Understand chisel language thoroughly 04. Chisel Foundation (I) - signal type and constant

Chisel Basics （ One ）—— Signal types and constants

At first, it must be an introduction Chisel The basic content of ,《Chisel Basics 》 This part mainly introduces two basic components in digital design ： Combinational circuits and triggers . Neither of these basic components is complicated , But together, you can get a large scale 、 It is also a powerful digital circuit .

In the digital logic circuit system , Generally, binary signals are used , That is to say, only two possible values are allowed for a single bit or a single signal , That's what we all know 0 and 1. But in digital design , Some other terms may be used , such as ： low （low）/ high （high）, false （false）/ really （true）, Or set invalid （de-asserted）/ Set valid （asserted）. These groups of terms correspond to 0 and 1, But pay attention to the last group de-asserted and asserted, I translate it here as set invalid and set valid , The reason is that in some scenarios, the low level is effective , In some scenarios, high level is effective , So they and 0/1 The corresponding relationship of depends on the specific scenario .

In the first part, let's take a brief look Chisel Signal types and constants in , In the last article, there is also a related introduction , It also includes and Verilog Contrast between .

Three basic signal types

Chisel There are three basic data types to describe signals 、 Combinational logic and registers ：Bits、UInt and SInt. among UInt and SInt from Bits Expanded .

These three types are signals / Vector of bits .UInt Used to represent the bit vector corresponding to an unsigned integer , and SInt Represents the bit vector corresponding to a signed integer , and Chisel The signed integer encoding method used is Binary complement , I learned this in computer general courses .

Below Chisel The code defines a 8 position Bits、 One 8 Bit unsigned integer and a 10 Bit signed integer ：

Bits(8.W)
UInt(8.W)
SInt(10.W)

The width of these bit vectors is through Chisel Width type Width To define the , expression n.W You can put a Scala The integer is converted to a Width, Then it is used for the definition of bit vector ：

n.W
Bits(n.W)

Be careful ： Current version Chisel Medium Bits There is no corresponding operation , So it's of little use to us , Just know there's such a thing .

Chisel Constant

Constants can be used like Width Define methods to define , Direct use Scala The integer of is converted to Chisel Type is OK ：

0.U		//  Defined a UInt Constant of type 0
-3.S	//  Defined a SInt Constant of type -3

When defining constants, you can also take the width , Use or Chisel Of Width type ：

3.U(4.W)	//  A bit width is defined as 4 Of UInt Type constant 3

Maybe you think 3.U and 4.W This kind of notation is very strange , Then you can think of it as an integer constant followed by a type . This notation is similar to C、Java and Scala Medium 3L, Indicates a value of 3 Long integer .

A common problem when defining a specified width constant

We may define a constant with a specified width , Forget the width behind .W identifier , such as 1.U(32). This representation Can't Define a 32 Constant of bit width 1, contrary , expression (32) Will be interpreted as 1.U This number is in position 32 Bit extraction on , That is, you will get a single bit constant 0, Obviously, it's not what we expected , Therefore, we must not omit .W！

Chisel Use other base numbers to represent constants

Chisel It's from Scala Extended ,Scala Type inference in Chisel Benefit a lot , Therefore, the type information in many places can be omitted . This type of inference on bit width also applies ,Chisel It will automatically infer the correct bit width , So from this point of view Chisel Describing hardware circuits is also better than Verilog and VHDL More concise 、 More readable .

For constants in base numbers other than decimal , Constants are defined by strings , Determine the base according to the character at the beginning of the string . among h It's hexadecimal ,o It's octal ,b For binary , The following code uses these three bases to represent constants 255：

"hff".U
"o377".U
"b1111_1111".U

It can be noted that , The above definitions are useless (n.W) To specify the width , Because this can be omitted ,Chisel The minimum width will be automatically inferred to represent this constant , Here is all 8 position （255 The binary representation of 8 Bit binary number ）.

You can also notice ,"b1111_1111" Underline in , Here is for grouping numbers , Make the code more readable , In the compiler's view, underscores are omitted , such as b_1_1_11_1111 and b11111111 Is equivalent .

Chisel of use ASCII Characters represent constants

ASCII code （ Content of computer general course ） The character of can also be used for Chisel Constant definition of ：

val aChar = 'A'.U

The result is UInt<7>(65), namely 7 Bit unsigned number 65. Note that strings cannot be used in this way , such as ：

val aChar = "AA".U		//  Incorrect usage 

Report error as chisel3.internal.ChiselException: Invalid base A, That is, for strings ,Chisel The first character will be regarded as the base of a hexadecimal string .

Chisel Logical value in

To represent logical values ,Chisel Types are defined in Bool.Bool The value of type can represent a true perhaps false Value , Define a Bool Variables of type can be like this ：

Bool()

And constant Bool Value can be passed Scala The logical value is converted to , Method is xx.B：

true.B
false.B