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An embeddable ARM board, the ARMiGo

Posted: March 10th, 2014 | Author: | Filed under: Uncategorized | Comments Off on An embeddable ARM board, the ARMiGo

This is a guest post by Ken Boak (@monsonite).


ARMiGo2As integrated circuits get smaller, and more powerful, it becomes increasingly difficult for the hobbyist to utilise them easily. Soldering tiny SMT parts is a challenge for many – but an amazingly useful skill, once mastered.

32 bit ARM microcontrollers are now virtually the same cost as older 8 bit devices, yet offer a many fold increase in performance, as a result of their 32 bit architecture and faster clock speed. In addition to this, they come with a rich set of peripherals, making them ideal for more demanding applications.

For the hobbyist who may have first gained exposure to microcontrollers via 8 bit products such as the Arduino, the move to 32 bit may appear a little daunting.  However we all have to progress and keep up with the times, and so now the time has come to move up to the “big school”.

For a particular microcontroller to gain traction in the hobbyist community, it has to meet a few basic requirements:

  1. Low cost development boards
  2. Free to use, open source toolchain
  3. Lots of online code examples, projects, forum support
  4. Support from a vibrant user community
  5. Easy to use and incorporate into projects
  6. Easy to upgrade to different devices from within the same microcontroller family

1.  Low Cost Hardware

After some investigation, and triggered into action by exceptionally low cost development boards, I have settled on the ST Microelectronics STM32F range of microcontrollers as being the basis for future projects.

These micros are already being used extensively in a range of hobbyist boards including Maple, Netduino, Olimex, Expruino, mbed, MicroPython and STM’s own range of very low cost Discovery boards, which are priced around the  $11 – $18 range.

A recent offering from mbed in collaboration with ST is offering boards for as little as $10, and come complete with a detachable ST Link programmer, which can be used for programming other boards. These boards are now available from suppliers including Newark,  Mouser, Digikey.

Low cost hardware is the new trend, with dev kits often being sold at close to cost price, so gone are the days when you forked out $30 for your first Arduino, and then another $30 when you wanted to build it into a more permanent project.

2.  Open Source Toolchain

Free to use toolchains are nothing new. Microchip realised in the 1990’s that they would sell more chips if they made the coding tools free. Fortunately the ARM microcontroller is supported by GCC, and there is a free to use IDE,  called CooCox CoIDE, which supports a wide range of ARM chips from several main manufacturers.

3. Lots of Online Support and Code Examples

Getting started with ARM chips is as easy as dowloading the free tools, installing some drivers and learning the basics from a wide range of code examples easily found online.  The STM32 range of microcontrollers have been available for about 4 years now, and so there is a rapidly growing resource of online code examples, reference designs and supporting documentation. ST Microelectronics produces a firmware pack for all of its evaluation and Discovery boards, and this is a good place to start for code examples. Other online examples,  which are easily Googled, are downloadable from repository sites such as Github amongst others. YouTube is also a good place to search for STM32 projects. As the processors are considerably more powerful than 8 bit devices, they are often used with LCD colour displays, and there are several examples of these on YouTube.

4. A Large User Community.

All popular devices build up a loyal user community and the STM32 is no exception.  The popularity is driven by low cost hardware and free tools, and is ever expanding as new products based on the STM32 are released.  Again a simple Google search will often turn up a blog from an enthusiast who has already covered a lot of ground and is happy to share it. 

5. Easy to Incorporate into Projects.

This is where things are not so clear-cut.  The problem with most dev boards is that they are either too big, or use header pin layouts that are incompatible with breadboard or stripboard.  This can be a major inconvenience, and often force the user to buy some form of shield for mounting other hardware expansion, or resort to using female/male jumper leads, so that the headers can be jumpered into a breadboard. Either way the result is not ideal, and a rat’s nest of jumper wires is neither permanent nor easy to work with.

6. Easy to move between devices in the STM32 family range.

There are now a considerable number of devices in the STM32Fxxx family, based broadly around the Cortex M0, M3 and M4 ARM architectures. Whilst to the newcomer the range of different parts may seem confusing, STM have designed the family in such a way which makes it relatively easy to move between parts.

The first thing to realise is that they are all microcontrollers, with on chip SRAM, and only the larger packaged parts will support external memory.

Secondly, all parts share identical, or very similar I/O and peripheral architectures, so if you are using USART 1, or Timer 2 on a 48 pin pack, you will find the same peripherals, and more, in the 64, 100 and 144 pin packages.

Thirdly, you will find that you can switch between family members – because specific packages share common pin-outs. This means if you have designed a pcb for a 48 pin STM32F103, and you want to move up to the STM32F303, then you will find all the ports, clock lines and power pins are exactly the same – so no pcb changes are needed.

Here are some guidelines to selecting a part:

  1. How much I/O will my application require?  The GPIO ports are 16 bits wide, and devices are made with between 2 and 8 ports.  The 48 pin package provides 2 ports PortA and PortB,  the 100 pin package provides 5 ports PortA – PortE.
  2. Choose the appropriate amount of flash memory and SRAM.  The smaller parts have 128kB of flash and 32kB of SRAM. The largest parts have 2MB of flash and 256k of SRAM.
  3. Choose the Cortex family member. M0 is the cheapest with the slowest clock, and generally without USB. M3 is a good starting point with a 72MHz clock and on chip, full speed USB.  M4 is similar to M3, but supports clock speeds up to 180MHz and has a floating point unit (FPU) which may be useful for intensive maths code such as used in robotics and flight controllers.

 Getting Started.

 Initially I bought a Discovery F3 development board, which uses a 100 pin STM32F303 processor and has  compass, accelerometer and gyro devices on the board. It was exceptionally good value at under $11 from Newark.  However, whilst it provided a platform to get me started writing code and getting the framework to support my application built, it was just too big and the double row headers are not the friendliest of connectors to use. I didn’t need all of the  80 lines of I/O provided by the 100 pin device,  so I settled on the 48 pin part, which somewhat smaller, cheaper and more manageable in terms of pcb layout.

 My solution was to create a 2″ x 0.75″ (51mm x 19mm)  double sided pcb, which mounts the STM32F303 microcontroller,  its clock and reset circuits and USB/programming connectors. This small breakout pcb, converts the pins from the 48 pin LQFP to an easier to use 40 pin dual in line module, and provide the minimum of features to get the STM32 to load and run code.

This approach is not new and has been used in minimalist products such as the Maple Mini and the Arduino Pro Mini. Micro and Nano, all of which have been around for some time.

I decided to name the board “ARMiGo” in respect to its user friendliness and I designed the ARMiGo to be as open and flexible as possible, so that it can be used as a core for incorporation into other designs.  The essential parts of that core are the microcontroller, the 8MHz crystal, the reset circuit, USB connection and programming header. 

The first of the prototypes arrived this week, and now working running some simple test code.

ARMiGo uses the STM32F303 Cortex M4 ARM device which runs at a maximum of 72MHz.  The pcb also supports the STM32F103 which is based on the M3 core.

The board is the same size as a standard 40 pin DIL format IC – making it ideal for breadboarding, and small enough to be used as a plug in module in a 40 pin socket, on a larger board.

All 35 I/O lines of the ARM chip are brought out to standard 0.1″ (2.54mm) spaced headers.

The 5 pin right angle header on the left accepts the clock and data signals from the “ST-Link” programmer/debugger device.  These are available very cheaply but it is relatively simple to use the embedded STM bootloader and program it via either the mini-B USB connector on the right or via one of the USART channels.

There is a really rich mix of on-chip peripherals, which gives these small ARM parts tremendous flexibility – including the following:

  • Full speed USB interface
  • 4  Fast (5 Msps)  12bit ADCs, each with up to 4 input channels
  • 4 Programmable gain op-amps and 7 comparators
  • 2  12 bit DACs
  • 3   USARTS
  • 3   SPI
  • 2   I2C
  • RTC with 32768 Hz oscillator and dedicated output pin for “alarm”
  • 10 timer channels,  2 basic,  6 general purpose and 2 advanced:
    5  General purpose  16 bit timer channels with up to 4 outputs for PWM generation, timing, counting etc.
  • 1 general purpose 32 bit counter/timer – for optical encoder reading etc
  • 2 advanced 16 bit timers for complimentary PWM generation etc.

In addition, the analogue section of the IC contains programmable gain op-amps and comparators which feed the ADC channels, and can be used to replace external analogue circuitry.

This small 48 pin packaged part contains 128KB of Flash and 40KB of SRAM – of which 8K can be battery backed up when the rest of the IC is powered down.