How to delete clock signal on chisel3 top module?

Question

I have a blinking led design who use a differential clock input (xilinx AC701 kit). To instantiate the Xilinx differential buffer I'm using a BlackBox as explained by jkoening here:

class Top extends Module {
  val io = IO(new Bundle{
    val clock_p = Input(Clock())
    val clock_n = Input(Clock())
    val led = Output(Bool())
  })

  val ibufds = Module(new IBUFDS)
  ibufds.io.I := io.clock_p
  ibufds.io.IB:= io.clock_n

  val blink = Module(new Blink)
  blink.clock := ibufds.io.O
  io.led := blink.io.led  

}

That works, but On the Top verilog module I have a useless clock input :

module Top(
  input   clock,
  input   reset,
  input   io_clock_p,
  input   io_clock_n,
  output  io_led
);
...

Then on the target only io_clock_p and io_clock_n are used for clock input. clock signal is useless. Is there a proper way to hide it ?

Why aren't you asking the same about reset? I'm guessing clock and reset are hard-coded to always exist, because they are so fundamental. — Jonathon Reinhart
In fact, I'm asking the same about reset. clock and reset not always exist, they aren't exist in blackbox for example. — FabienM

FabienM FabienM · Accepted Answer · 2017-04-27T07:45:27

After discussion with Chisel3 team a feature has been added to support this.

The solution is to use a RawModule:

class Top extends RawModule {
  val clock_p = IO(Input(Clock()))
  val clock_n = IO(Input(Clock()))
  val led = IO(Output(Bool()))

  val ibufds = Module(new IBUFDS)
  ibufds.io.I := clock_p
  ibufds.io.IB:= clock_n

  withClockAndReset(ibufds.io.O, false.B) {
    val blink = Module(new Blink)
    led := blink.io.led
  }
}

A raw module has no implicit signal, and name are reflected «has it» in verilog generated file, no «io_» in prefix.

The source code of full blinking led project is available on this github project (blinking led project : blp).

Here is a french description of how to do it. To use this feature, last git master version of chisel3 must be used.

How to delete clock signal on chisel3 top module?

2 Answers