Chisel Shiftregister Example

Question

i'm very new with scala and Chisel. I tried to create a Shiftregister example with dynamic size, but i'm not sure if the following code is correct. It would be nice if someone could review it:

import Chisel._

class Shiftregister(length: Int) extends Module {
    val io = new Bundle {
        val clk        = UInt(INPUT,  1)
        val load       = UInt(INPUT,  1) // 1 read from s_data_in, 0 read from p_data_in
        val s_data_in  = UInt(INPUT,  1)
        val s_data_out = UInt(OUTPUT, 1)
        val p_data_in  = UInt(INPUT,  length)
        val p_data_out = UInt(OUTPUT, length)
    }

    val bitfield = Reg(init = UInt(length))

    when (io.load.toBool()) {
        bitfield := Cat(io.s_data_in, bitfield(length, 1))
    }
    .otherwise {
        bitfield := io.p_data_in
    }
    io.p_data_out := Reg(next = bitfield)
    io.s_data_out := Reg(next = bitfield(0))
}

class ShiftregisterTest(c: Shiftregister) extends Tester(c, Array(c.io)) {
    defTests {
        true
    }
}

object Shiftregister {
    def main(args: Array[String]): Unit = {
        chiselMainTest(Array[String]("--backend", "c", "--genHarness", "--v"), () => Module(new Shiftregister(16))){c => new ShiftregisterTest(c)}
    }
}

I tried to create an equivalent of the following VHDL code:

LIBRARY ieee;
USE ieee.std_logic_1164.ALL;

ENTITY shiftregister IS
    GENERIC(
        length: positive
    );
    PORT(
        clk: IN STD_LOGIC;
        load: IN STD_LOGIC; -- 1 read from s_data_in, 0 read from p_data_in
        s_data_in: IN STD_LOGIC := '0';
        s_data_out: OUT STD_LOGIC;
        p_data_in: IN STD_LOGIC_VECTOR(length-1 DOWNTO 0) := (others => '0');
        p_data_out: OUT STD_LOGIC_VECTOR(length-1 DOWNTO 0)
    );
END ENTITY shiftregister;

ARCHITECTURE synthesis OF shiftregister IS
    SIGNAL bitfield: STD_LOGIC_VECTOR(length-1 DOWNTO 0);
BEGIN
    PROCESS (clk) IS
    BEGIN
        IF RISING_EDGE(clk) THEN
            IF load = '0' THEN
                bitfield <= p_data_in;
            ELSE
                bitfield(length-1 DOWNTO 0) <= s_data_in & bitfield(length-1 DOWNTO 1);
            END IF;
        END IF;
    END PROCESS;
    p_data_out <= bitfield;
    s_data_out <= bitfield(0);
END ARCHITECTURE synthesis;

Answer 1

Here are my comments:

First, "clk" is unnecessary, as clock is implicit in Chisel.

Second, you should probably be using Bool() instead of UInt(width=1) for some of your signals.

val load = Bool(INPUT)

Although that's admittedly a stylistic opinion, but it prevents from needing to do the .toBool cast later.

Third, this line does not do what you are intending:

val bitfield = Reg(init = UInt(length))

That is creating a register that is initialized on reset to a UInt() of value "length". Instead, to create a register of width "length" do this:

val bitfield = Reg(outType=UInt(width=length))

You can also just use

val bitfield = Reg(UInt(width=length))

As the default parameter to Reg() is the "type" of register you want to create. However, IMO, that can be a bit ambiguous. If you want to initialize the register to 0, then do the following:

val bitfield = Reg(init = UInt(0, length))