1

For more complicated structures we can use a K

map. For example:

Exercise 10.3 Implement the Boolean function

F= (0, 1, 3, 6, 7, 8, 11, 12, 14) using an 8 x 1 multiplexer using D as the input variable.

If D is the input variable then we are looking for all the 8 possible combinations of A, B and C.

e.g. 0 0 0 is A B C and 1 1 0 is A B C etc.

Lecture 11

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A B A B A B A B

C D

C D

C D

C D 1

1

1

1

1

1

1

1

1

Step 1 Mark 1s in the appropriate square of the K map

Step 2 Group together the 8 ABC terms and write out what the values of D and NOT D are

3

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A B C NOT D D Data Entry 0 0 0 1 1 1 0 0 1 0 1 D 0 1 0 0 0 0 0 1 1 1 1 1 1 0 0 1 0 NOT D 1 0 1 0 1 D 1 1 0 1 0 NOT D 1 1 1 1 0 NOT D

If both D and NOT D are present (1,1) then the data entry is 1

If neither D nor NOT D is present (0,0) then the data entry is 0

If only one entry is present (either (0,1) or (1,0)) (then term with 1 is the data entry)

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Exercise 11.1

Redo the exercise 10.3 using A as the data inputs and B C D as the control lines. You can use the same K map.

Show that the data entry values are (0-7):

1, NOT A, 0, 1, A, 0, 1 and NOT A

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Exercise 11.2

Redo the exercise 10.3 using C and D as the data inputs and A and B You can use the same K map.

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Question: If B C and D are the control lines, how would yourepresent a term such as

B C D on a K map.

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For the function F = (0, 1, 3, 6, 7, 8, 11, 12, 14) we used an 8 x 1 multiplexer. This is better than using a 16 x 1 multiplexer.

If we used a 16 x 1 then with D as the input variable we would simply have

D0 = 1, D1 = 1,

D2 = 0, D3 = 1,

D4 = 0, D5 = 0,

D6= 1, D7 = 1,

D8 = 1, D9 = 0,

D10 = 0, D11 = 1,

D12 = 1, D13 = 0,

D14 = 1, D15 = 0

We got these numbers from the values of D and NOT D in the truth table.

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So for a 4 variable system using a 16 x 1 MUX ….

In the second assignment we will explore another example of this and will related to the earlier combinaitonal logic functions.

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The opposite of the multiplexer circuit, is the demultiplexer.

This circuit takes a single data input and one or more address inputs, and selects which of multiple outputs will receive the input signal.

Y

Out 1

Out 2

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A 2-to-4 line decoder/demultiplexer is shown below.

A

Out 1IN

Out 0

B

Out 2

Out 3

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Fixed function to Programmable Logic Devices

Fixed function logic and programmable logic are two broad categories of digital IC – with the logic functions of the former being set by the manufacturer and are classified by their complexity.

Small scale integration (SSI) : up to 12 equivalent (the basic gates.)

Medium scale integration (MSI): from 12 - 90 equivalent gates circuits. (encoders, multiplexer and arithmetic circuits).

Large scale integration (LSI): 100 – 9999 equivalent gates per chip. An example would be memories

Very-large scale integration (VLSI) 10,000-99,999 equivalent chips

Ultra large scale integration (ULCI) has over 100,000 equivalent chips.

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• Fixed function chips are those designed by the chip

manufacturer = mask programmable devices.

• Once produced they cannot be altered by the user.

• There are other types of chip available which can

programmed by the user and sometimes reprogrammed

by the user – (field) Programmable logic devices. (PLDs)

• Classified by their architecture – internal functional

arrangement of their array: – the AND array and the OR

array.

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Programmable Read Only Memory, PROMs

A Programmable Read Only Memory consists of a set of fixed AND gates connected to a decoder and a programmable OR array.

The PROM is used primarily used as an addressable memory not as a logic device.

The data stored by a ROM is permanent and cannot be changed furthermore ROM is an example of non-volatile memory i.e. the contents are preserved even if no power is applied.

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• The truth table is implemented by a matrix.

• The required connections at the intersections being achieved by means of suitable electronic components.

• The n inputs are fed into a decoder and the output of the decoder form the matrix rows.

•The output of the circuit is made from the columns of the matrix.

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inputs

A

C

B

3-to-8linedecoder

D0 D1 D2 D3

01234567

Exercise 11.3. How could we use this arrangement to implement the following functions

D0= A B C + A B C D1 =A B C + A B C + A B C

D2=A B C +A B C + A B C D3 = A B C + A B C + A B C + A B C

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Summary

1. A multiplexer is a deice that takes several inputs and puts them onto a single line at different times.

2. What signal is passed is determined by the logic used.

3. For a 4 variable MUX, we can have 16 inputs using 1 and 0s or 8 lines using a single variable or 4 lines using two variables as the data entry.

4. The opposite to a MUX is a demuliplexer.

5. PROM - fixed function memory using an address decoder.

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From ROMs to PALs and PLAs

The AND-OR programmable architecture devices can be summarised as follows

1. Fixed AND and programmable OR (PROM)

2. Programmable AND - fixed OR (PAL)

3. Programmable AND - programmable OR (PLA)

Lecture 12

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A PROM employs an address decoder,

the PLD employs a programmable address matrix (AND matrix).

There are two main types of PLD.

1. A basic PAL (programmable array logic) device consists of an array of programmable AND gates whose those outputs are connected to a fixed array of OR gates.

2. A PLA (programmable logic array ) – Programmable ANDs and programmable OR arrays.

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Every input and its complement can be connected to or disconnect from every AND gate. This can be represented in a couple of different ways.

A B

Y=ABX

X

A B

Y=ABX X

A NOT A function can also be represented using this shorthand notation

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Example 12.1 : Implement the logical functions F0= A B + B C and

F1 = A C + A B C using a 3 x 4 PAL

A A B B C C

Programmable AND arrayFixed OR array

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PLA (programmable logic array).

A A B B

Programmable AND array Programmable OR array

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Example 12.2 :

Implement the following logical output expressions using

(i) An 8x3 ROM(ii) A 3 input, 3 output PAL with a 6 product lines, 2 per lines per output(iii) A 3 input, 3 output PLA with a maximum of 4 product lines

Y1= B C + A B C + A B C

Y2 = B C + A B C + A B C

Y3 = A C + A B C

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inputs

A

C

B

3-to-8linedecoder

Y1 Y2 Y3

01234567

PROM – we need a 3-to-8 decoder

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PAL: For the PAL we are told we have a maximum of 6 product lines with 2 per output.

1. For Y1

A B A B A B A B

C

C

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A B A B A B A B

C

C

A B A B A B A B

C

C

2. For Y2

3. For Y3

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A A B B C C

Programmable AND array Fixed OR array

Y1 Y2 Y3

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PLA: We note that the terms derived from the K map are

This means that although we have 6 terms since 1 of them is repeated twice so we need only a maximum of 4 product lines

A C (three times), A B, A B and B C

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A A B B C C

Programmable AND array Programmable OR array

Y1 Y2 Y3

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Exercise 12.3 Implement the following logical output expressions using

(i) An 8x3 ROM(ii) A 3 input, 3 output PAL with a 6 product lines, 2 per lines per output(iii) A 3 input, 3 output PLA with a maximum of 4 product lines

F1 = A B C + A B C + A B C + A B C

F2 = A B C + A B C + A B C

F3 = A B C + A B C + A B C + A B C

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Summary

1. PROM – address decoder

2. A PAL (programmable array logic) - programmable AND and fixed OR gates.

3. A PLA (programmable logic array )

– Programmable ANDs and programmable OR arrays.