How to use a vector as input in Chisel

Question

I am designing a Chisel module with the following code:

import chisel3._
import chisel3.util._

class DisplayDriver extends Module {
    val io = IO(new Bundle {
        val digits = Input(Vec(4, UInt(4.W)))

        val leds = Output(UInt(7.W))
        val selector = Output(UInt(4.W))
    })

    // Divisor de frecuencia
    val freqDiv = RegInit(0.U(11.W))
    val tick = freqDiv === (4000 - 1).U

    freqDiv := freqDiv + 1.U;
    when (tick) {
        freqDiv := 0.U
    }

    // Multiplexor de dígitos
    val digitSel = RegInit(0.U(2.W))
    when (tick) {
        digitSel := digitSel + 1.U
    }

    val digit = UInt(4.W)
    digit := io.digits(digitSel)

    // Decodificador para el selector
    io.selector := 0.U
    switch (digitSel) {
        is (0.U) { io.selector := "b1110".U }
        is (1.U) { io.selector := "b1101".U }
        is (2.U) { io.selector := "b1011".U }
        is (3.U) { io.selector := "b0111".U }
    }

    // Decodificador para los leds
    io.leds := 0.U
    switch (digit) {
        is (0.U) { io.leds := "b0000001".U }
        is (1.U) { io.leds := "b1001111".U }
        is (2.U) { io.leds := "b0010010".U }
        is (3.U) { io.leds := "b0000110".U }
        is (4.U) { io.leds := "b1001100".U }
        is (5.U) { io.leds := "b0100100".U }
        is (6.U) { io.leds := "b0100000".U }
        is (7.U) { io.leds := "b0001111".U }
        is (8.U) { io.leds := "b0000000".U }
        is (9.U) { io.leds := "b0000100".U }
        is (10.U) { io.leds := "b0001000".U }
        is (11.U) { io.leds := "b1100000".U }
        is (12.U) { io.leds := "b0110001".U }
        is (13.U) { io.leds := "b1000010".U }
        is (14.U) { io.leds := "b0110000".U }
        is (15.U) { io.leds := "b0111000".U }
    }
}

class Top extends Module {
    val io = IO(new Bundle {
        val leds = Output(UInt(7.W))
        val selector = Output(UInt(4.W))
    })

    val displayDriver = Module(new DisplayDriver())
    displayDriver.io.digits(3) := 1.U
    displayDriver.io.digits(2) := 2.U
    displayDriver.io.digits(1) := 3.U
    displayDriver.io.digits(0) := 4.U
    io.leds := displayDriver.io.leds
    io.selector := displayDriver.io.selector
}

But I get the following error when running it:

[error] (run-main-0) chisel3.package$ExpectedHardwareException: data to be connected 'UInt<4>' must be hardware, not a bare Chisel type. Perhaps you forgot to wrap it in Wire(_) or IO(_)?
[error] chisel3.package$ExpectedHardwareException: data to be connected 'UInt<4>' must be hardware, not a bare Chisel type. Perhaps you forgot to wrap it in Wire(_) or IO(_)?

which apparently points to the line

digit := io.digits(digitSel)

I have tried many combinations with Wire around io.digits but it still does not compile. Indeed, following other examples, I am not able to see why I'm getting this error, since the vector is inside an IO. What is happening?

Answer 1

The error comes from the left hand side of the connect operation: digit should be defined as Wire.

    val digit = Wire(UInt(4.W))
    digit := io.digits(digitSel)