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Project Notes

#368 GranularSynth

Building an ATmega328 granular synth module in an Altoids can .. because granular synthesis is a basically how they make Altoids, right?


Here’s a quick demo .. if you can tolerate 5 minutes of me fiddling around!



I found out about the “Granular Synth” from Notes & Volts, and it’s history goes back to the Auduino developed by what appers to be the defunct “tinkerit”.


GranularSynth.ino is my adaptation of the original Auduino code. I have dropped support for processors other than the ATmega328.

Pin mapping used by the sketch is as follows:

Pin Function
Analog 0 Grain 1 pitch
Analog 1 Grain 1 decay
Analog 2 Grain 2 decay
Analog 3 Grain 2 pitch
Analog 4 Grain repetition frequency
Digital 3 Audio out
Digital 13 LED; ICSP SCK
Digital 12 ICSP MISO
Digital 11 ICSP MOSI
Digital 1 ICSP RESET

Audio Generation

The “audio” signal generated on pin 3 is actually a PWM signal, constructed based on the settings of the 5 pots.

PWM on pin 3 uses:

  • Timer 2 (8-bit timer)
  • OCR2B compare register
  • OC2B timer output
  • see “12. 8-bit Timer/Counter0 with PWM” in the Atmel ATmega datasheet

PWM is configured with registers TCCR2A/B

  • TCCR2 _BV(WGM20): PWM, Phase Correct; TOP=0xFF; updates OCR2 at TOP; TOV set on BOTTOM
  • TCCR2 _BV(COM2B1): Clear OC0B on Compare Match when up-counting.
  • TCCR2 _BV(CS20): no pre-scaling of the clock

In Phase Correct mode, the timer counts from 0 to 255 and back down to 0. Because it counts up and down and no prescaler, the frequency of the PWM time is 16Mhz/1/256/2 = 31.25kHz.

The output turns off as the timer hits the output compare register (OCR2B) value on the way up, and turns back on as the timer hits the output compare register value on the way down.

The sketch hooks TIMER2_OVF_vect and enables interrupts:


When the interrupt is triggered, the sketch fiddles the output compare register (OCR2B) given the current settings of the 5 pots. It also directly toggles the LED each time a full sync cycle is complete.

The granular synthesis algorithm is implemented with two “grains”:

  • each has a frequency and decay set by the corresponding pot.
  • each interrupt, the grain counters are updated, and combined.
  • the mixed signal (value) is used to adjust the output compare register

The master frequency control is used to adjust the counter limit at which time the grains are re-synchronised.

Breadboard Construction

I started with a breadboard and an Uno for initial tests. The potentiometers I’m using are a selection of 5kΩ, 10kΩ and 20kΩ linear. I don’t believe it is particularly important to have them matched, as they are just voltage dividers. I fact higher values are preferred - at 5kΩ, 5 in parallel effectively put 1kΩ across the 5V supply and that’s 5mA of wasted power.




Warning: output impedence

The circuits used here send the ATmega328 output pin straight to the audio out (left) channel with no protection. This means care is required not to plug it directly into any low impedence device; doing so could damage the microprocessor.

Putting it in a Can

To fit in an Altoids can, I replaced the Arduino Uno with:

  • ATmega328P with 16MHz crystal
  • 7805 regulator
  • ICSP header
  • LED for pin 13

Potentiometers and LED are mounted on the lid, and the electronics on two sections of protoboard. It just fits in nicely!




Finished and under test:


Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.