Decoders

Decoders• A decoder is multiple-input, multiple-output logic circuit

that converts coded inputs into coded outputs.• Input code with fewer bits than the output bits.

– Typically n inputs decoder has 2n outputs• 2-to-4, 3-to-8, 4-to-16, etc.

• There is a one-to-one mapping.

Decoders• General decoder structure

• Typically n inputs decoder has 2n outputs– 2-to-4, 3-to-8, 4-to-16, etc.

Binary 2-to-4 decoder

Note “x” (don’t care) notation.

2-to-4-decoder logic diagram

Decoder applications• Microprocessor memory systems

– selecting different banks of memory• Microprocessor input/output systems

– selecting different devices• Microprocessor instruction decoding

– enabling different functional units• Memory chips

– enabling different rows of memory depending on address• Lots of other applications

Decoding Circuits• Have to be able to decode particular combinations of input

signals.– Need to decode the address lines to determine where the

data is to go.– Used to transfer data from or to memory or peripherals.– Take a number of input signals and provide enough

outputs to indicate what the input was.

Decoding Circuits• If the input is two binary signals, there would have to be

four outputs. • One output for each input combination.

Two Bit Decoder

D0

D1

3

2

1

0

D1 D0 0 1 2 3

0 0

0 1

1 0

1 1

MSI 2-to-4 decoder

• Input buffering (less load)• NAND gates (faster)

Decoder Symbol

More decoder symbols

Complete 74x139 Decoder

3-to-8 decoder

74x138: 3-to-8-decoder symbol

Decoder cascading

4-to-16 decoder

More cascading

5-to-32 decoder

Decoder ICs• 74138Octal decoder (3-line-to-8-line)• 74154hex decoder (4-line-to-16-line)• 7442 BCD (Binary Coded Decimal) decoder (4-to-10)• 7447 BCD to seven-Segment decoder (4-line-to-7-line)

Binary encoders

Three-state buffers• Output = LOW, HIGH, or Hi-Z.

• Can tie multiple outputs together, if at most one at a time is driven.

Three-state buffers• When the enable input is not asserted, the device output “floats”; that is, it goes to a high-

impedance (Hi-Z), disconnected state and functionally behaves as if it weren’t even there.

Different flavors

timing• Typically three-state devices are designed so that they go

into the Hi-Z state faster than they come out of the Hi-Z state.

• That ensures the first device to get off the party line before the second one gets on.

• Otherwise excessive current will flow.• The safe way to use three-state devices is to design control

logic that guarantees a dead time, during which no one is driving the party line.