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2. Encoders and Decoders
Active high/Active low inputs
• Active high inputs
need a +5V for the
pin to be considered
as logic high
• Active low inputs
need to be tied to the
0V for a logic high to
be applied.
+5V
0V
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Encoders• Used to convert from one numerical system to
another
• Decimal numbers are encoded to a binary
number
• Application
Keypad Encoder Display
Encoder• 74147 is a priority encoder
• Accepts a decimal input and gives the equivalent binary output
• The higher order input presented to the chip will be selected if two inputs are pressed at the same time
• E.g. 8 has priority over 7 etc
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Truth table
74147 example application block
diagram
Keypad
(Decimal)
74147
(Decimal to
BCD
Converter)
BCD output
(inverted)
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Typical circuit
Active Low
inputs
Active Low outputs
Limitations of the 74147
• Limited to 10 decimal inputs only
• Does not indicate when a switch has been
pressed
• The limitations are overcome by the
74HC148 encoder
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74HC148
• The 74HC148 provides the option to
cascade encoders
• They are
– Inputs can be enabled/disabled
– Signals when any input is pressed
– Cascadable to encode more than 8 inputs
74HC148
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74HC148
Application block diagram
Keypad
74148
(Decimal to
BCD
Converter)
74148
(Decimal to
BCD
Converter)
16 bit output
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Application circuit – Two 74HC148 cascaded together
Decoder
• Decoding is the process of translating data from a form to another
• Most common– Binary to Digital outputs
– Binary to 7 Segment displays to indicate a numerical value
• Applications– Decode and address from the PC Address bus to select a particular device
– To select different devices one at a time
• Example 74HC42,74HC138
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Use of decoders
Input
(Encode)Process Decode Display
74HC42 – BCD to decimal decoder
• Mutually exclusive outputs
• 1-of-8 de-multiplexing capability
• Outputs disabled for input codes above
nine
• Output capability: standard
• Integrated circuit category: MSI
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74HC42 – BCD to decimal decoder
Pin layout Function diagram
74HC42 – BCD to decimal decoder
Truth Table
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74HC42 – BCD to decimal decoder
Internal logic diagram
74HC42 – BCD to decimal decoder
Application circuit
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74HC138
• Decoders come in many different forms!
• Sometimes a single IC needs to be
selected from a group of IC’s
• 74HC138 features• Demultiplexing capability
• Multiple input enable for easy expansion
• Ideal for memory chip select decoding
• Active LOW mutually exclusive outputs
• Integrated circuit category: MSI
74HC138
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74HC138
74HC138 – Application circuit
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74HC138 – Alternative application
circuitDriven from 5V
Application using am encoder to
display the number of days in a month
