Category Archives: Buffer

Asynchronous, Buffer, Concepts, Inverter, Latch, Logic Gate, Uncategorised

Concepts V: Set-Reset Latch

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Set-Reset Latch

After learning how a C-element can be produce with concepts, we can introduce other gates. In this case another fairly standard gate, a latch.

The operation of a latch is to hold a given value. There are several different types of latch, but in this case we will talk about a Set-Reset latch (SR Latch). This latch has two inputs, one will set the latch when high, causing its one output to be high. The second input will reset the latch when it is high, causing its output to be low.

So lets see how we can specify this using concepts.

Concepts

For this, let’s start off by referring to the two inputs as a and b. The output is c.

So, first of all, lets think about how we can set the latch:

set = a+ ⇝ c+ ⋄ a- ⇝ c-

And it’s that simple. Simply put, when one of the outputs, a in this case, goes high, we want the output, c, to go high.

So we have described the interaction for set using one of the inputs and the output, now lets describe the reset action:

reset = b+ ⇝ c- ⋄ b- ⇝ c+

This will cause the output to go low when the second input, b, goes high.

These two concepts describe the operation of a SR-latch. But we still need to think about how the SR-latch will act initially, such as when it first receives power.

Unless specifically required, we should assume that the output of the latch should be low at startup. Because of this, we don’t want it to be set initially by a being high, so we can set this to 0 initially also.

However, with the second input, b, setting this high at this point would ensure that the output will reset to 0 at start up, but if the initial state of c is already set to low initially, we shouldn’t need to set b to high, so we will set the initial state of this to low too.

The initial state concept will be as follows:

initialState = initialise(a, 0) ⋄ initialise(b, 0) ⋄ initialise(c, 0)

Now let’s create the scenario concept for the SR-Latch:

SRLatch = set ⋄ reset ⋄ initialState

And this can be converted to a Signal Transition Graph, so this specification can be verified, simulated, tested and then synthesised. The STG for a SRLatch will be as follows:

STG_SR_Latch

This is a fairly simple STG, but simulation would prove that setting a high will allow c to transition high. Once it has transitioned high, it cannot transition low until b has been set high.

For some simpler understanding of how this latch works, we can change some of the signal names. In this case, we will change a to s, which stands for set. b will be set to r, which stands for reset. Finally, we can call c, q. q is a standard output symbol for latches.

This will change the concepts to the following:

set = s+ ⇝ q+ ⋄ s- ⇝ q-
reset = r+ ⇝ q- ⋄ r- ⇝ q+
initialState = initialise(s, 0) ⋄ initialise(r, 0) ⋄ initialise(q, 0)
SRLatch = set ⋄ reset ⋄ initialState

This will produce the following STG:STG_SR_Lath2

Changing the names of the signals does not affect the operation.

Now the SR-Latch has been defined, if a user has signals which will be inputs to a latch, they can use the concept and pass their signals into this, in the order of s, r and q.  For example someone can use the concept:

SRLatch(x, y, z)

In this case, x will be the set signal, y is the reset and z will be the output.

Some reuse

As is usually the case with concepts, we can define a SR-Latch using previously defined gates as concepts.

First of all, if we look at the set concept, it is clear that s and q form a buffer. So this can be rewritten as:

set = buffer(s, q)

The reset concept can also be redefined. In this case, it acts as an inverter:

reset = inverter(r, q)

As the initial state doesn’t need to be changed, we can compose these concepts as above to produce the SRLatch concept:

SRLatch = set ⋄ reset ⋄ initialState

Or we can reduce the number of concepts defined and compose these as follows:

SRLatch = buffer(s, q) ⋄ inverter(r, q) ⋄ initialState

There are of course several ways of specifying an SRLatch using concepts, but these are two of the simpler ways, either using signal-level concepts, or predefined gate-level concepts. The preference is entirely on the user.

Finally

In this post, we have explained another simple logic gate. The discussion includes the basic operation being specified as signal-level concepts, or by taking this a step higher and using pre-defined gates.

We have also shown how while the concept defines certain signal names, these aren’t set. A default SRLatch concept will use these signal names, but if these signals are not used in a design, a user can pass the desired signal names into the concept, and these names will replace the default ones.

This blog series will continue soon, with the inclusion of further logic gates and how they can be defined simply.

Asynchronous, Buffer, Concepts, Inverter

Concepts III: Initial States

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Initial States

Now we have introduced a couple of simple gates, a buffer and an inverter, we should now discuss an important part of Signal Transition Graphs, and their implementation using Concepts. These are initial states.  

Initial states are used in Signal Transition Graphs to show what state a signal is in, high or low, when the system being designed starts. This is important as STGs require initial states for simulation, verification and synthesis.

In the previous posts, we have assumed that all signals involved in the STGs are initially set to 0, and this has been indicated by placing a token in the places which are before each signals + transition (as above), but this assumption cannot be applied to every STG.

Example – A Handshake

So now let’s use a previous example, a Handshake, and discuss what we can do to ensure there are initial states in an STG produced using concepts.

The concept used to describe a handshake is as follows:

handshake(a,b) = buffer(a,b) ⋄ inverter(b,a)

This concept captures the interactions of signals a and b, using previously defined buffer and inverter concepts, but this does not explain the state of these signals when the handshake starts.

Initial state concepts can be defined similarly to other concepts, and thus can be composed with other concepts as normal. For this example, let’s say the two signals involved in handshake are both initially 0.

initialState = initialise(a,0) ⋄ initialise(b,0)

There is some syntax to discuss here:

initialise(a,0) – This is used to express an initial state. The state of the signal, a, will be initially set to 0.

The concept, initialState, implies that the initial state of signals a and b will be 0, so the first transition of each can be +, from low to high.

Now we can include this concept with the handshake concept:

system = handshake(a,b) ⋄ initialState

Passing this concept into the algorithm will produce the same STG as in the previous post:

bufferAndInverter00

And this can be resynthesized to produce the same handshake STG:

handshake00

However, what if the initial states are not both 0? For this, let’s redefine the initial state concept and compose this with a handshake concept:

initialState = initial(a,0) ⋄ initial(b,1)
system = handshake(a,b) ⋄ initialState

Now, a should initially be 0, and b should initialy be 1. The STG produced from these concepts is:

BufferAndInverter01This is the same as the previous version, but the tokens have shifted. After resynthesis, the STG is as follows:BufferAndInvertorRe01Again, very similar to the previous post-resynthesis STG. But in this case, we can see that b- is the first transition, instead of a+.

No initial state concepts

So, what if a set of concepts doesn’t contain any initial state concepts. In this case, the STG produced may still be a correct implementation of the system, but a user may wish to do some testing with options for initial states open.

When this occurs, the algorithm has the option of adding some extra transitions to allow a user some options. So, for example, let’s define a handshake, but no initial states:

system = handshake(a,b)

In this case, we make no assumptions of initial states, but how does this affect the STG produced?

BufferInverterNoInit

The STG produced is very similar to the previous STG, the concepts used to describe which features initial states. In this case however, the places a_0, a_1, b_0 and b_1 are all free from tokens. In order to select an initial state, the algorithm adds in an extra place and two extra transitions per signal, in order to allow  a choice of each signal’s initial state.

Using this example, when a user starts the simulation, the first transitions to be enabled will be the transitions either high or low from the initial state selections, allowing the user to select a- to set this signal to 0 initially, or a+ to set it to 1 initially, and this is the same as with signal b.

After this choice is made, the initial state selection will no longer be usable until the simulation is started again. The user can test how the system runs using these initial states, and then restart to test another combination of initial states. After the initial states have been selected, the user can either change this STG, removing the initial state selections and adding tokens into the correct place, or they can add the initial states to the list of concepts, and re-compose these for a new STG.

DISCLAIMER – An STG featuring initial state selection is not a correct STG, it violates properties of STGs which are necessary in order to be used with tools which perform useful functions like synthesis, for example. Currently, this method of testing for initial states should be used only for simulation, to test how differing initial states can affect a system. Any future developments are made on how to include a choice  of initial state in a correct STG, a follow-up post will be made.

Finally

In this and the previous two posts, we have covered the basics of concepts, and the design process of various simple gates.  Now we can move onto discussing some more complicated designs, and how we can use pre-existing concepts, and define new ones.

The next blog post will be Concepts IV: A C-element. This will be the first in a set of posts describing some standard logic gates, and from this we will work up to some more complex examples, using some of the previously described gates.

Asynchronous, Buffer, Concepts, Logic Gate

Concepts I: A buffer

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A Buffer

In this post, we will discuss one of the most simple logic gates, a buffer.

buffer_schematic

A buffer takes an input signal, and outputs a signal in the same state as the input signal, the output simply follows the input. For example, if the input signal changes from 0 to 1, the output will eventually change from 0 to 1. We say the output changes ‘eventually’ because the output changes after some unknown delay. It is possible for the input to change from 0 to 1, and then back to 0 very quickly, before we see any changes on the output. But intuitively, we want to be able to say that if the input transitions from 0 to 1, then stays 1 long enough, the output will follow and also transition to 1.

So how can we design these using concepts? We need to describe the causal relationship between the input and output; what occurs in the input to cause a transition in the output? For a buffer, this is when a signal transition occurs on the input, this needs to be reflected on the output.  This can be described as:

buffer(in, out) = in+ ⇝ out+ ⋄ in- ⇝ out-

There are some operators to discuss here:

⇝ – Shows the causal relationship
⋄  – The composition operator

This concept therefore implies that in+ causes out+, this is composed with in- causing out-.  This covers all the operations of the buffer.

But now, how does this turn into a Signal Transition Graph, which we can simulate, verify and test to ensure it works correctly? This is performed using an algorithm:

Buffer0

First of all, the algorithm takes the list of concepts and finds places one of these cycle for each individual signal. This is to keep with the property of consistency, where positive (+) and negative (–) transitions of each signal must always alternate. This is also in-keeping with the possibility for a signal to transition one way and back quickly without affecting the others, as with the example of the input transitioning high and then low before the output changes.

buffer1

Next, the algorithm uses the first concept, in+ ⇝ out+, to show this causality in the STG. This concept shows how the input rising causes the output to rise, so this is viewed in an STG by connecting the in_1 place, which shows when the input has transitioned high, to the transition of out+. This is done using an arc commonly known as a read-arc. This is because a one way arc connecting these STG components would mean that when out+ occurs, a token in in_1 is consumed, blocking in- from being enabled. A read-arc allows out+ to simply check for a token in in_1 for it to occur without consuming it.

Buffer2

In the final step for designing a buffer, another read arc is used which represents the concept of in- ⇝ out-, connecting in_0 place with the out- transition. This conversion of a concept to an STG is now complete.

Note that in the above STGs we assume that all signals are initialised to 0.  In a later blog post we will see how to use concepts to specify initial states of all signals in a compositional manner.

Reuse

Now we have defined a buffer, we can reuse this. As said earlier in this post, a concept can be defined as several concepts. Therefore, let use an example of a circuit which uses buffers.

double_buffer_schematic

This circuit can be produced from the following concept:

doubleBuffer = buffer(a,b) ⋄ buffer(b,c)

In this circuit, a is an input, c is an output, and b is an internal signal connecting the output of buffer_1 to the input of buffer_2. We could have described this using signal-level concepts, which would be a longer description that that of the one above. Since we have defined a buffer, we can simply reuse this definition in order to save time on describing concepts. This produces the following Signal Transition Graph:

doubleBufferSTG

Comparing this STG to the STG for a single buffer, it is possible to see that a and b in this STG form one buffer, and b and c form another buffer.  Again, we assume that all signals are initially 0.

Finally

In this first blog post, we have discussed how to describe a simple logic gate, a buffer, in terms of concepts. How these concepts are then converted into a Signal Transition Graph is also described. Another useful aspect of concepts is their reusability, and how a defined concept can be reused is displayed in an example using two buffers.

The following post, Concepts II: An inverter will discuss another simple logic gate, an inverter. After having defined these two gates, a further post will explain how initial states work, whether defined as part of a concept or not, using both a buffer and an inverter as examples.