Digital Electronics/Lecture number systems and logic states

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Overview[edit | edit source]

This lecture will introduce some basic number system concepts and introduce number systems useful in electrical and computer engineering.

The decimal number system[edit | edit source]

In childhood, people are often taught the fundamentals of counting by using their fingers. Counting from one to ten is one of many milestones a child achieves on their way to becoming educated members of society. We will review these basic facts on our way to gaining an understanding of alternate number systems.

The child is taught that the fingers and thumbs can be used to count from one to ten. Extending one finger represents a count of one; two fingers represents a count of two, and so on up to a maximum count of ten. No fingers (or thumbs) refers to a count of zero.

The child is later taught that there are certain symbols called digits that can be used to represent these counts. These digits are, of course:

Ten, of course, is a special case, since it is comprised of two digits.

Before we go deeper, we need a few fundamental definitions.

Digit[edit | edit source]

A digit is a symbol given to an element of a number system.

Radix Notation[edit | edit source]

The radix, or base of a counting system is defined as the number of unique digits in a given number system.

Back to our elementary example. We know that our hypothetical child can count from zero to ten using their fingers and thumbs. There are ten unique digits in this counting system, therefore the radix of our elementary counting system is ten.

We represent the radix of our counting system by putting the radix in subscript to the right of the digits. For example,

represents 3 in decimal (base 10).

Our special case (ten) illustrates a fundamental rule of our number system that was not readily apparent - what happens when the count exceeds the highest digit? Obviously, a new digit is added, to the left of our original digit which is "worth more", or has a higher weight than our original digit. (In reality, there *always* are digits to the left; we simply choose not to write those digits to the left of the first nonzero.)

When our count exceeds the highest digit available, the next digit to the left is incremented and the original digit is reset to zero. For example:

Because we are dealing with a base-10 system, each digit to the left of another digit is weighted ten times higher. Using exponential notation, we can imagine the number 10 as representing:

The fundamentals of decimal arithmetic will not be expanded on in this lecture.

The binary number system[edit | edit source]

It is widely believed that the decimal system that we find so natural to use is a direct consequence of a human being's ten fingers and thumbs being used for counting purposes. One could easily imagine that a race of intelligent, six-fingered beings could quite possibly have developed a base-six counting system. From this perspective, consider the hypothesis: the most intuitive number system for an entity is that for which some natural means of counting exists.

Since our focus is electronic and computer systems, we must narrow our focus from the human hand to the switch, arguably the most fundamental structure that can be used to represent a count.

The switch can represent one of two states; either open, or closed. If we return to our original definition of a digit, how many digits are required to represent the possible states of our switch? Clearly, the answer is 2. We use the binary digits zero and one to represent the open and closed states of the switch.

Counting in Binary[edit | edit source]

Counting in binary like base 10 can be accomplished with your fingers with some differences. In decimal base 10 each finger represents a number in the first digit 1, 2, 3, 4, 5, 6, 7, 8, 9 ,10 . When counting in binary each finger represents a separate digit so counting is more like this.

each number is a finger
dont use thumbs for binary
value decimal binary
right    left 
hand   hand
0 00000 00000 0000
1 00000 01000 0001
2 00000 01100 0010
3 00000 01110 0011
4 00000 01111 0100
5 00000 11111 0101
6 00010 11111 0110
7 00110 11111 0111
8 01110 11111 1000
9 11110 11111 1001
10 11111 11111 1010

WHOA, hold on, that pattern doesn't make sense? Well when you consider that each digit in binary is worth 2 times more than the previous (right to left always) then it makes sense. Lets compare number lines.


128 ,64 ,32 ,16 ,8 ,4 ,2 ,1


1 ,2 ,3 ,4 ,5 ,6 ,7 ,8 ,9 ,10 ,11 ,12 ,13 ,14 ,15 ,16 ,17 ,18 ,19 ,20 ,21 ,22 ,23 ,24 ,25 ,26 ,27 ,....


Bits and bytes[edit | edit source]

A bit is a digit in the binary counting system.

A nybble (also spelled nibble) is a binary number consisting of four bits.

A byte, or octet is a binary number consisting of eight bits.

From our earlier definition of radix, the binary system has a radix of two. We use the radix in the subscript much like we do with decimals:

Counting[edit | edit source]

Similar to decimals, binary digits are weighted. Each bit is weighted twice as much as the bit to the right of it:

Counting from one to ten (base 10) in binary yields:

Basic binary arithmetic[edit | edit source]

Binary addition, subtraction, multiplication and division operations work essentially the same as they do for decimals.

For addition, you add equally-weighted bits, much like decimal addition (where you add equally-weighted digits) and carry as required to the left.

As you can see, a carry is generated in the column which increments the column.

Subtraction works just like decimal arithmetic, using borrowing as required.

Here, a borrow is required, reducing the column to and changing the column to .

Multiplication is straightforward also:

Division is left as an exercise to the student [hint: use long division].

Conversion between binary and decimal[edit | edit source]

When you consider a binary number in exponential form, you can easily perform a decimal conversion:

...simply add up the factors.

To convert from decimal to binary, you can repeatedly divide the decimal by two until the result of the division is zero. Starting from the rightmost bit, write 1 if the division has a remainder, zero if it does not. For example, to convert the decimal 74 into binary:

* 74 / 2 = 37 remainder 0; ->       0
* 37 / 2 = 18 remainder 1; ->      10
* 18 / 2 =  9 remainder 0; ->     010
*  9 / 2 =  4 remainder 1; ->    1010
*  4 / 2 =  1 remainder 0; ->   01010
*  2 / 2 =  1 remainder 0; ->  001010
*  1 / 2 =  0 remainder 1; -> 1001010

The octal number system[edit | edit source]

The name octal implies eight, if you consider that an octagon has eight sides. Octal has a radix of eight, and uses the following octal digits:

An octal number has the subscript 8.

Counting[edit | edit source]

Counting from one to ten (base 10) in octal yields:

Octal arithmetic[edit | edit source]

Octal arithmetic, like binary arithmetic, follows the same rules and patterns of decimal arithmetic. As an exercise, verify the following:

Conversion between binary and octal[edit | edit source]

Each octal digit is representable by exactly three bits. This becomes obvious when you consider that the highest octal digit is seven, which can be represented in binary by .

To convert a binary number to octal, group the bits in groups of three starting from the rightmost bit and convert each triplet to its octal equivalent.

To convert an octal number to binary, simply write the equivalent bits for each octal number.

Conversion from decimal to octal[edit | edit source]

The repeated-division method described for binary will also work for octal, simply by changing the divisor to eight. To convert 67 (base 10 into octal):

* 67 / 8 =  8 remainder 3; ->       3
*  8 / 8 =  1 remainder 0; ->      03
*  1 / 8 =  0 remainder 1; ->     103

The hexadecimal number system[edit | edit source]

The most commonly-used number system in computer systems is the hexadecimal, or more simply hex, system. It has a radix of 16, and uses the numbers zero through nine, as well as A through F as its digits:

Hex numbers can have the subscript 16, but more often have a leading *0x* to indicate their type.

Counting[edit | edit source]

Counting from zero to twenty (base 10) in hex yields:

Hexadecimal arithmetic[edit | edit source]

Hex arithmetic, yet again, follows the same rules and patterns of decimal arithmetic. As an exercise, verify the following:

Conversion between hexadecimal and binary[edit | edit source]

Each hex digit is representable by exactly four bits. This becomes obvious when you consider that the highest hex digit represents fifteen, which can be represented in binary by .

To convert a binary number to hex, group the bits in groups of four starting from the rightmost bit and convert each group to its hex equivalent.

To convert an hex number to binary, simply write the equivalent bits for each hex number.

Representing Logic States[edit | edit source]

high 1 or onlow 0 or off






X 100M
X 100K
X 100
X 100m
X 100μ






X 100M
X 100K
X 100
X 100m
X 100μ

VCC to Vhigh in = logic high
vlow in to gnd = logic low

technology type AC, HC, AHC, H ACT, HCT, AHCT, FCT F, S, AS, LS, ALS LV LV, LVC, ALVC
vcc 5v 5v 5v-4.5v 3.6v-3v 3.6v-3v
vhigh Out 4.7v 4.7v 3.3v or 2.4v 2.4v 3.5v
vhigh In Min 3.7v 2.0v 2.0v 2.0v 2.6v
vlow In Max 1.3v .8v .8v .8v .72v
vlow Out .2v .2v .35v .4v .54v

Most 5v logic will have no problem with a 5V high or a 0V low. You should always refer to the data sheet before choosing your device.[1]

The range in between "vhigh lower" and "vlow higher" should be avoided and as little time as possible should ever be spent here.

Exercises[edit | edit source]

1. Convert the following binary numbers to decimal:

2. Convert the following decimal numbers to binary:

3. Add the following numbers:

Further reading[edit | edit source]

  1. Mano, M. Morris. (2001) Digital Design, 3rd edition (Prentice Hall); ISBN 978-0130621214
  2. Floyd, Thomas L. (2002) Digital Fundamentals, 8th edition (Prentice Hall); ISBN 978-0130942005

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