Decoders and Multiplexers 91509 L 15 Decoders Multiplexers

Decoders and Multiplexers 9/15/09 - L 15 Decoders, Multiplexers Copyright 2009 - Joanne De. Groat, ECE, OSU 1

Class 15 – Decoders, Multiplexers o Decoders Multiplexers o Material from section 3 -7 and 3 -9 of text o 9/15/09 - L 15 Decoders, Multiplexers Copyright 2009 - Joanne De. Groat, ECE, OSU 2

Decoder background o o Given a n-bit binary number there are 2 n things it can represent or lines that can be activated Activated can mean pulling the signal high (Active High) or pulling the signal low (Active Low) 9/15/09 - L 15 Decoders, Multiplexers Copyright 2009 - Joanne De. Groat, ECE, OSU 3

Decoder definition o o o Decoding is the conversion of an n-bit input code to an m-bit output code with n ≤ m ≤ 2 n, such that each valid code work produces a unique output code. Decoding in performed by a logic circuit called a decoder. A decoder may have unused bit combinations on its inputs. For these no valid output occurs. 9/15/09 - L 15 Decoders, Multiplexers Copyright 2009 - Joanne De. Groat, ECE, OSU 4

n-to-m-line decoders o o o Circuit has n inputs and m outputs and m ≤ 2 n Start with n=1 and m=2 This a 1 -to-2 Line decoder – exactly one of the output lines will be active. 9/15/09 - L 15 Decoders, Multiplexers Copyright 2009 - Joanne De. Groat, ECE, OSU 5

n-to-m-line decoders o o o A decoder when n=2 and m=4 A 2 -to-4 line decoder Note that only one output is ever active 9/15/09 - L 15 Decoders, Multiplexers Copyright 2009 - Joanne De. Groat, ECE, OSU 6

Can be seen to create structure o For the 2 -to-4 line o Contains 2 2 -to-1 line decoders and the additional logic. 9/15/09 - L 15 Decoders, Multiplexers Copyright 2009 - Joanne De. Groat, ECE, OSU 7

Higher levels of structure o A 3 -to-8 line decoder o Can be extended to even high levels 9/15/09 - L 15 Decoders, Multiplexers Copyright 2009 - Joanne De. Groat, ECE, OSU 8

And on the other hand o Encoding – An encoder is a digital function that performs the inverse operation of a decoder. An encoder has 2 n (or fewer) inputs and n output lines that indicate which single bit of the input is active. 9/15/09 - L 15 Decoders, Multiplexers Copyright 2009 - Joanne De. Groat, ECE, OSU 9

Octal to binary Encoder o Table 3 -7 of text. 9/15/09 - L 15 Decoders, Multiplexers Copyright 2009 - Joanne De. Groat, ECE, OSU 10

Selecting o o o The Multiplexer – a digital building block for selecting and routing signals The select signal, S, routes input I 0 or I 1 to the output Y in this 2 -to-1 multiplexer. Y = S’ I 0 + S I 1 9/15/09 - L 15 Decoders, Multiplexers Copyright 2009 - Joanne De. Groat, ECE, OSU 11

Multiplexing in general o o You have 2 n input lines and a n-bit selection control input which routes that specific input to the output. Have seen a 2 -to-1 mux. – The circuit 9/15/09 - L 15 Decoders, Multiplexers Copyright 2009 - Joanne De. Groat, ECE, OSU 12

Also common is the 4 -to-1 mux o o o In the 2 -to-1 mux there is a 1 -to-2 decoder and then the 2 enabling and gates In a 4 -to-1 mux you have a 2 -to-4 decoder to select which of the 4 inputs, I 0, I 1, I 2, I 3 is routed to the output Y. 4 -to-1 mux n n n 2 select inputs 4 data line inputs 1 output 9/15/09 - L 15 Decoders, Multiplexers Copyright 2009 - Joanne De. Groat, ECE, OSU 13

Typical occurances o o The size for typical use can be seen from the configurations that were available in SSI/MSI components. Available n n o 1 -to-2, 2 -to-4, 3 -to-8, and 4 -to-16 line decoders 2 -to-1, 4 -to-1, 8 -to-1 multiplexers Larger sizes, when needed could be implemented from the basic units. 9/15/09 - L 15 Decoders, Multiplexers Copyright 2009 - Joanne De. Groat, ECE, OSU 14

Class 15 assignment o o Covered sections 3 -7 and 3 -10 Problems for hand in n o Problems for practice n o none 3 -28. 33, 37, 44, 46, 48 Reading for next class: sections same 9/15/09 - L 15 Decoders, Multiplexers Copyright 2009 - Joanne De. Groat, ECE, OSU 15