Multiplexers ECE 352 Digital System Fundamentals Multiplexers 1

Multiplexers ECE 352 Digital System Fundamentals Multiplexers 1

Multiplexers (Muxes) Multiplexers • Selection: based on the select inputs, choose one from several data inputs to drive an output 2 – Output is equal to one of the data inputs – Select inputs indicate which data input • Typically: n select lines, 2 n data input lines Data Inputs and Labels Output Select Input

Multiplexers Multiplexer Labeling • Data inputs must be labeled to indicate which is selected for a given select value • Select inputs must be labeled so we know the ordering of bits within the select value – Exception: if there is only one select bit Data Inputs and Labels 3 Select Inputs and Labels

Multiplexer Structure • Can always build things by understanding their truth tables and then building the logic functions… Multiplexers 2: 1 multiplexer with 1 -bit select signal S 4: 1 multiplexer with 2 -bit select S Y = S 1·S 0·D 3 4

Multiplexer Logic Function • The two-level logic function for any mux output Y is really quite easy to remember Multiplexers – Write an OR of all minterms of the select inputs – AND each minterm with the corresponding data input 5 • Example: An 8 -to-1 mux has a 3 -bit select input S = S 2 S 1 S 0

Other Ways To Build Muxes • Build larger muxes using smaller muxes – Repeated refinement of the selection code Multiplexers • Decode select signals into individual enables for each data input 6 First-level muxes each choose an input based on select bit 0, select bit 1 chooses between the results

Hierarchical Mux Example Multiplexers D 3 D 1 1 0 7

Multiplexers Concept: Value-Fixing • Can set one or more inputs of a logic circuit to a constant (fixed) value (meaning, 0 or 1) • We might do this to make the logic circuit behave in a particular way – Example: set one input of a 2 -input XOR gate to 1 to make the XOR act like an inverter – Example: set one input of a 3 -to-8 decoder to 0 to make the 3 -to-8 decoder act like a 2 -to-4 decoder • Need to make sure to use the correct 4 outputs based on which input was set to 0! – Example: implement a logic function with a multiplexer by fixing multiplexer data inputs • This is how the FPGAs used in this class implement logic 8

Example: Logic With Muxes 1 1 Multiplexers 1 Truth Table 1 0 Function B Inputs A 9 B

Example: Logic With Muxes Multiplexers A B C 10 F 0 0 0 0 C 0 0 1 1 0 1 1 0 0 1 0 1 1 0 1 1 1 1 0 Identify what function is for each set of AB values 0 1 C F AB

Multiplexers ECE 352 Digital System Fundamentals Multiplexers 11