Warning

The SpinalHDL fixed point support is only partially used/tested, if you have any bug with it or you think that an functionality is missing, please create a github issue. Please, do not use undocumented features.

UFix/SFix

Description

The UFix and SFix types correspond to a vector of bits that can be used for fixed point arithmetic.

Declaration

The syntax to declare a fixed point number is as follows:

Unsigned Fixed Point

Syntax

bit width

resolution

max

min

UFix(peak:ExpNumber, resolution:ExpNumber)

peak-resolution

2^resolution

2^peak-2^resolution

0

UFix(peak:ExpNumber, width:BitCount)

width

2^(peak-width)

2^peak-2^(peak-width)

0

Signed Fixed Point

Syntax

bit width

resolution

max

min

SFix(peak:ExpNumber, resolution:ExpNumber)

peak-resolution+1

2^resolution

2^peak-2^resolution

-(2^peak)

SFix(peak:ExpNumber, width:BitCount)

width

2^(peak-width-1)

2^peak-2^(peak-width-1)

-(2^peak)

Format

The chosen format follows the usual way of defining fixed point number format using Q notation. More information there).

For example Q8.2 will mean an fixed point of 8+2 bits, where 8 bit are used for the natural part and 2 bits for the fractional part. If the fixed point number is signed, one more bit is used for the sign.

The resolution is defined as being the smallest power of two that can be represented in this number.

Examples

// Unsigned Fixed Point
val UQ_8_2 = UFix(peak = 8 exp,resolution = -2 exp)
val UQ_8_2 = UFix(8 exp, -2 exp)

val UQ_8_2 = UFix(peak = 8 exp,width = 10 bits)
val UQ_8_2 = UFix(8 exp, 10 bits)

// Signed Fixed Point
val Q_8_2 = SFix(peak = 8 exp,resolution = -2 exp)
val Q_8_2 = SFix(8 exp, -2 exp)

val Q_8_2 = SFix(peak = 8 exp,width = 11 bits)
val Q_8_2 = SFix(8 exp, 11 bits)

Assignments

Valid Assignments

An assignment to a fixed point value is valid when there is no bit loss. A bit loss occurrence will result in an error.

If the source fixed point value is too big, the .truncated function will allow you to resize the source number to match the destination size.

Example

val i16_m2 = SFix(16 exp,-2 exp)
val i16_0  = SFix(16 exp, 0 exp)
val i8_m2  = SFix( 8 exp,-2 exp)
val o16_m2 = SFix(16 exp,-2 exp)
val o16_m0 = SFix(16 exp, 0 exp)
val o14_m2 = SFix(14 exp,-2 exp)

o16_m2 := i16_m2            // OK
o16_m0 := i16_m2            // Not OK, Bit loss
o14_m2 := i16_m2            // Not OK, Bit loss
o16_m0 := i16_m2.truncated  // OK, as it is resized
o14_m2 := i16_m2.truncated  // OK, as it is resized

From a Scala constant

It is allowed to assign a Scala Double or a Scala BigInt into a fixed point.

Example

val i4_m2 = SFix(4 exp,-2 exp)
i4_m2 := 1.25    //Will load 5 in i4_m2.raw
i4_m2 := 4       //Will load 16 in i4_m2.raw

Raw value

The integer representation of the fixed point number can be read or written using the raw property.

Example

val UQ_8_2 = UFix(8 exp, 10 bits)
UQ_8_2.raw := 4        //Assign the value corresponding to 1.0
UQ_8_2.raw := U(17)    //Assign the value corresponding to 4.25

Operators

The following operators are available for the UFix type

Arithmetic

Operator

Description

Returned resolution

Returned amplitude

x + y

Addition

Min(x.resolution, y.resolution)

Max(x.amplitude, y.amplitude)

x - y

Subtraction

Min(x.resolution, y.resolution)

Max(x.amplitude, y.amplitude)

x * y

Multiplication

x.resolution * y.resolution)

x.amplitude * y.amplitude

x >> y

Arithmetic shift right, y : Int

x.amplitude >> y

x.resolution >> y

x << y

Arithmetic shift left, y : Int

x.amplitude << y

x.resolution << y

x >>| y

Arithmetic shift right, y : Int

x.amplitude >> y

x.resolution

x <<| y

Arithmetic shift left, y : Int

x.amplitude << y

x.resolution

Comparison

Operator

Description

Return type

x === y

Equality

Bool

x =/= y

Inequality

Bool

x > y

Greater than

Bool

x >= y

Greater than or equal

Bool

x > y

Less than

Bool

x >= y

Less than or equal

Bool

Type cast

Operator

Description

Return

x.asBits

Binary cast in Bits

Bits(w(x) bits)

x.asUInt

Binary cast in UInt

UInt(w(x) bits)

x.asSInt

Binary cast in SInt

SInt(w(x) bits)

x.asBools

Cast into a array of Bool

Vec(Bool,width(x))

x.toUInt

Return the corresponding UInt (with truncation)

UInt

x.toSInt

Return the corresponding SInt (with truncation)

SInt

x.toUFix

Return the corresponding UFix

UFix

x.toSFix

Return the corresponding SFix

SFix

Misc

Name

Return

Description

x.maxValue

Return the maximum value storable

Double

x.minValue

Return the minimum value storable

Double

x.resolution

x.amplitude * y.amplitude

Double