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This article will show you how to make state machines several different ways. It assumes a good understanding of the various components and of electrics in general. Please see the referenced pages if you need more info. State machines are usually a college-level concept but I will attempt to present it in a manner that does not require advanced math or logic. However, it is still an advanced article and not for noobs.

(Authored by Stanimus)


Introduction to State Machines

A state machine can control different electric elements at one time and will change them in a certain order, depending on timing and control inputs. For more details on state machines, see other sources. Because of the nature of a state machine, there is a wide variety of ways to design and implement them. This article will certainly NOT try to cover them all. I will try to cover the ones that are easiest to implement in SC electrics. If you have questions or specific needs, you may contact me (assuming I'm available) and I'll try to help.


Every state machine has three basic stages. The first is the input encoding. This takes all relevant inputs including a clock or counter and converts them to a format the actual controller requires. It also must take into consideration the 'initial conditions' of the machine. The second stage is the actual controller logic. This takes those inputs and determines what the next state should be. The last stage is the decoder and just takes the controller output and converts it to the signals that go to the components that the machine operates. These would be the doors, lights, spikes, pistons, furniture or whatever, that you want to affect with this machine.

When designing state machines, you need to clearly lay out what you want to happen, in what order and/or in response to which inputs. This plan is your state machine description and is necessary to be able to design the controller properly.


Sequential State Machines

The simplest state machine is timing related only and is controlled by a clock signal. Of course there must be at least one input to get the machine started. These are called 'sequential' state machines because the same things happen in the same order, every time it's started. The description is usually along the lines of: -When started, activate output-1 -Wait 1/2 second, activate output-2 -Wait 1/2 second, deactivate output-1 -Wait 2 seconds, deactivate output-2 -End

There are (at least) two ways to implement a sequential state machine.

One typical example is when controlling a series of pistons. The above sequence might be used to control two pistons, each one assigned to an output. The 'start' may be the signal from a pressure plate. Since all changes are made based on timing alone, there are no other inputs except the clock. This machine may be implemented several different ways in Survivalcraft.

One way would be by using a series of variable delay gates. This will work fine unless you run into difficulties. Then it can be tough to make changes while keeping track of the delays for all the different parts. Plus, it could take up a lot of space unnecessarily.

Instead, first we will look at using a Memory Bank as the controller:


Memory Bank Controller

A timing-only sequential state machine can easily use the bank as the controller stage. Typically you will have a gated clock running a counter for the controller input.


Input Stage

There are three steps for the input stage in this machine. First we need to start the machine, then we need to let it run, then we need to stop it. Simple - sort of. When we use a bank as the controller stage, it is easy to use a counter as the core of the input stage. We can control the counter in different ways - some more complex than others, not all are useful with every state description, either.

For the first example, I will use a 'constant clock' input stage. In this design, a clock is applied to a counter and the output of that counter is the controller stage input, so everything happens at specific times and in a specific order. The 'start' signal simply connects the clock to the counter and lets it start counting from '0'. Each value of the counter specifies a state of the controller. When the machine is done counting, the clock is disconnected and the counter is (usually) reset to '0'.

Implementation

To implement this design, we need to control the clock going to the counter. Since there will be separate start and stop signals, we'll need a latch. The start signal goes to the 'S' input of the latch and the stop signal goes to the 'R' input. The latch output then goes to an AND gate which controls the flow of the clock signal. The cheapest way to do this is to use a NAND gate in the clock generator. By 'cheap' I mean that it uses the least number of gate. A clock generator then needs only the NAND and one Adjustable Delay Gate. However, in this case cheapest is not best. The circuit does not start up properly this way. Instead, we'll use a NOT in the clock generator and a separate AND gate to control the clock.

The clock output is fed to the counter and the counter's output is the state number. Each different value represents a 'different' state of the machine. The states may not actually be different - that depends on what the machine is intended to do. In this example, there will be some 'wait states' - a sequence of states that are identical and just waiting for a certain amount of time to pass.

The last section needed will disable the clock (via the SR latch) and reset the counter (if needed). In this simplest case, we can just let the counter run through its full count and stop. That way we can use the overflow signal from the counter as the clock disable. The counter will automatically be at '0' so we don't need anything more to reset it. The down side to this is only that the machine can't be restarted before the full count is complete. In many applications, this is a non-issue. This is a picture of the final input stage circuit:

SM ex1p1

The display is the output which goes to the next stage.


Controller Stage

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