Registers in Spinal | SpinalHDL documentation

Introduction

Creating register is very different than VHDL/Verilog.
In Spinal, you don’t have process/always blocks. Registers are explicitly defined at the declaration.
This difference against traditional event driven HDL has a big impact :

You can assign registers and wires in the same scope, you don’t have to split your code between process/always blocks
It make things much more flexible (see Functions)

The way how clock and reset wire are managed are explained in the Clock domain chapter.

Instantiation

There is 4 ways to Instantiate a register :

Syntax	Description
Reg(type : Data)	Register of the given type
RegInit(resetValue : Data)	Register loaded with the given `resetValue` when a reset occur
RegNext(nextValue : Data)	Register that sample the given `nextValue` each cycle
RegNextWhen(nextValue : Data, cond : Bool)	Register that sample the given `nextValue` when a condition occurs

There is some usage examples :

//UInt register of 4 bits    
val reg1 = Reg(UInt(4 bit))  

//Register that sample toto each cycle  
val reg2 = RegNext(reg1 + 1)    

//UInt register of 4 bits initialized with 0 when the reset occur
val reg3 = RegInit(U"0000")
reg3 := reg2
when(reg2 === 5){
  reg3 := 0xF
}

//Register toto when cond is True
val reg4 = RegNextWhen(reg3,cond)

The code above will infer the following logic :

Note: The reg3 example show how you can assign the value of a RegInit register. But this it’s also right to use the same syntax for all others (Reg,RegNext,RegNextWhen).
As for combinatorial assignments, the rule is ‘Last assignment win’, but if no assignment is done, the register keep its value.

Also, RegNext is an abstraction which is build over the Reg syntax. The two followings sequances of code are strictly equivalents :

//Standard way
val something = Bool
val value = Reg(Bool)
value := something

//Short way
val something = Bool
val value = RegNext(something)

Reset value

In addition of the RegInit(value : Data) syntax which directly create the register with a reset logic, you can also set the reset value by calling the init(value : Data) function on the register.

//UInt register of 4 bits initialized with 0 when the reset occur
val reg1 = Reg(UInt(4 bit)) init(0)

If you have a Bundle register, you can use the init function on each elements of that Bundle.

case class ValidRGB() extends Bundle{
  val valid = Bool
  val r,g,b = UInt(8 bits)
}

val reg = Reg(ValidRGB())
reg.valid init(False)  //Only the valid of that register bundle will have an reset value.

Initialization value for simulation purposes

For register that doesn’t need a reset value in RTL, but need an initialization value for simulation (avoid x-propagation), you can ask for an initialization random value by calling the randBoot() function.

// UInt register of 4 bits initialized with a random value
val reg1 = Reg(UInt(4 bit)) randBoot()

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