# 5. The Digital Abstraction

Having explored ways to use a binary channel capable of transferring bits between a transmitter and receiver, we now turn to the problem faced by the channel itself: physical representation of individual ones and zeros in a form that allows them to be manipulated and communicated reliably. In dealing with this problem, we will confront the deceptively complex problem of representing discrete variables as continuously-variable physical quantities; moreover, we will establish one of the major abstractions of digital systems: the*combinational device*.

## 5.1. Why digital?

In classical physics, measureable quantities that we might use to represent information -- position, voltage, frequency, force, and many others -- have values that vary continuously over some range of possibilities. The values of such variables are*real numbers:*even over a restricted range, e.g. the interval between 0 and 1, the number of possible values of a real variable is

*infinite*, implying that such a variable might carry arbitrarily large amounts of information. This may appear as an advantage of continuous variables, but it comes at a serious cost: it obscures the

*actual*information content of the variable. This limitation of the engineering discipline surrounding analog systems results in symptoms like the degraded video content of our video toolkit example of Section 4.4.

### 5.1.1. Role of Specifications

Informally, engineering involves building things: assembling components to make a system that performs some useful function. We formalize this process via*specifications*, both for the system and its components. Specifications abstract the external function performed by a module, and isolate that function from the internal details of the module's implementation. Ideally, a module's specifications provide all of the information an engineer

*using*that module as a component needs to know, without encumbering that user-level view with details relevent to the

*implementation*of the module.

Given well-designed specifications, the engineering of a system becomes the task of finding an arrangement of interconnected components that is guaranteed to meet the system specification, assuming that each of the component modules meets its module specification. Well-engineered systems are tantamount to a proof that the system "works", assuming that each of the modules work, where our standard of working is set by the specifications. While the proof is rarely formalized explicitly, it is implicit in the choices made by the system's designer.

If we are to build systems of arbitrary complexity, it is important
that specifications of each module characterize that module's behavior --
viz., the information flowing into and out of each module -- *precisely*
rather than approximately. Specifications that can only be approximated
in real-world implementations, such as those for our analog video toolkit,
result in systems that obey their specifications only approximately;
as these systems are combined to make yet bigger systems, the approximations
continue to degrade. Such models can and are used for systems of limited
complexity; but to remove this complexity limit, we need an abstraction
whose specifications can be simply and precisely realized in practice.

## 5.2. Representing discrete values: Part 1

An enabling step toward the goal of simple, precisely realizable specifications is to adopt an abstract model based on*discrete*rather than

*continuous*(real) variables. Although other possibilities exist, we choose the binary symbols $\{0, 1\}$ as the two distinct values that each variable may assume, so that each variable carries at most a single bit of information. The move from the unbounded information content of a continuous variable to the finite -- indeed tiny -- single bit of a binary variable represents a serious compromise in the capabilities of our model: it implies, for example, that each module's inputs and outputs each carry finite information. Paying that price, however, allows us to build devices whose specifications are obeyed precisely, and to combine the devices (and their specifications) to build systems of arbitrary complexity that obey their specifications precisely.

Given that the real-world physical variables we must use to implement our devices use continuous variables, translating our discrete-value models to real-world implementations requires a scheme for representing discrete values as real quantities like voltage. We must somehow represent 0 and 1 as distinct quantities, with no intermediate alternatives. Moreover, we must assure that the representation of a 0 is never mistaken for a 1, and vice versa. In an electronic system whose wires carry voltages between 0 and 1, we might choose that voltage less than $0.5v$ represent 0 and those above $0.5$ represent 1; however, then voltages arbitrarily close to the $0.5v$ threshold could easily be misinterpreted.

**Simple binary representation**

*forbidden zone*in our range of voltages, as shown in the diagram to the right. In this scheme, sufficiently low voltages (perhaps those below $0.1v$) represent a logical 0, sufficiently high voltages (perhaps those above $0.9v$) represent a 1, and those between represent neither a 0 or a 1.

While the forbidden zone allows us to reliably distinguish logical 0's from 1's,
it introduces a complication: not every voltage in our systems will represent
valid logic values. Indeed, given the continuous nature of our underlying
physics, the voltage on a wire cannot change between representing 0 and 1
without at least temporarily going through the forbidden zone. We will address
this issue shortly, via a discipline that effectivly allows us to avoid asking
the question *"which logic level does this voltage represent?"* at time when it
may be invalid.

## 5.3. Combinational Devices

Armed with a plausible approach to implementing the digital (discrete) $\{0, 1\}$ values of our model, we now introduce an abstraction for its major building block: A*combinational device*is a component that has

- one or more digital
*inputs*; - one or more digital
*outputs*; - a
*functional specification*that details the logic value of each output for each combination of input values; - a
*timing specification*consisting, at minimum, of a*propagation delay*: an upper bound $t_{pd}$ on the time required for the device to compute the specified output values from an arbitrary set of stable, valid inputs.

The combinational device is our model for *combinational* modules,
whose outputs are determined by their current inputs after a propagation
delay.

**Combinational Device**

*truth table*specifying outputs for each of the $2^3$ input combinations, and whose timing specification is a 3-nanosecond upper bound on the time a valid output will appear after valid inputs are applied.

### 5.3.1. The Static Discipline

The specifications of a combinational device represent a contract between that device and its system context:*each output will be the valid logic value dictated in its specifcation whenever its inputs have been valid for at least the specified propagation delay*.

*static discipline*, and it plays a critical role in our model: it allows us to avoid the propagation of invalid representations of logic values throughout our systems. It is

*static*because it deals with equilibrium values at inputs and outputs (after propagation delays, which allow transients to settle); and the term

*discipline*implies that its a self-imposed engineering constraint on the design of combinational devices.

### 5.3.2. Combinational Circuits

A compelling property of the combinational device abstraction as a building block is the ability to build complex combinational devices by combining simpler ones. In particular, we can specify a new combinational device $S$ by a diagram of interconnected components such that- Each component is itself a combinational device;
- Each component input is connected to either
- an input to the system $S$;
- an output of another component; or
- a constant $0$ or $1$.

- Each output of the system $S$ is connected to either
- an input to the system $S$;
- an output of a component; or
- a constant $0$ or $1$.

- The circuit contains no
*directed cycles*, i.e, there is no cyclic path within the diagram that goes from inputs to outputs through one or more components.

Such a diagram is called a *combinational circuit*, and conforms to
our definition of a combinational device.

This analysis generalizes to combinational circuits of arbitrary
size. In general we can procede through the circuit from inputs
to outputs, assigning to each node in the circuit a functional
specification (e.g., a *truth table*
giving its value for each combination
of circuit inputs) and a timing specification, viz. an upper bound
on the time at which the signal will be valid after valid inputs
have been applied to the circuit. We will see many examples of
such analyses in the next chapter.

The propagation delay specification is an *upper bound* on
the time at which a combinational device's outputs will become valid:
it constitutes a performance guarantee by the device to its user.
The specification may be chosen conservatively, although to extract
the best performance from our systems, we tend to prefer the smallest
propagation delay specifications that we can reliably guarantee.
For a combinational circuit $C$, a *lower* bound on this value --
the tightest propagation delay we may specify -- can be computed as
follows:

- Consider each path from an input of $C$ to an output of $C$.
For each such path, compute the
*cumulative*propagation delay along that path -- i.e., the sum of the propagation delays for each component on the path. - Take, as the propagation delay specification for $C$, the
*maximum*of these cumulative propagtion delays along each input-output path.

Recall that our combinational device definition requires a timing
specification that includes *at minimum* a single, summary
propagation delay. It is sometimes useful to supply more detail
in the specification; we may, for example, specify individual
propagation delays for each input/output pair.

## 5.4. Representing discrete values: Part 2

We now return to the problem of representing $0$ and $1$ logic values as voltages. Recall that we introduced a forbidden zone between ranges of voltages representing each discrete logic value, in order to reliably discriminate between representations of $0$ and $1$; this forced the distinction between valid and invalid representations on which the static discipline of our combinational device abstraction depends.However, this new distinction -- valid/invalid representations -- confronts another difficult decision problem. We want our systems to work reliably in real-world situations in which our control over voltages on wires and their detection is imperfect: environmental noise, and manufacturing variation among devices, will perturb actual voltages from our design ideals to some limited extent.

While we have noted some real-world sources of noise in Section Section 4.5.1, we will model these imperfections here by the simple injection of a bounded amount of noise into the wires that interconnect components in our combinational circuits.

We might ask whether this noise-tolerant model of wires obeys the static discipline we impose on combinational devices -- i.e., does a valid representation at $V_{in}$ guarantee a valid representation at $V_{out}$?

*any*nonzero amount of noise, we can construct an example of this sort in which a valid input is corrupted by our wire to an invalid output. Any realistic model of wires -- one that reflects any imperfections in their capacity to convey voltages between devices -- will suffer from this defect.

### 5.4.1. Noise Margins

Unfortunately, we are stuck with this noise susceptibility of wires, and hence with the connections within our combinational circuits. Our recourse is to build the combinational devices themselves in such a way that they tolerate some amount of noise on the connections between them. We do this by establishing different standards of logical validity to be applied to the*outputs*of combinational devices that we require of their

*inputs*: a combinational device must accept sloppy input representations but produce squeaky-clean representations at its output. The idea is that a valid signal at the output of a device will appear as a valid at a connected device input, despite a modest amount of noise introduced by the connection.

We can accomplish this goal by interspersing finite
*noise margins* between the forbidden zone and each valid logic
representation, as shown below:

We characterize our enhanced mapping between voltages and logic levels by four numeric parameters:

**$V_{ol}$:**The

*highest*voltage that represents a valid $0$ at an output

**$V_{il}$:**
The *highest* voltage that represents a valid $0$ at an input

**$V_{ih}$:**
The *lowest* voltage that represents a valid $1$ at an input

**$V_{oh}$:**
The *lowest* voltage that represents a valid $1$ at an output

*restore*signals that have been degraded by noise to pristine $0$s and $1$s. This is something that a wire, or any passive device, cannot do: it requires that the marginal signals be

*amplified*by increasing their distance from the forbidden zone.

## 5.5. A Combinational Buffer

The need for noise margins to maintain signal validity, we have seen, rules out the use of a passive device (like a simple wire) as a combinational device that performs in our digital domain the function of the analog $Copy$ operator described in Section 4.4.**Buffer**

When using combinational modules like the buffer as digital building blocks, we typically represent them using their logic symbol and think about their behavior in our abstracted world of $1$s and $0$s. When considering their implementation, however, we often need to pierce through the digital abstraction and consider the voltages at their input and output terminals that represent the $1$s and $0$s that are being processed.

*voltage transfer characteristic*, a simple graph showing, for each input voltage $V_{in}$ with some range, the

*equilbrium*output voltage $V_{out}$ if $V_{in}$ is applied and time is allowed for the $V_{out}$ to settle. Note that this curve does not involve time -- it assumes that for each $V_{in}$ we apply, $V_{out}$ will stabilize to some fixed value eventually. When we consider the voltage transfer characteristic of a combinational device, we generally assume that this settling time is covered by its propagation delay specification.

What can we say about the voltage transfer characteristic of a combinational buffer? From its functional specification, we know that a low input voltage should produce a low output voltage, and a high input voltage should produce a high voltage on its output. Given the parameters $V_{ol}$, $V_{il}$, $V_{ih}$, and $V_{oh}$ that define our logic representation, we can mark these values on both the input and output axes and reticulate our voltage input/output plane into a number of distinct regions.

**Buffer VTC**

*cannot*go through if the device it represents is to obey the static discipline: those regions which represent a

*valid input*and an

*invalid output*. Such a shading is shown on the curve to the right. The avoidance of these regions is a simple consequence of the

*valid input implies valid output*static discipline rule: if the transfer curve goes through such a region, then there is a valid input voltage we can apply which will yield an

*invalid*output voltage, violating our rule.

For a combinational buffer, what we require of the voltage transfer characteristic is

- That it obeys the functional specification (e.g., any valid 0 input yields a valid 0 output); and
- That it avoids regions that correspond to invalid outputs and valid inputs.

*every*single-input logic function that might be performed by a device under this logic mapping.

**Inverter**

*inverter*-- the other "interesting" single-input logic function, whose symbol and truth table are shown to the right.

**Inverter VTC**

## 5.6. Gain and nonlinearity requirements

The voltage transfer characteristics for each of these devices -- the buffer and inverter -- are subject to certain geometric constraints imposed by the exclusion of the shaded regions. In each case, the output actually depends upon the input; this implies that the transfer curve must go between valid $0$ and $1$ representations as the input voltage varies. This transition can only be made in the forbidden zone, between the two shaded regions of our voltage input/output plane: any other path would violate the static discipline for some range of input voltages.Note the dimensions of the rectangular gap between the shaded regions: its width is $V_{ih}-V_{il}$, and its height is $V_{oh}-V_{ol}$. The height is necessarily greater than the width, since the height includes our noise margins $V_{il}-V_{ol}$ and $V_{oh}-V_{ih}$ that are excluded by the width. We conclude that the transfer curve must enter this tall, thin rectangle at the top (or bottom) and leave at the bottom (or top) to make the transition between $0$ and $1$. It follows that this region of the curve must be more vertical than horizontal: the magnitude of its average slope as it traverses the forbidden zone must be greater than one.

The slope of a voltage transfer curve reflects the *gain* of
the device it describes -- the ratio between an output voltage change
and the change in input voltage that caused it.
The static discipline and its requirement of non-zero noise margins
dictates that combinational devices exhibit gain greater than 1 (or
less than -1) in the forbidden zone; this is the mechanism by which
they restore marginally valid inputs to produce solid outputs.
The gain requirement for combinational devices reinforces our earlier
observation that a passive device (such as a wire) does not satisfy
the static discipline; some active amplification is required to
avoid degeneration of logic values.

It is also noteworthy that, since the input and output voltage ranges are identical, the transfer curve must "flatten out" outside of the forbidden zone to compensate for the high gain in that region. Like the example VTCs for the buffer and inverter, the slope (gain) is low for valid $0$ and $1$ inputs, but high between them. The curve must be nonlinear, since its slope necessarily varies over the range of voltages that may be applied. This, along with the requirement for amplification (gain), rules out the use of a simple linear device like a resistor as a combinational device.

## 5.7. Constraints on logic mapping parameters

Digital designers don't ordinarily get to choose the logic mapping parameters $V_{ol}$, $V_{il}$, $V_{ih}$, and $V_{oh}$; they are specifications dictated by the family of logic devices being used. Since our goal is a single logic representation scheme to be used by a wide range of devices and operating circumstances, the parameters for a logic family tend to be chosen conservatively. The choice is heavily dependent on the underlying technology being used, as well as other factors such as anticipated range of environmental conditions (temperature, electrical noise, etc) and variability of the manufacturing process.
For reliable operation, it is desireable to maximize the noise
margins. If we model noise as the injection of a bounded perturbation
$V_{noise}$ in connections between devices, we may view *noise immunity*,
defined as the lesser of the two noise margins, as a limit to
the magnitude of $V_{noise}$ that our family will tolerate. For this
reason, we prefer to equalize the noise margins to the extent possible.

The choice of the output parameters $V_{ol}$ and $V_{oh}$ is
restricted to the voltages that can be reliably produced by the
technology of the logic family.
Given values for the output parameters, the choice of
valid *input* ranges dictated by $V_{il}$ and $V_{ih}$
is subject to two opposing goals: (a) the desire to maximize
noise margins, and
(b) the need for every device in the family to obey the static
discipline (i.e., for its transfer curve to avoid forbidden regions).

**Ideal inverter VTC**

*infinite*gain at $V_{in}=V_T$, and zero gain at other input voltages. Given this characteristic, we could expand the noise margins by moving $V_{il}$ and $V_{ih}$ closer to the $V_T$ threshold voltage, expanding our shaded regions until they almost touch the transfer curve. The extent to which we can do this is limited, however, by the variability of the manufacturing process and operating conditions. If $V_T$ differs from device to device due to uncontrollable factors (as it will), or is temperature dependent (as it is likely to be), our conservative selection of $V_{il}$ and $V_{ih}$ must allow for these variations.

## 5.8. Combinational Timing

Our combinational device abstraction provides a simple guarantee on the timing of output signal validity: whenever all inputs to a combinational device have been stable and valid for the specified propagation delay for that device, each output is guaranteed to be a valid representation of the value dictated by the functional specification for the device. Beyond this promise, there are*no*validity guarantees on the outputs; in particular, unless all inputs have been valid for at least a propagation delay, an output may be a valid $0$, a valid $1$, or invalid.

**Inverter timing**

*timing diagram*such as that shown to the right. The diagram shows the timing of an inverter whose input $A$ is an "ideal" voltage waveform having two instantaneous transitions; each transition causes the inverter output $B$ to assume a new valid logic value after the prescribed propagation delay $t_{PD}$. Note that our timing diagram abstracts away details of the output signal behavior during the propagation delay: the crosshatched regions of the output waveform indicate, symbolically, that the output voltage is unspecified during these intervals. It may assume the valid output value earlier than required; it may oscillate between valid $0$ and $1$ during this period; or it may assume an invalid level in the forbidden zone. In the real-world circuits we build, we will typically see an exponential decay to the valid output value, but our abstraction leaves this detail completely unspecified.

As the diagram indicates, the duration of each unspecified interval is bounded by $t_{PD}$; keep in mind that this propagation delay is a device specification, and applies to all instances of the inverter and all operating conditions anticipated in its design. It is chosen to be conservative, providing a guaranteed upper bound to the settling time of the output.

**Cascaded inverters**

*valid*. If the input transition takes finite time (which, realistically, is always the case) we must allow time for a valid logic value to be reached before starting the $t_{PD}$ clock. To illustrate, consider adding a second inverter to our little test circuit, resulting in the timing diagram shown above. The output of the second inverter is unspecified for a $t_{PD}$ while its input $B$ -- the output of the first inverter -- is unspecified, and for a $t_{PD}$ following that interval.

## 5.9. Optional stronger guarantees

While the combinational device abstraction as described thus far serves the majority of our needs well, there are occasional circumstances where stronger guarantees are necessary. We will encounter an important such case in Chapter 8, when we use combinational devices to build digital memory elements.
In this section, we introduce two *extensions* to the combinational
device abstraction to deal with such extenuating circumstances. Each
amounts to an additial guarantee that can be specified for a combinational
device, extending its timing or functional specification, respectively,
beyond our usual standard.

### 5.9.1. Contamination Delay

Our combinational device abstraction provides for a*propagation delay*specification, explicitly recognising the physical reality that it takes some finite time for information in the inputs of a device to propagate to its outputs. The propagation delay is an upper bound on a period of output

*invalidity*; this specification is conservatively chosen, since invalidity of logic signals can cause systems to fail.

One can argue for recognition of another, symmetric physical reality:
given a device with valid inputs and outputs, it takes finite time
for the fact that an input has become *invalid* to propagate
to an output and contaminate its validity. Our model makes the
pessimistic assumption that any invalid input *immediately*
contaminates all outputs: all output guarantees are instantly voided
by any invalid input. This pessimism is, again, conservative:
systems can fail due to *invalidity* of logic levels, not
due to undocumented *validity* for a brief period after an
input change.

It is occasionally useful, however, to rely on an output remaining valid for a brief interval after an input becomes invalid. To support these infrequent needs, we introduce a new, optional specification that can augment a combinational device:

*$t_{CD}$: a specified*

**contamination delay***lower bound*on the time a valid output will persist following an input becoming invalid.

In the usual case, we assume that the contamination delay of a combinational
device is *zero* -- that is, that an invalid input immediately
contaminates (renders invalid) all outputs. In cases where we require
a nonzero contamination delay, it reflects a minumum time it will take
an *invalid* input to propagate to (and contaminate) an output.

**Inverter Contamination Delay**

*invalid*, while $t_{PD}$ starts when the input becomes

*valid*.

In Section 5.3.2, we noted that an acyclic circuit
whose components are combinational devices is itself a combinational
device, and observed that an appropriate specification for the propagation
delay of the circuit is the maximum cumulative propagation delay over
every input-to-output path through the circuit. This maximum corresponds
to our understanding of *propagation delay* as an *upper bound*
on the time for valid input values to propagate to each output.

In contrast, we can specify a *contamination delay* for the circuit
as the *minimum* cumulative contamination delay over every input-to-output
path through the circuit; this stems from our definition of contamination delay
as a *lower bound* on the time for an *invalid* input value
to propagate to any output.

We will see many examples of combinational circuits and their timing specifications in Chapter 7.

### 5.9.2. Lenient combinational devices

Combinational devices are not required to produce valid outputs unless each of their inputs are valid; using the terminology of denotational semantics, they compute*strict*functions of their inputs. This characteristic of combinational devices works fine for most of our logic needs, and in the vast majority of cases, all inputs to a combinational device will be valid for a propagation delay before we depend on the validity of its output.

However, it is possible (and occasionally useful) to make stronger guarantees, particularly in situations where the output of a function can be deduced when input values have been only partially specified.

**Combinational**

OR

OR

*independent*of the input $B$.

It is possible to build combinational devices that exploit situations
in which the output is determined by a subset of the input values,
and guarantee valid outputs in such cases. We call such devices
*lenient* (or non-strict) combinational devices, and define
them as follows:

**is a combinational device whose output is guaranteed valid whenever any combination of inputs sufficient to determine the output value has been valid for at least $t_{PD}$.**

*lenient combinational device*Notice that a lenient combinational device is, in fact, a combinational device; it is one offering an additional guarantee that it will produce a valid output when only a subset of its inputs are valid, to the extent possible.

**Lenient OR**

*don't care*values in the input columns, as shown to the left. Here the * in an input column can be read as

*"any value, valid or not"*; the "* 1" line, for example, explicitly guarantees that so long as $B=1$ the output $Y$ will be $1$.

Lenience is a convenient property, and is occasionally essential; we will see such a case in Chapter 8. Happily, basic gates made using our technology of choice as described in Chapter 6 are lenient, although more complex circuits are often not lenient unless special care is taken in their design. We will revisit this topic in Chapter 7.

## 5.10. Chapter Summary

In this chapter, we have taken a first major step toward the engineering abstraction that underlies digital system engineering: the representation of the*discrete*variables of our logic model using underlying continuously variable physical parameters. Our representation conventions take care to allow manufacturable devices to conform to realizable specifications made in the logic domain: it is robust in the presence of noise, parasitics, and manufacturing variations that inevitably plague real-world systems.

Key elements of this engineering discipline include:

- Our 4-parameter mapping convention, using $V_{ol}$, $V_{il}$,
$V_{ih}$, and $V_{oh}$ to partition a continuous range of voltages
into five distinct regions which define
*valid*representations of logical $1$s and $0$s; - The separation of valid $0$ and $1$ representations by voltage
ranges which do
*not*represent $1$ or $0$; - The
*combinational device*, an abstract model of logic elements whose specifications include a*functional specification*(e.g., a truth table) and a*propagation delay*$t_{pd}$, and which guarantees valid outputs whenever all inputs have been valid for at least $t{pd}$; - The incorporation of
*noise margins*in our logic mapping, requiring the outputs of combinational devices to adhere to stricter validity standards than the inputs they accept. This important property forces each combinational device to restore marginally valid logic signals to unequivocally valid $1$s and $0$s; - The fact that acyclic circuits whose components are combinational devices are, themselves, combinational devices. This gives us a simple construction rule for building large systems from a repertoire of small building blocks;
- An optional
*contamination delay*specification, which provides an optional guarantee that outputs of a combinational device remain valid for a short interval after inputs become invalid; and - The optional stronger guarantee offered by a
*lenient*combinational device, which guarantees valid outputs in cases where functionally irrelevent inputs are invalid.

The combination of these ingredients results in a simple engineering model that largely allows the designer of a system to ignore the complexities of voltages, phase, noise, and other real-world complications, working entirely in the abstracted world of $1$s, $0$s, and truth tables.

Copyright © 2016 M.I.T. Department of Electrical Engineering and Computer Science

Your use of this site and materials is subject to our terms of use.