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What are the potential risks when implementing a 12-states ?First of all I will need then 4 flip flops to draw my circuit giving a total of 16 states machines meaning that 4 states will be unused .
What are the problems I might envision in a system that can exist in 4 more states than are actually intended to be used ?
There are several ways to code state machines.
The simplest is 'one hot', where your 12 states would use 12 flip flops, only one of which would be active at any time. This is simple to debug, and avoids much decoding.
To save flip flops, you could use a binary code to use only four of them. What you save in flip flops, you may lose in decoding circuitry. If your state machine has a linear sequence through the states, then a Gray-type encoding could avoid race conditions decoding to unwanted output glitches.
With any coding scheme, what do you do if the SM ends up in an illegal state? One option is to decode all illegal states as a prompt transition to the idle state, or reset, or power-on. This would make recovery automatic, but would mask what happened. Another possibility would be for them to stick in the illegal state forever more, so you'd have something to debug.
It depends on whether this is a life-critical system, and whether failure would be merely inconvenient, or very important.
If you're doing this in code (e.g. a microcontroller), having unused states isn't a problem at all. Most of the time it's the cheapest/smallest/lowest consumption solution, as micros are getting smaller and cheaper, while discrete components (or LSI ICs) are bigger, and PCB plus the bigger box costs money.
If you're using hardware (or maybe an FPGA or something similar), encoding your states in a way to ignore e.g. the last bit sometimes (12 "real" states with 16 numbers) saves you some gates and helps with the "illegal" state problem.
E.g. Let's say your states are encoded like this: 0000, 0001, 0010, 0011, 0100, 0101, 0110, 0111, 100x, 101x, 110x, 111x. Now you can use fewer gates to check your state, because you know if your highest bit is 1, your lowest doesn't matter.
