different ways to implement mux

mr_vasanth · Mar 10, 2014

Method1:

Code:

assign y = sel ? a : b;

Method2:

Code:

case sel
1'b0: y = b;
1'b1: y = a;
endcase

Method3:

Code:

if (sel)
y = a;
else
y = b;

What is the difference between the implementation of mux using these three different methods ?
How do these methods treat 'x' and 'z' on sel input ?

After synthesis, will all these three methods produce same netlist ?

viveks2585 · Mar 11, 2014

Hi,

Using conditional operator, there will be an issue with select lines if they are x or z.
But when you use if else for implementing the mux, then x in select line then it will execute else condition.

For beter understanding, just implement the design and run the simulation once. Always a case block should have default statement in it.

Thanks,
Vivek.S

mr_vasanth · Mar 23, 2014

Found the difference between if-else and ternary/conditional operator based mux implementation.

1. if-else
during simulation, if the sel line is 'x' then the output y will be b.

2. ternary/conditional operator
during simulation, if the sel line is 'x' then the output y will be 'x'.

Coding Guideline: Never use “if-else” statement in combinatorial logic. Instead, use ternary/conditional “?” operator.

can someone explain how the case statement based mux will behave if the sel is 'x' ?

jt_eaton · Mar 23, 2014

mr_vasanth said:
Found the difference between if-else and ternary/conditional operator based mux implementation.

1. if-else
during simulation, if the sel line is 'x' then the output y will be b.

2. ternary/conditional operator
during simulation, if the sel line is 'x' then the output y will be 'x'.

Coding Guideline: Never use “if-else” statement in combinatorial logic. Instead, use ternary/conditional “?” operator.

can someone explain how the case statement based mux will behave if the sel is 'x' ?

The case statement will output your default value. That is why you need a default.

If sel = x, a=1 and b=1 then the ternary will deliver an x even though hardware will deliver a 1. That is not good. You should also distinguish between glitching and glitchless muxes.

John Eaton

mr_vasanth · Mar 24, 2014

John, I understand your point about default case statement. But what will be the output if the case statement does not have a default case statement.

different ways to implement mux

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