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According to this answer (correct me if I'm wrong), copyleft software licenses don't do much when applied to hardware. Unlike software which is protected by copyright in any form, HDL/CAD hardware is transformed to "circuit" / "netlist" / "layout" when compiled, and these cannot be copyrighted. As a result, one can make a hardware device based on modified open-source blocks, and they are not required to release their modifications if they produce and distribute a physical product (unless the hardware blocks are also protected by a patent):

Legally, you can use copyright to protect your drawings, but you can't protect the actual circuit, nor the PCB layout, and therefore you can't insist on share-alike clauses to be followed. What we are left with is a "social contract", i.e. a statement that a design is shared for general use but please be nice and share your modifications.

But there's TAPR license which seems to require exactly that:

 * You may distribute products you make to third parties, if you either include the documentation on which the product is based, or make it available without charge for at least three years to anyone who requests it.

Another example is Arduino (licensed under CC BY-SA) which states in their FAQ:

Deriving the design of a commercial product from the Eagle files for an Arduino board requires you to release the modified files under the same Creative Commons Attribution Share-Alike license. You may manufacture and sell the resulting product.

Did I understand these clauses correctly and are they really enforceable?

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Modern electronic products are a combination of hardware and software.

Firmware is software that is "burnt" into hardware, that may or may not be modifiable, which controls the functionality of the hardware. Whether the software is stored in a temporary medium such as hard drive or flash memory, or permanently etched into an ASIC, it is still software, developed using source code like any other software.

If you build a desktop computer and install GPL software on it you need to provide access to the source code. Get a raspberry pi and install a linux kernel onto a flash card and you still need to comply with the GPL. Using the same software to burn an eprom or program an FPGA doesn't get around the need to comply with the GPL terms, you are distributing a binary copy that was generated from the source code.

Using dedicated languages to design hardware components has little difference to installing software on a temporary storage device, only the method of storage differs. If the source used to create the component is licensed under terms that require providing access to the source code, it should be just as enforceable as any "standard" software distribution.

A persons ability to make use of HDL files may still be rather limited today, but then walking around with a computer in your hand was an absurd thought 20 years ago. Hardware you design today could be the basis of consumer personalised hardware next year.

Of course I am not a lawyer and my opinion has little merit, until someone takes it to court and gets a legal ruling one way or the other.

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    I certainly like the spirit of your answer, but is it really the case in real world? You seem to stretch the term "software" beyond that everyone would univocally agree with. – Dmitry Grigoryev Jun 12 at 8:11
  • @DmitryGrigoryev Digital hardware can control/monitor some electrons to determine an on/off binary state. Software controls a CPU to make decisions based on the state of these hardware components that then control other components. C/C++ source gets compiled into binary form that a CPU uses to dynamically control its decisions, HDL files are compiled into a hardware design to manufacture circuitry that performs the same decisions. At this lowest level it is harder to define a difference than to list similarities. – sambler Jun 12 at 8:58
  • The difference is that "software" has a very narrow legal definition which allows for copyright laws to apply to something which is not normally subject to copyright. Copyright generally doesn't apply to utilitarian items, which a piece of silicon controlling a bunch of electrons certainly is. – Dmitry Grigoryev Jun 12 at 9:22

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