Printing in Chisel

Chisel provides the printf function for debugging purposes. It comes in two flavors:


Chisel also supports printf in a style similar to Scala’s String Interpolation. Chisel provides a custom string interpolator cf which follows C-style format specifiers (see section C-style below).

Note that the Scala s-interpolator is not supported in Chisel constructs and will throw an error:

class MyModule extends Module {
  val in = IO(Input(UInt(8.W)))
  printf(s"in = $in\n")
// error: The s-interpolator prints the Scala .toString of Data objects rather than the value of the hardware wire during simulation. Use the cf-interpolator instead. If you want an elaboration time print, use println.
//   printf(s"in = $in\n")
//   ^^^^^^^^^^^^^^^^^^^^^

Instead, use Chisel’s cf interpolator as in the following examples:

val myUInt = 33.U
printf(cf"myUInt = $myUInt") // myUInt = 33

Note that when concatenating cf"..." strings, you need to start with a cf"..." string:

// Does not interpolate the second string
val myUInt = 33.U
printf("my normal string" + cf"myUInt = $myUInt")

Simple formatting

Other formats are available as follows:

val myUInt = 33.U
// Hexadecimal
printf(cf"myUInt = 0x$myUInt%x") // myUInt = 0x21 // myUInt = 0x21
// Binary
printf(cf"myUInt = $myUInt%b") // myUInt = 100001 // myUInt = 100001
// Character
printf(cf"myUInt = $myUInt%c") // myUInt = !

Aggregate data-types

Chisel provides default custom “pretty-printing” for Vecs and Bundles. The default printing of a Vec is similar to printing a Seq or List in Scala while printing a Bundle is similar to printing a Scala Map.

val myVec = VecInit(5.U, 10.U, 13.U)
printf(cf"myVec = $myVec") // myVec = Vec(5, 10, 13) // myVec = Vec(5, 10, 13)

val myBundle = Wire(new Bundle {
  val foo = UInt()
  val bar = UInt()
}) := 3.U := 11.U
printf(cf"myBundle = $myBundle") // myBundle = Bundle(a -> 3, b -> 11)

Custom Printing

Chisel also provides the ability to specify custom printing for user-defined Bundles.

class Message extends Bundle {
  val valid = Bool()
  val addr = UInt(32.W)
  val length = UInt(4.W)
  val data = UInt(64.W)
  override def toPrintable: Printable = {
    val char = Mux(valid, 'v'.U, '-'.U)
    cf"Message:\n" +
    cf"  valid  : $char%c\n" +
    cf"  addr   : $addr%x\n" +
    cf"  length : $length\n" +
    cf"  data   : $data%x\n"

val myMessage = Wire(new Message)
myMessage.valid := true.B
myMessage.addr := "h1234".U
myMessage.length := 10.U := "hdeadbeef".U


Which prints the following:

  valid  : v
  addr   : 0x00001234
  length : 10
  data   : 0x00000000deadbeef

Notice the use of + between cf interpolated “strings”. The results of cf interpolation can be concatenated by using the + operator.


Chisel provides printf in a similar style to its C namesake. It accepts a double-quoted format string and a variable number of arguments which will then be printed on rising clock edges. Chisel supports the following format specifiers:

Format Specifier Meaning
%d decimal number
%x hexadecimal number
%b binary number
%c 8-bit ASCII character
%% literal percent

It also supports a small set of escape characters:

Escape Character Meaning
\n newline
\t tab
\" literal double quote
\' literal single quote
\\ literal backslash

Note that single quotes do not require escaping, but are legal to escape.

Thus printf can be used in a way very similar to how it is used in C:

val myUInt = 32.U
printf("myUInt = %d", myUInt) // myUInt = 32