# What’s new in MyHDL 0.4: Conversion to Verilog

Author: | Jan Decaluwe

## Introduction

MyHDL 0.4 supports the automatic conversion of a subset of MyHDL code to synthesizable Verilog code. This feature provides a direct path from Python to an FPGA or ASIC implementation.

MyHDL aims to be a complete design language, for tasks such as high level modeling and verification, but also for implementation. However, prior to 0.4 a user had to translate MyHDL code manually to Verilog or VHDL. Needless to say, this was inconvenient. With MyHDL0.4, this manual step is no longer necessary.

## Solution description

The solution works as follows. The hardware description should be modeled in MyHDL style, and satisfy certain constraints that are typical for implementation-oriented hardware modeling. Subsequently, such a design is converted to an equivalent model in the Verilog language, using the function `toVerilog` from the MyHDLlibrary. Finally, a third-party synthesis tool is used to convert the Verilog design to a gate implementation for an ASIC or FPGA. There are a number of Verilog synthesis tools available, varying in price, capabilities, and target implementation technology.

The conversion does not start from source files, but from a design that has been elaborated by the Python interpreter. The converter uses the Python profiler to track the interpreter’s operation and to infer the design structure and name spaces. It then selectively compiles pieces of source code for additional analysis and for conversion. This is done using the Python compiler package.

## Features

### The design is converted after elaboration

Elaboration refers to the initial processing of a hardware description to achieve a representation of a design instance that is ready for simulation or synthesis. In particular, structural parameters and constructs are processed in this step. In MyHDL, the Python interpreter itself is used for elaboration. A `Simulation` object is constructed with elaborated design instances as arguments. Likewise, the Verilog conversion works on an elaborated design instance. The Python interpreter is thus used as much as possible.

### The structural description can be arbitrarily complex and hierarchical

As the conversion works on an elaborated design instance, any modeling constraints only apply to the leaf elements of the design structure, that is, the co-operating generators. In other words, there are no restrictions on the description of the design structure: Python’s full power can be used for that purpose. Also, the design hierarchy can be arbitrarily deep.

### Generators are mapped to Verilog always or initial blocks

The converter analyzes the code of each generator and maps it to a Verilog `always` blocks if possible, and to an `initial` block otherwise. The converted Verilog design will be a flat “net list of blocks”.

### The Verilog module interface is inferred from signal usage

In MyHDL, the input or output direction of interface signals is not explicitly declared. The converter investigates signal usage in the design hierarchy to infer whether a signal is used as input, output, or as an internal signal. Internal signals are given a hierarchical name in the Verilog output.

### Function calls are mapped to a unique Verilog function or task call

The converter analyzes function calls and function code to see if they should be mapped to Verilog functions or to tasks. Python functions are much more powerful than Verilog subprograms; for example, they are inherently generic, and they can be called with named association. To support this power in Verilog, a unique Verilog function or task is generated per Python function call.

### If-then-else structures may be mapped to Verilog case statements

Python does not provide a case statement. However, the converter recognizes if-then-else structures in which a variable is sequentially compared to items of an enumeration type, and maps such a structure to a Verilog case statement with the appropriate synthesis attributes.

### Choice of encoding schemes for enumeration types

The `enum` function in MyHDL returns an enumeration type. This function takes an additional parameter `encoding` that specifies the desired encoding in the implementation: binary, one hot, or one cold. The Verilog converter generates the appropriate code.

## The convertible subset

### Introduction

Unsurprisingly, not all MyHDL code can be converted to Verilog. In fact, there are very important restrictions. As the goal of the conversion functionality is implementation, this should not be a big issue: anyone familiar with synthesis is used to similar restrictions in the synthesizable subset of Verilog and VHDL. The converter attempts to issue clear error messages when it encounters a construct that cannot be converted.

In practice, the synthesizable subset usually refers to RTL synthesis, which is by far the most popular type of synthesis today. There are industry standards that define the RTL synthesis subset. However, those were not used as a model for the restrictions of the MyHDL converter, but as a minimal starting point. On that basis, whenever it was judged easy or useful to support an additional feature, this was done. For example, it is actually easier to convert `while` loops than `for` loops even though they are not RTL-synthesizable. As another example, `print` is supported because it’s so useful for debugging, even though it’s not synthesizable. In summary, the convertible subset is a superset of the standard RTL synthesis subset, and supports synthesis tools with more advanced capabilities, such as behavioral synthesis.

Recall that any restrictions only apply to the design post elaboration. In practice, this means that they apply only to the code of the generators, that are the leaf functional blocks in a MyHDL design.

### Coding style

A natural restriction on convertible code is that it should be written in MyHDL style: cooperating generators, communicating through signals, and with `yield` statements specifying wait points and resume conditions. Supported resume conditions are a signal edge, a signal change, or a tuple of such conditions.

### Supported types

The most important restriction regards object types. Verilog is an almost typeless language, while Python is strongly (albeit dynamically) typed. The converter has to infer the types of names used in the code, and map those names to Verilog variables.

Only a limited amount of types can be converted. Python `int` and `long` objects are mapped to Verilog integers. All other supported types are mapped to Verilog regs (or wires), and therefore need to have a defined bit width. The supported types are the Python `bool` type, the MyHDL `intbv` type, and MyHDL enumeration types returned by function `enum`. The latter objects can also be used as the base object of a `Signal`.

`intbv` objects must be constructed so that a bit width can be inferred. This can be done by specifying minimum and maximum values, e.g. as follows:

    index = intbv(0, min=0, max=2**N)

Alternatively, a slice can be taken from an `intbv` object as follows:

    index = intbv(0)[N:]

Such as slice returns a new `intbv` object, with minimum value `0` , and maximum value `2**N`.

### Supported statements

The following is a list of the statements that are supported by the Verilog converter, possibly qualified with restrictions or usage notes.

The `break` statement.

The `continue` statement.

The `def` statement.

The `for` statement.
  The only supported iteration scheme is iterating through sequences of integers returned by built-in function `range` or MyHDLfunction `downrange`. The optional `else` clause is not supported.
The `if` statement.
  `if`, `elif`, and `else` clauses are fully supported.

The `pass` statement.

The `print` statement.
  When printing an interpolated string, the format specifiers are copied verbatim to the Verilog output. Printing to a file (with syntax `’>>’`) is not supported.
The `raise` statement.
  This statement is mapped to Verilog statements that end the simulation with an error message.

The `return` statement.

The `yield` statement.
  The yielded expression can be a signal, a signal edge as specified by MyHDL functions `posedge` or `negedge`, or a tuple of signals and edge specifications.
The `while` statement.
  The optional `else` clause is not supported.

## Methodology notes

### Simulation

In the Python philosophy, the run-time rules. The Python compiler doesn’t attempt to detect a lot of errors beyond syntax errors, which given Python’s ultra-dynamic nature would be an almost impossible task anyway. To verify a Python program, one should run it, preferably using unit testing to verify each feature.

The same philosophy should be used when converting a MyHDL description to Verilog: make sure the simulation runs fine first. Although the converter checks many things and attempts to issue clear error messages, there is no guarantee that it does a meaningful job unless the simulation runs fine.

### Conversion output verification

It is always prudent to verify the converted Verilog output. To make this task easier, the converter also generates a test bench that makes it possible to simulate the Verilog design using the Verilog co-simulation interface. This permits one to verify the Verilog code with the same test bench used for the MyHDL code. This is also how the Verilog converter development is being verified.

### Assignment issues

#### Name assignment in Python

Name assignment in Python is a different concept than in many other languages. This point is very important for effective modeling in Python, and even more so for synthesizable MyHDL code. Therefore, the issues are discussed here explicitly.

Consider the following name assignments:

    a = 4
    a = ``a string''
    a = False

In many languages, the meaning would be that an existing variable `a` gets a number of different values. In Python, such a concept of a variable doesn’t exist. Instead, assignment merely creates a new binding of a name to a certain object, that replaces any previous binding. So in the example, the name `a` is bound a number of different objects in sequence.

The Verilog converter has to investigate name assignment and usage in MyHDL code, and to map names to Verilog variables. To achieve that, it tries to infer the type and possibly the bit width of each expression that is assigned to a name.

Multiple assignments to the same name can be supported if it can be determined that a consistent type and bit width is being used in the assignments. This can be done for boolean expressions, numeric expressions, and enumeration type literals. In Verilog, the corresponding name is mapped to a single bit `reg`, an `integer`, or a `reg` with the appropriate width, respectively.

In other cases, a single assignment should be used when an object is created. Subsequent value changes are then achieved by modification of an existing object. This technique should be used for `Signal` and `intbv` objects.

#### Signal assignment

Signal assignment in MyHDL is implemented using attribute assignment to attribute `next`. Value changes are thus modeled by modification of the existing object. The converter investigates the `Signal` object to infer the type and bit width of the corresponding Verilog variable.

#### `intbv` objects

Type `intbv` is likely to be the workhorse for synthesizable modeling in MyHDL. An `intbv` instance behaves like a (mutable) integer whose individual bits can be accessed and modified. Also, it is possible to constrain its set of values. In addition to error checking, this makes it possible to infer a bit width, which is required for implementation.

In Verilog, an `intbv` instance will be mapped to a `reg` with an appropriate width. As noted before, it is not possible to modify its value using name assignment. In the following, we will show how it can be done instead. Consider:

    a = intbv(0)[8:]

This is an `intbv` object with initial value `0` and bit width 8. The change its value to `5`, we can use slice assignment:

    a[8:] = 5

The same can be achieved by leaving the bit width unspecified, which has the meaning to change “all” bits:

    a[:] = 5

Often the new value will depend on the old one. For example, to increment an `intbv` with the technique above:

    a[:] = a + 1

Python also provides augmented assignment operators, which can be used to implement in-place operations. These are supported on `intbv` objects and by the converter, so that the increment can also be done as follows:

    a += 1

## Converter usage

We will demonstrate the conversion process by showing some examples.

### A small design with a single generator

Consider the following MyHDL code for an incrementer module:

    def inc(count, enable, clock, reset, n):
        """ Incrementer with enable.

        count -- output
        enable -- control input, increment when 1
        clock -- clock input
        reset -- asynchronous reset input
        n -- counter max value
        """
        def incProcess():
            while 1:
                yield posedge(clock), negedge(reset)
                if reset == ACTIVE_LOW:
                    count.next = 0
                else:
                    if enable:
                        count.next = (count + 1) % n
        return incProcess()

In Verilog terminology, function `inc` corresponds to a module, while generator function `incProcess` roughly corresponds to an always block.

Normally, to simulate the design, we would “elaborate” an instance as follows:

    m = 8
    n = 2 ** m

    count = Signal(intbv(0)[m:])
    enable = Signal(bool(0))
    clock, reset = [Signal(bool()) for i in range(2)]

    inc_inst = inc(count, enable, clock, reset, n=n)

`incinst` is an elaborated design instance that can be simulated. To convert it to Verilog, we change the last line as follows:

    inc_inst = toVerilog(inc, count, enable, clock, reset, n=n)

Again, this creates an instance that can be simulated, but as a side effect, it also generates an equivalent Verilog module in file . The Verilog code looks as follows:

    module inc_inst (
        count,
        enable,
        clock,
        reset
    );

    output [7:0] count;
    reg [7:0] count;
    input enable;
    input clock;
    input reset;

    always @(posedge clock or negedge reset) begin: _MYHDL1_BLOCK
        if ((reset == 0)) begin
            count <= 0;
        end
        else begin
            if (enable) begin
                count <= ((count + 1) % 256);
            end
        end
    end

    endmodule

You can see the module interface and the always block, as expected from the MyHDL design.

### Converting a generator directly

It is also possible to convert a generator directly. For example, consider the following generator function:

    def bin2gray(B, G, width):
        """ Gray encoder.

        B -- input intbv signal, binary encoded
        G -- output intbv signal, gray encoded
        width -- bit width
        """
        Bext = intbv(0)[width+1:]
        while 1:
            yield B
            Bext[:] = B
            for i in range(width):
                G.next[i] = Bext[i+1] ^ Bext[i]

As before, you can create an instance and convert to Verilog as follows:

    width = 8

    B = Signal(intbv(0)[width:])
    G = Signal(intbv(0)[width:])

    bin2gray_inst = toVerilog(bin2gray, B, G, width)

The generated Verilog code looks as follows:

    module bin2gray_inst (
        B,
        G
    );

    input [7:0] B;
    output [7:0] G;
    reg [7:0] G;

    always @(B) begin: _MYHDL1_BLOCK
        integer i;
        reg [9-1:0] Bext;
        Bext[9-1:0] = B;
        for (i=0; i<8; i=i+1) begin
            G[i] <= (Bext[(i + 1)] ^ Bext[i]);
        end
    end

    endmodule

### A hierarchical design

The hierarchy of convertible designs can be arbitrarily deep.

For example, suppose we want to design an incrementer with Gray code output. Using the designs from previous sections, we can proceed as follows:

    def GrayInc(graycnt, enable, clock, reset, width):

        bincnt = Signal(intbv()[width:])

        INC_1 = inc(bincnt, enable, clock, reset, n=2**width)
        BIN2GRAY_1 = bin2gray(B=bincnt, G=graycnt, width=width)

        return INC_1, BIN2GRAY_1

According to Gray code properties, only a single bit will change in consecutive values. However, as the `bin2gray` module is combinatorial, the output bits may have transient glitches, which may not be desirable. To solve this, let’s create an additional level of hierarchy and add an output register to the design. (This will create an additional latency of a clock cycle, which may not be acceptable, but we will ignore that here.)

    def GrayIncReg(graycnt, enable, clock, reset, width):

        graycnt_comb = Signal(intbv()[width:])

        GRAY_INC_1 = GrayInc(graycnt_comb, enable, clock, reset, width)

        def reg():
            while 1:
                yield posedge(clock)
                graycnt.next = graycnt_comb
        REG_1 = reg()

        return GRAY_INC_1, REG_1

We can convert this hierarchical design as before:

    width = 8
    graycnt = Signal(intbv()[width:])
    enable, clock, reset = [Signal(bool()) for i in range(3)]

    GRAY_INC_REG_1 = toVerilog(GrayIncReg, graycnt, enable, clock, reset, width)

The Verilog output code looks as follows:

    module GRAY_INC_REG_1 (
        graycnt,
        enable,
        clock,
        reset
    );

    output [7:0] graycnt;
    reg [7:0] graycnt;
    input enable;
    input clock;
    input reset;

    reg [7:0] graycnt_comb;
    reg [7:0] _GRAY_INC_1_bincnt;

    always @(posedge clock or negedge reset) begin: _MYHDL1_BLOCK
        if ((reset == 0)) begin
            _GRAY_INC_1_bincnt <= 0;
        end
        else begin
            if (enable) begin
                _GRAY_INC_1_bincnt <= ((_GRAY_INC_1_bincnt + 1) % 256);
            end
        end
    end

    always @(_GRAY_INC_1_bincnt) begin: _MYHDL4_BLOCK
        integer i;
        reg [9-1:0] Bext;
        Bext[9-1:0] = _GRAY_INC_1_bincnt;
        for (i=0; i<8; i=i+1) begin
            graycnt_comb[i] <= (Bext[(i + 1)] ^ Bext[i]);
        end
    end

    always @(posedge clock) begin: _MYHDL9_BLOCK
        graycnt <= graycnt_comb;
    end

    endmodule

Note that the output is a flat “net list of blocks”, and that hierarchical signal names are generated as necessary.

### Optimizations for finite state machines

As often in hardware design, finite state machines deserve special attention.

In Verilog and VHDL, finite state machines are typically described using case statements. Python doesn’t have a case statement, but the converter recognizes particular if-then-else structures and maps them to case statements. This optimization occurs when a variable whose type is an enumerated type is sequentially tested against enumeration items in an if-then-else structure. Also, the appropriate synthesis pragmas for efficient synthesis are generated in the Verilog code.

As a further optimization, function `enum` was enhanced to support alternative encoding schemes elegantly, using an additional parameter `encoding`. For example:

    t_State = enum('SEARCH', 'CONFIRM', 'SYNC', encoding='one_hot')

The default encoding is `’binary’`; the other possibilities are `’onehot’` and `’onecold’`. This parameter only affects the conversion output, not the behavior of the type. The generated Verilog code for case statements is optimized for an efficient implementation according to the encoding. Note that in contrast, a Verilog designer has to make nontrivial code changes to implement a different encoding scheme.

As an example, consider the following finite state machine, whose state variable uses the enumeration type defined above:

    FRAME_SIZE = 8

    def FramerCtrl(SOF, state, syncFlag, clk, reset_n):

        """ Framing control FSM.

        SOF -- start-of-frame output bit
        state -- FramerState output
        syncFlag -- sync pattern found indication input
        clk -- clock input
        reset_n -- active low reset

        """

        index = intbv(0, min=0, max=8) # position in frame
        while 1:
            yield posedge(clk), negedge(reset_n)
            if reset_n == ACTIVE_LOW:
                SOF.next = 0
                index[:] = 0
                state.next = t_State.SEARCH
            else:
                SOF.next = 0
                if state == t_State.SEARCH:
                    index[:] = 0
                    if syncFlag:
                        state.next = t_State.CONFIRM
                elif state == t_State.CONFIRM:
                    if index == 0:
                        if syncFlag:
                            state.next = t_State.SYNC
                        else:
                            state.next = t_State.SEARCH
                elif state == t_State.SYNC:
                    if index == 0:
                        if not syncFlag:
                            state.next = t_State.SEARCH
                    SOF.next = (index == FRAME_SIZE-1)
                else:
                    raise ValueError("Undefined state")
                index[:]= (index + 1) % FRAME_SIZE

The conversion is done as before:

    SOF = Signal(bool(0))
    syncFlag = Signal(bool(0))
    clk = Signal(bool(0))
    reset_n = Signal(bool(1))
    state = Signal(t_State.SEARCH)
    framerctrl_inst = toVerilog(FramerCtrl, SOF, state, syncFlag, clk, reset_n)

The Verilog output looks as follows:

    module framerctrl_inst (
        SOF,
        state,
        syncFlag,
        clk,
        reset_n
    );
    output SOF;
    reg SOF;
    output [2:0] state;
    reg [2:0] state;
    input syncFlag;
    input clk;
    input reset_n;

    always @(posedge clk or negedge reset_n) begin: _MYHDL1_BLOCK
        reg [3-1:0] index;
        if ((reset_n == 0)) begin
            SOF <= 0;
            index[3-1:0] = 0;
            state <= 3'b001;
        end
        else begin
            SOF <= 0;
            // synthesis parallel_case full_case
            casez (state)
                3'b??1: begin
                    index[3-1:0] = 0;
                    if (syncFlag) begin
                        state <= 3'b010;
                    end
                end
                3'b?1?: begin
                    if ((index == 0)) begin
                        if (syncFlag) begin
                            state <= 3'b100;
                        end
                        else begin
                            state <= 3'b001;
                        end
                    end
                end
                3'b1??: begin
                    if ((index == 0)) begin
                        if ((!syncFlag)) begin
                            state <= 3'b001;
                        end
                    end
                    SOF <= (index == (8 - 1));
                end
                default: begin
                    $display("Verilog: ValueError(Undefined state)");
                    $finish;
                end
            endcase
            index[3-1:0] = ((index + 1) % 8);
        end
    end
    endmodule

## Known issues

Negative values of `intbv` instances are not supported.
  The `intbv` class is quite capable of representing negative values. However, the `signed` type support in Verilog is relatively recent and mapping to it may be tricky. In my judgment, this was not the most urgent requirement, so I decided to leave this for later.
Verilog integers are 32 bit wide
  Usually, Verilog integers are 32 bit wide. In contrast, Python is moving toward integers with undefined width. Python `int` and `long` variables are mapped to Verilog integers; so for values wider than 32 bit this mapping is incorrect.
Synthesis pragmas are specified as Verilog comments.
  The recommended way to specify synthesis pragmas in Verilog is through attribute lists. However, my Verilog simulator (Icarus) doesn’t support them for `case` statements (to specify `parallelcase` and `fullcase` pragmas). Therefore, I still used the old but deprecated method of synthesis pragmas in Verilog comments.
Inconsistent place of the sensitivity list inferred from `alwayscomb`.
  The semantics of `alwayscomb`, both in Verilog and MyHDL, is to have an implicit sensitivity list at the end of the code. However, this may not be synthesizable. Therefore, the inferred sensitivity list is put at the top of the corresponding `always` block. This may cause inconsistent behavior at the start of the simulation. The workaround is to create events at time 0.
Non-blocking assignments to task arguments don’t work.
  I didn’t get non-blocking (signal) assignments to task arguments to work. I don’t know yet whether the issue is my own, a Verilog issue, or an issue with my Verilog simulator Icarus. I’ll need to check this further.