Cheep Mod - a low cost platform for Euro-Rack digital audio.
Cheep Mod Front - fully assembled
What is it?
This is a small 6HP Euro-Rack format module with 3 CV inputs, 3 pots, Digital
(Sync) input, 12-bit DAC Audio output and Digital (gate) output.
I do a lot of development for various Euro-Rack manufacturers but I've never
built an entire module myself. This project was an experiment on several fronts -
what's the cheapest yet most capable MCU to use (the one used here costs $0.50)
while still providing features that allow advanced DSP, and how well does it work
to use PCB manufacturing processes to create a modular front panel? Considering
the advanced synthesis algorithms I've been running on this hardware and the
decent appearance and assembly of the panel I consider Cheep Mod a success on
- 32-bit ARM Cortex-M0 CPU rated for 48MHz clock
- 4kB SRAM
- 16kB Flash
- 5 Timers
- 1 SPI
- 1 I2C
- 1 USART
- 15 GPIO pins (4 are 5V tolerant)
- 1 12-bit SAR ADC with 11 input channels
- 3 front-panel pots
- 3-position toggle switch for mode selection
- 3 +/-5V CV inputs
- 1 digital (sync/trigger/gate) input
- 1 12-bit 200kHz DAC +/-5V (audio/CV) output
- 1 digital (sync/trigger/gate) output
- 1 6-pin ARM SWD connector (ST-Link V2 format) on back
- Standard 16-pin shrouded Euro-Rack power:
- 24mm deep
The STM32 family of parts are supported by a variety of commercial IDEs, all
of which are available in low-cost or limited free versions from the
I prefer to use open-source tools so I've set up a development environment
based on the GNU C compiler for ARM, available for free from the Launchpad
project. For downloading to the target I use OpenOCD and the ST-Link V2 SWD
interface that's available on most all of the inexpensive STM32 Discovery boards.
To accelerate development I also use the STM32 Standard Peripheral Library as
a starting point for my code and then optimize out the heavyweight functions
as required. Links to all of these are here:
- 2015-04-30: Started Schematic, order parts.
- 2015-05-02: Order PC boards.
- 2015-05-18: PC boards arrive, assembled, working.
- 2015-05-22: Created web page.
- 2015-05-26: Synth apps work. Added BOM.
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