1. EET 1131 Unit 8 Code Converters, Multiplexers, and Demultiplexers
Read Kleitz, Chapter 8, skipping Sections 8-2 and 8-4.
Homework #8 and Lab #8 due in a week and a half.
Quiz when Homework #8 is due.
-Handouts: Practice sheets for comparator, comparator timing diagram, decoder, encoder, Gray code, MUX, DEMUX.
Comparator: 7485
Decoders: 74138, 7442, 74154
Encoders: 74147, 74148
Code converters: 7447, 74184

2. Types of Chips
Here are the kinds of chips we’ll study in the coming weeks:
Comparators
Decoders
Encoders Chapter 8
Code converters
Multiplexers
Demultiplexers
Latches & Flip-flops Chapter 10
Counters Chapter 12
Shift registers Chapter 13
Multivibrators Chapter 14
Memory Chapter 16
Day 1 of this unit=1st two; day 2=2nd two; day 3=3rd two.

3. Types of Chips (Continued)
For each type of chip listed on previous slide, you should understand:
What that type of chip does, and why it’s useful.
How you could build such a circuit out of gates.
Specific details of actual chips in each category.

4. Example Solution Comparators
The function of a comparator is to compare the magnitudes of two binary numbers to determine the relationship between them. In the simplest form, a comparator can test for equality using XNOR gates.
Example
How could you test two 4-bit numbers for equality?
Solution
AND the outputs of four XNOR gates. A1 B1 A2 B2
-Have them do two examples: 1.) A=1001, B=1001; 2.) A=1001, B=0101
Output A3 B3 A4 B4

5. Comparators
IC comparators provide outputs to indicate which of the input numbers is larger or if they are equal. Cascading inputs are provided to expand the comparator to larger numbers. A1 A0 A2 A3 B1 B0 B2 B3
Cascading inputs
COMP
A = B
A < B
A > B 3 A B
Outputs
-Note numbering of input bits: A0 and B0 are LSBs, A3 and B3 are MSBs.
-Have them do practice sheet: A=1001, B=0101
The IC shown is the 4-bit 7485.

6. Comparators
IC comparators can be expanded using the cascading inputs as shown. The lowest order comparator has a HIGH on the A = B input.
Outputs A1 A0 A2 A3 B1 B0 B2 B3
COMP
A = B
A < B
A > B 3 A B A5 A4 A6 A7 B5 B4 B6 B7
+5.0 V
LSBs
MSBs
-Note numbering of input bits: A0 and B0 are LSBs, A7 and B7 are MSBs.
-Have them do practice sheet: 1) A= , B= ; 2) A= , B=

7. Comparator Chip 7485 Four-bit magnitude comparator
-Do practice timing diagram for 7485.

8. Enable Pins
Many of the chips we’ll study have enable inputs. Depending on the logic level at this pin, the chip is either enabled or disabled.
When the chip is enabled, it performs its intended function and the outputs behave as you would expect.
When the chip is disabled, then (usually) all outputs are forced to their inactive state, regardless of the other inputs to the chip.
Common names for enable pins include EN, G (for “gate”), and CS (for “chip select”).
-Example: enable line on a 2-bit comparator chip. 8

9. Active-High versus Active-Low Pins
Each input pin and output pin on a chip is either active-high or active-low.
In a logic symbol:
Active-low pins are marked with a bubble or triangle.
Active-high pins have no bubble or triangle.
Active-high pins: the pin is active when there’s a HIGH on that pin.
Many chips have active-low pins: the pin is active when there’s a LOW on that pin.
-Input example: enable line on a comparator chip.
-Output example: output pins on a comparator.

10. Example: Decoder
-Note different pin names as well as different conventions for active-low.
-Note: Individual pins are either active-high or active-low. It’s not correct to refer to a chip as being active-high or active-low.
From Floyd, p From Texas Instrument datasheet

11. Decoders, Encoders, & Code Converters
Decoders convert a binary code into a single active output representing the code’s value.
Encoders generate a coded output from a single active input line.
Code converters take one input code (such as BCD) and convert it to another code (such as binary).
-Demo a decoder with 5 students: me as 2-bit input, one student as decoder, the other four as outputs.
-Demo an encoder with 5 students: four students as inputs, one student as encoder.

12. Decoders
A decoder is a logic circuit that detects the presence of a specific combination of bits at its input. Two simple decoders that detect the presence of the binary code 0011 are shown. The first has an active HIGH output; the second has an active LOW output. A0 A0 A1 X A1 X A2 A2 A3 A3
Active HIGH decoder for 0011
Active LOW decoder for 0011

13. Decoders
Assume the output of the decoder shown is a logic 1. What are the inputs to the decoder?
Question

14. Decoders
IC decoders have multiple outputs to decode any combination of inputs. For example the hex decoder shown here has 16 outputs – one for each combination of binary inputs.
Question
For the input shown, what is the output?

15. Decoders
X/Y
A specific integrated circuit decoder is the 74154, a 4-to-16 decoder. It includes two active LOW chip select lines which must be at the active level to enable the outputs. These lines can be used to expand the decoder to larger inputs. A0 A1 A2 A3
CS1
CS2 EN
74154

16. Octal Decoder 3 data input pins for input code. 8 output pins.
Also called 1-of-8 decoder or 3-line-to-8-line decoder.
May have other inputs and outputs too, such as enable inputs.
Example chip: 74138
-Follow link for pin-out and logic symbol.
-Active-high data inputs, active-low outputs, mixture of enable inputs.
-Pins are laid out sensibly.
-Do timing diagram for

17. Hex Decoder 4 data input pins for input code. 16 output pins.
Also called 1-of-16 decoder or 4-line-to-16-line decoder.
May have other inputs and outputs too, such as enable inputs.
Example chip: 74154
-Follow link for pin-out and logic symbol.
-Active-high data inputs, active-low outputs, active-low enable inputs.
-Pins are laid out sensibly.

18. BCD Decoder 4 data input pins for input code. 10 output pins.
Also called 1-of-10 decoder or 4-line-to-10-line decoder.
May have other inputs and outputs too, such as enable inputs.
Example chip: 74LS42
- Follow link for pin-out and logic symbol.
-Active-high data inputs, active-low outputs, no enable inputs.
-Pins are laid out sensibly.

19. Encoders
An encoder accepts an active logic level on one of its inputs and converts it to a coded output, such as BCD or binary. 1
The basic logic diagram is shown. This encoder has an input for each decimal digit, and four outputs that represent the binary code for the active input digit.
There is no zero input because the outputs are all LOW when the input is zero. A0 2 3 A1 4 5 A2 6 7
Go straight to next slide. 8 A3 9

20. Example Solution Encoders
Show how the BCD encoder converts the decimal number 3 into a BCD 0011.
Example
The top two OR gates have ones as indicated with the red lines. Thus the output is 0011.
Solution 1 1 1 A0 2 3 1 A1
-Do it on practice sheet.
-Note what happens if inputs 3 and 5 are high at same time. 4 5 A2 6 7 8 A3 9

21. Encoders
The is an example of an IC encoder. It is has ten active-LOW inputs and converts the active input to an active-LOW BCD output.
VCC
This device offers additional flexibility in that it is a priority encoder. This means that if more than one input is active, the one with the highest order decimal digit will be active.
HPRI/BCD
Decimal input
BCD output
-Could call this a 10-line-to-4-line encoder.
-Introduce concept of priority encoder.
74HC147
The next slide shows an application …
GND

22. Encoders Keyboard encoder
VCC
Encoders
Keyboard encoder
HPRI/BCD
BCD complement of key press
74HC147
The zero line is not needed by the encoder, but may be used by other circuits to detect a key press.

23. BCD Encoder 10 input pins. 4 output pins for output code.
Also called 10-line-to-4-line encoder.
May have other inputs and outputs too, such as enable inputs.
Example chip: 74147
-Follow link for pin-out and logic symbol.
-Active-low data inputs, active-low outputs, no enable inputs.
-Pins are not laid out sensibly.
-Priority encoder.
-Do timing diagram for

24. Octal Encoder 8 input pins. 3 output pins for output code.
Also called 8-line-to-3-line encoder.
May have other inputs and outputs too, such as enable inputs.
Example chip: 74148
-Follow link for pin-out and logic symbol.
-Active-low data inputs, active-low outputs, other inputs & outputs.
-Pins are not laid out sensibly.
-Priority encoder.

25. Different Numeric Codes
Several different codes exist for using 1s and 0s to represent positive integers.
Standard binary code
Example: In standard binary, 15 is 1111.
Binary-coded decimal (BCD)
Example: In BCD, 15 is
Gray code
Example: In Gray code, 15 is 1000.

26. Four-Bit Gray Code
The key feature of Gray code is that only one bit changes when we increase a number by one.
This is not true of standard binary.

27. Why is Gray Code Useful?
Gray code is used for rotary encoders that sense the angular position of a shaft or axle.
From Wikipedia article on rotary encoders:
Do Gray code practice sheet.
Standard 3-bit binary code: no good!
3-bit Gray code: better!

28. Code Converters
If a digital system needs to handle numbers using two different codes, it needs circuitry to convert between the two codes.
Examples of code converters:
74184 BCD-to-binary and binary-to-BCD converter
Binary-to-Gray code or Gray-code-to-binary converters (see next slide)

29. Gray Code/Binary Converters
Figure Binary-to-Gray-code converter
Figure Gray-code-to-binary converter

30. Question Multiplexers
A multiplexer (MUX) selects one data line from two or more input lines and routes data from the selected line to the output. The particular data line that is selected is determined by the select inputs.
Two select lines are shown here to choose any of the four data inputs.
Select
inputs S0 1 S1 D0
Data
output
Question
Data
inputs D1
-MUX = Data selector; DEMUX = data distributor.
-Introduce examples of MUXes and DEMUXes as traffic routers: such as phone lines from Dayton to Phoenix.
-Data can be analog or digital.
-Do practice sheet. D2
Which data line is selected if S1S0 = 10? D3 D2

31. Some Multiplexer Chips
74150 (16-input MUX)
74151 (8-input MUX)
74153 (dual 4-input MUX)
74157 (quad 2-input MUX)
-Discuss common control block on symbols for ‘53 and ‘157.
-Do timing diagram for and troubleshooting for MUXes.

32. Demultiplexers
A demultiplexer (DEMUX) performs the opposite function from a MUX. It switches data from one input line to two or more data lines depending on the select inputs.
The 74LS138 was introduced previously as a decoder but can also serve as a DEMUX. When connected as a DEMUX, data is applied to one of the enable inputs, and routed to the selected output line depending on the select variables. Note that the outputs are active-LOW as illustrated in the following example…
Data select lines
Data outputs
Enable
inputs
First go back to previous MUX slide and sketch a hypothetical DEMUX.
74LS138

33. Example Solution Demultiplexers A A A G G LOW LOW G Y Y Y Y Y Y Y YA
Demultiplexers 1 A
Determine the outputs, given the inputs shown. 2
Example G 1
Solution G
LOW 2A
The output logic is opposite to the input because of the active-LOW convention. (Red shows the selected line). G
LOW 2B Y Y 1
Data select lines Y 2 Y
Data outputs
-Do practice sheet. 3 Y 4
Enable
inputs Y 5 Y 6 Y
74LS138 7

34. Some Demultiplexer Chips
74138 (3-line to 8-line Decoder/DEMUX)
74154 (4-line to 16-line Decoder/DEMUX)
74139 (dual 2-line to 4-line Decoder/DEMUX)
-Do troubleshooting exercise for 74139/154.