for triangle and sawtooth waveforms, the formula is so simple that it would not be worth bothering with a lookup table.

Another approach is to use a 1-bit DAC approach, as described in theory here, and in an interesting example which is PIC specific here which is designed for simulating speech, so would only be usable for low audio frequencies.

Whilst a filter is part of the solution, it would not be enough to simply add an inductor on the output pin, and that would not be recommended on a digital IO pin because it creates a flyback voltage when it is switched off which will damage the output transistors.