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TL;DR: Grey market clones can sometimes have significant functional differences compared to the original product.

A while ago I found an Arduino Due board on Amazon that was selling for about half the price of the official one so thinking I had stumbled on a great deal I bought a few. After delivery I flashed one of the example sketches that ships with the Arduino IDE to them in order to verify they work and then forgot about them for a while. Lately I've been learning more about writing firmware for ARM Cortex-M cores and given the Arduino Due has a Cortex-M3 I pulled one of the boards out to use as a testbed. I quickly ran into an unexpected issue when trying to configure the clock system to use the external 32 kHz crystal oscillator where it would never lock on to the desired frequency. After verifying the code was correct using multiple online references I decided to measure the crystal on the board itself only to discover it was actually missing!

I was aware when I bought them that Arduino boards have many clones due to the open source nature of their design and that many of the clones are not identical to the original. I naively assumed that any functional differences would be obvious and quickly detected by running the example sketches on the boards. That assumption turned out to be wrong. After more research it turns out there are at least two functional differences between these clones and the official boards which may not be obvious right away:

• A pull-up resister (R99) is missing. This leads to inconsistent power-on behavior which depends on how long the board has been powered up (i.e. warm reset). As far as I can tell the only mention of this is on this Copper Hill blog article.
• The 32 kHz oscillator (Y2) is missing. This means it cannot be used as an external reference for the ATSAM3XE slow clock. As far as I can tell several people have noticed this before as evidenced by Arduino forum posts such as this one and this one.

I'm not aware of any other differences but certainly I plan to be more careful going forward when my code doesn't behave as it should and not immediately rule out hardware issues. I also ordered an official board to use as a reference.