Adder, part 2

At this point, in order to provide a complete solution to the Adder problem, we will exercise very basic, but practical cases of Chisel features advertised in the Big Picture: bit width inference and parameterization.

Bit Width Specification

Reminding ourselves of the value definitions:

val in0 = Input(UInt(1.W))
val in1 = Input(UInt(1.W))
val out = Output(UInt(1.W))

we note the part not mentioned during previous explanations: UInt(1.W) - this is where the value type (UInt) and bit width (1.W) are determined.

If the bit width of a value wasn't explicitly specified, the Chisel compiler would have inferred the bit width based on the inputs that define the value.

Basic Parameterization

Taking another look at the module definition, i.e. Scala class definition in Chisel:

class Adder(val w: Int) extends Module...

note val w: Int - a parameter passed to the class constructor. This means that, for every instance of the class Adder, this parameter may be passed as a user-defined value.

Thinking about the solution

Let's use this possibility to create an agile class Adder that can be called upon to construct an adder circuit of arbitrary bit width - please take a moment to think of what should be modified in the three lines defining values within the io bundle.

Solution

The correct way to modify value definitions is:

val in0 = Input(UInt(w.W))
val in1 = Input(UInt(w.W))
val out = Output(UInt(w.W))

In this way, every time an Adder instance is constructed, user can, without changing the Chisel source code, choose the adder circuit bit width.

Testing

Please edit and save Adder.scala and navigate back to $LJHOME/chisel-tutorial and run again:

./run-problem.sh Adder

The output should provide:

Circuit state created
[info] [0.001] SEED 1514975326903
test Adder Success: 10 tests passed in 15 cycles taking 0.128609 seconds
[info] [0.075] RAN 10 CYCLES PASSED
Tutorials passing: 1
[success] Total time: 17 s, completed Jan 3, 2018 11:28:51 AM

If there are any issues reported instead, please provide feedback using either the User Experiences page or the Hangout meeting.

Next Steps

Please, continue on to the next step of the Learning Journey, design of a counter.