Version vom 8. März 2013, 21:25 Uhr

Why the Arduino Due

This was my selection criteria:

• Popular platform, well supported, reasonably priced.
• Open-source hardware.
  Hopefully this will lead to lower prices and longer availability.
• 32-bit CPU with good toolchain support.
• High Speed (480 Mbps) USB tranfers (effective throughput up to 35 MB/s or 280 Mbit/s).
  This is about time, the USB 2.0 spec was released in April 2000 (!).
  Most ARM microcontrollers support only the 11 Mbit/s mode (as of 2013).

Other candidates:

• The Beagle Bone was mostly sold out at the time.
• The Raspeberry Pi is designed to act as an USB Host, and not as an USB Device.

Specs

The Arduino Due has an AT91SAM3X8E microcontroller with the following features:

• ARM Cortex-M3 core:
  • Implements the ARMv7-M architecture, which is a 32-bit architecture.
  • Supports all of the base Thumb-2 instruction set, where instructions can be 16-bit or 32-bit long.
  • Supports only Thumb-2, does not support the original 32-bit ARM instruction set.
  • Hardware multiplication and division.
  • Memory Protection Unit with 8 regions.
  • Limited support for unaligned memory accesses.
  • Choice of several modes with different privilege levels (user, supervisor...).
  • Support for single-bit atomic operations with (bit-banding)
  • No floating point support.
• 84 MHz clock frequency.
• 2 x 256 KBytes Flash.
• 64 + 32 KBytes RAM.
• The "native" USB port and can do High Speed USB 2.0 (480 Mbit/s).
• JTAG port

Inadequate as a learning and hacking platform

The Arduino Due is surprisingly inadequate as a learning and hacking platform:

• It is relatively expensive compared to more powerful platforms like the Rasperry Pi, the Beagle Bone and many of the microcontroller evaluation boards.
  Due to the low availability 4 months after release date I ended up paying 59 € for it, and that hurts if you manage to fry it during your experiments.
• The development environment has no integrated debugger (!).
• Launching a program, however small, takes a few seconds.
  That is quite a long time for such a modern board, it goes on your nerves. Maybe it is due to the beta development environment version 1.5.2 I tested, but why is it still beta? The release date was 22nd October 2012, more than 4 months ago.
• It can only interface at 3.3 V level.
  The Bus Pirate has a CD4066B analog switch that uses an external reference voltage.
  There are warnings all over the place that you can damage the board with a 5 V level, but why is there no protection circuitry? For hackers and learners such a safety feature would be a welcomed addition. How about some integrated 5 V capability too? With a small CPLD or FPGA you could probably choose your voltage level on a pin basis.
• The board has a single user LED and no user buttons whatsoever, let alone any kind of display or sound ability.
  You need extra hardware for the most simple of tests.
• Serious hackers will need a JTAG-based hardware debugger, which costs at least 50 €.
  There are some cheap USB-Blaster clones for 10 € in Ebay, but I haven't been able to make mine work properly yet.
  The Arduino designers added a separate 8-bit AVR CPU in order to update the main microcontroller firmware over the "programming" USB port. What a complete waste of time! They could have added some existing JTAG-over-USB chip, or they could have made one themselves by connecting that little AVR to the JTAG wires.
• The JTAG header is smaller than usual, it has a 1.27 mm raster instead of the normal 2.54 mm.
  Most hackers will have to pay extra for an adapter.
• You'll probably need buy extra USB cables too.
  The board has 2 USB ports, the "native" USB connector is of type Micro-A, and the "programming" one is of type Micro-B. It may be my bad luck, but all devices I use have the Mini-B connector (Bus Pirate, USB-Blaster, AVR JTAGICE3, Wind River probe...), so I didn't have the right cable at hand. Of course there isn't one in the Arduino Due box either. Wasn't USB supposed to be standard? Apparently, there is a type of USB cable that fits in both Micro-A and Micro-B connectors, but the documentation offers no advice about this either. I wonder why there are 2 types of Micro USB connectors then, it's just confusing.
• Linux issues.
  • The Linux installation instructions are not together in a single page.
  • If the Arduino environment does not detect any suitable port, all ports options are greyed out with not further explanation.
    Also, after selecting the Arduino Due in the Tools/Board menu, it was not clear which option to select under Tools/Programmer.
  • It was not clear that the reason why your account may need "dialout" privileges is because /dev/ttyACM0 belongs to that group. One web page stated "No driver installation is necessary for Linux.", but fails to mention that you will probably need to configure things manually. I tried the suggested "sudo usermod", but it took me a while to realise that this change does not take place immediately. You probably don't have to restart the PC, but that is what I tried, and it did the trick.

Connecting over JTAG

When connected to the Arduino Due, OpenOCD discovers a JTAG IDCODE of 0x4BA00477. The JTAG Instruction Register has a length of 4 bits.

The IDCODE breakdown, using my script to decode JTAG IDCODEs, is as follows:

 Decoding of JTAG IDCODE 0x4BA00477 (1268778103, 0b01001011101000000000010001110111):
 Version:      0b0100  (0x4, 4)
 Part number:  0b1011101000000000  (0xBA00, 47616)
 Manufacturer: 0b01000111011  (0x23B, 571)  # Name: ARM Ltd.
 Leading bit:  1  # Always set to 1 according to the IEEE standard 1149.1