Bool

Description

The Bool type corresponds to a boolean value (True or False) or a single bit/wire used in a hardware design. While named similarly it should not be confused with Scala Boolean type which does not describe hardware but truth values in the Scala generator code.

An important concept and rule-of-thumb to understand is that the Scala Boolean type is used in places where elaboration-time HDL code-generation decision making is occurring in Scala code. Like any regular program it affects execution of the Scala program that is SpinalHDL at the time the program is being run to perform HDL code generation.

Therefore the value of a Scala Boolean can not be observed from hardware, because it only exists ahead-of-time in the SpinalHDL program at the time of HDL code-gen.

In scenarios where you might need this for your design, for example to pass a value (that maybe acting as a parameterized constant input) from Scala into your hardware design, you can type convert it to Bool with the constructor Bool(value: Boolean).

Similarly the value of a SpinalHDL Bool can not be seen at code-generation, all that can be seen and manipulated is the HDL construct concerning a wire and how it is routed (through modules/Components), driven (sourced) and connected (sunk).

The signal direction of assignment operators := is managed by SpinalHDL. The use of the Bool instance on the left-hand-side or the right-hand-side of the assignment operator := dictates if it is a source (provides state) or sink (captures state) for a given assignment.

Multiple uses of the assignment operator are allowed, such that it is normal for a signal wire to act as a source (provides a value to drive HDL state) to be able to connect and drive multiple inputs of other HDL constructs. When a Bool instance used as a source the order the assignment statements appear or are executed in Scala does not matter, unlike when it is used as a sink (captures state).

When multiple assignment operators drive the Bool (the Bool is on the left-hand-side of the assignment expression), the last assignment statement wins rule; take effect. The last would be the last to execute in Scala code. This matter can affect the layout and ordering of your SpinalHDL Scala code to ensure the correct precedence order is archived in the hardware design for assigning a new state to the Bool in hardware.

It may help to understand the concept with relating the Scala/SpinalHDL Bool instance as a reference to a HDL net in the net-list. Which the assignment := operator is attaching HDL constructs into the same net.

Declaration

The syntax to declare a boolean value is as follows: (everything between [] is optional)

Syntax	Description	Return
Bool()	Create a Bool	Bool
True	Create a Bool assigned with `true`	Bool
False	Create a Bool assigned with `false`	Bool
Bool(value: Boolean)	Create a Bool assigned with a value from a Scala Boolean type (true, false). This explicitly converts to `True` or `False`.	Bool

val myBool_1 = Bool()        // Create a Bool
myBool_1 := False            // := is the assignment operator (like verilog <=)

val myBool_2 = False         // Equivalent to the code above

val myBool_3 = Bool(5 > 12)  // Use a Scala Boolean to create a Bool

Operators

The following operators are available for the Bool type:

Logic

Operator	Description	Return type
!x	Logical NOT	Bool
x && y x & y	Logical AND	Bool
x \|\| y x \| y	Logical OR	Bool
x ^ y	Logical XOR	Bool
~x	Logical NOT	Bool
x.set[()]	Set x to True	Unit (none)
x.clear[()]	Set x to False	Unit (none)
x.setWhen(cond)	Set x when cond is True	Bool
x.clearWhen(cond)	Clear x when cond is True	Bool
x.riseWhen(cond)	Set x when x is False and cond is True	Bool
x.fallWhen(cond)	Clear x when x is True and cond is True	Bool

 val a, b, c = Bool()
 val res = (!a & b) ^ c   // ((NOT a) AND b) XOR c

 val d = False
 when(cond) {
   d.set()                // equivalent to d := True
 }

 val e = False
 e.setWhen(cond)          // equivalent to when(cond) { d := True }

 val f = RegInit(False) fallWhen(ack) setWhen(req)
 /** equivalent to
  * when(f && ack) { f := False }
  * when(req) { f := True }
  * or
  * f := req || (f && !ack)
  */

// mind the order of assignments!  last one wins
val g = RegInit(False) setWhen(req) fallWhen(ack)
// equivalent to g := ((!g) && req) || (g && !ack)

Edge detection

All edge detection functions will instantiate an additional register via RegNext to get a delayed value of the Bool in question.

This feature does not reconfigure a D-type Flip-Flop to use an alternative CLK source, it uses two D-type Flip-Flop in series chain (with both CLK pins inheriting the default ClockDomain). It has combinational logic to perform edge detection based on the output Q states.

Operator	Description	Return type
x.edge[()]	Return True when x changes state	Bool
x.edge(initAt: Bool)	Same as x.edge but with a reset value	Bool
x.rise[()]	Return True when x was low at the last cycle and is now high	Bool
x.rise(initAt: Bool)	Same as x.rise but with a reset value	Bool
x.fall[()]	Return True when x was high at the last cycle and is now low	Bool
x.fall(initAt: Bool)	Same as x.fall but with a reset value	Bool
x.edges[()]	Return a bundle (rise, fall, toggle)	BoolEdges
x.edges(initAt: Bool)	Same as x.edges but with a reset value	BoolEdges
x.toggle[()]	Return True at every edge	Bool

when(myBool_1.rise(False)) {
    // do something when a rising edge is detected
}


val edgeBundle = myBool_2.edges(False)
when(edgeBundle.rise) {
    // do something when a rising edge is detected
}
when(edgeBundle.fall) {
    // do something when a falling edge is detected
}
when(edgeBundle.toggle) {
    // do something at each edge
}

Comparison

Operator	Description	Return type
x === y	Equality	Bool
x =/= y	Inequality	Bool

when(myBool) { // Equivalent to when(myBool === True)
    // do something when myBool is True
}

when(!myBool) { // Equivalent to when(myBool === False)
    // do something when myBool is False
}

Type cast

Operator	Description	Return
x.asBits	Binary cast to Bits	Bits(1 bit)
x.asUInt	Binary cast to UInt	UInt(1 bit)
x.asSInt	Binary cast to SInt	SInt(1 bit)
x.asUInt(bitCount)	Binary cast to UInt and resize, putting Bool value in LSB and padding with zeros.	UInt(bitCount bits)
x.asBits(bitCount)	Binary cast to Bits and resize, putting Bool value in LSB and padding with zeros.	Bits(bitCount bits)

// Add the carry to an SInt value
val carry = Bool()
val res = mySInt + carry.asSInt

Misc

Operator	Description	Return
x ## y	Concatenate, x->high, y->low	Bits(w(x) + w(y) bits)
x #* n	Repeat x n-times	Bits(n bits)

val a, b, c = Bool()

// Concatenation of three Bool into a single Bits(3 bits) type
val myBits = a ## b ## c

MaskedBoolean

A masked boolean allows don’t care values. They are usually not used on their own but through MaskedLiteral.

// first argument: Scala Boolean value
// second argument: do we care ? expressed as a Scala Boolean
val masked = new MaskedBoolean(true, false)