Nano function generator - Ep.1

	NanoRama Enter Your Project for a chance to win a Nano Grand Prize bundle for the most innovative use of Arduino plus a $400 shopping cart! Submit an EntrySubmit an Entry  Back to homepage	Project14 Home
		Monthly Themes
		Monthly Theme Poll

I think Nanorama initiative is a great idea, so I would really love to contribute, so this is a project I started to work on.

Unfortunately, because of lockdown, I don’t have full access to my laboratory, but nothing can stop a maker (well, except may be the maker’s wife, but that’s another matter), thus I can not test on a real board: I will provide more details as soon as restrictions will fade

From time to time, while debugging embedded software, I need to run the same sequence of digital inputs. For example, if you have an issue when a particular sequence of events occurs, or when a certain sequence of buttons is pressed by the user. So, why not build a programmable function generator?

I want to do something lean-and-mean, because it’s something I will use just few times. I do not want to connect a display, make up all the menus with the buttons to navigate, etcetera.

Do you remember the old good piece of hardware like this one?

where the only interface was a serial connection and menus were built using VT100 escape sequences?

Well, I want to build an interface like that: I will configure the function generator by means of a VT100 terminal interface

Luckily, there is a library that wraps VT100 escape sequences into a set of handy functions. This is the starting point to build a menu-based UI

Since I have some time to spend on this project, my function generator will implement the following functions

1. Sine wave
2. Square wave (PWM)
3. Triangle wave
4. Sawtooth wave
5. User-defined

I have a spare Arduino Nano, so this will be the target board for this project. After all, I think it is a good choice because it is small in size and the same board is shipped in 3.3V and 5V version. This means I can use the same firmware to generate signal with different amplitudes

The Arduino Nano, however, does not have a DAC converter, so the analog waveforms must be generated by means of a PWM and a low-pass filter.

As a quick reminder, the dc value, given the on and off time, can be calculated using the following formula

You can find a great explanation at this link

https://create.arduino.cc/projecthub/Arduino_Scuola/build-a-simple-dac-for-your-arduino-4c00bd

The PWM frequency on Arduino Nano is 480 Hz (or 960 Hz on pin 5 and 6). So we need to design a low pass filter that cuts well below this frequency (let’s say about 10 Hz – this means that the max frequency of a sin wave we can generate should be less than about 2 Hz in order not to attenuate the signal)

I am going to use a passive filter, because I expect to have loads that takes less than 10mA. So the circuit for the low-pass filter is very simple:

The cut-off frequency is given by the formula

With R = 6,8 kOhm and C = 2.2 uF, the cut-off frequency is 10,64 Hz, which is good

Back to the code, the analog waveforms are generated using lookup tables generated using this site

https://www.daycounter.com/Calculators/

Each table has 128 samples. Every main cycle, I perform the following code

I should evaluate whether to use a linear interpolation between two table’s point to get a better accuracy

For the user-defined functions, we can enter a sequence of high and low and the duration of each step

The source (still work in progress) is available on github

https://github.com/sataur-code/nano-function-generator