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AR# 51835

Design Assistant for Vivado Synthesis - Help with SystemVerilog Processes Support


This answer record describes SystemVerilog Processes supported by Vivado Synthesis, and also provides coding examples for them. These coding examples are attached to this answer record. The answer record also contains information related to known issues and good coding practices.

Note: Each coding example can be used to directly create a Vivado project. Please refer to the header in each source file for the SystemVerilog constructs demonstrated in each example.


SystemVerilog Processes that are Supported by Vivado Synthesis

Please refer to Table 1-1 at the end of this answer record for the coding examples related to Processes.

1. Always Procedure

Verilog-2001 has an always statement which defines static processes. System Verilog adds specialized blocks for combinational, latch and sequential logic.

Vivado Synthesis supports the following four always process statements:

1.1. always
   This block has a syntax and usage which is same as the one defined in Verilog-2001 standard without any separate exception for Vivado Synthesis.

1.2. always_comb
   This block is defined only in System Verilog and is a procedure for modeling combinational logic behavior. For example:
   a = b & c;

   The always_comb procedure provides functionality that is different than a normal always procedure:
   There is an inferred sensitivity list that includes the expressions.
   The variables written on the left-hand side of assignments shall not be written to by any other process.
   The procedure is automatically triggered once at time zero, after all initial and always blocks have been started, so that the outputs of the procedure are consistent with the inputs.

1.3. always_latch
   This block is defined only in System Verilog and is a procedure for modeling latch logic behavior. For example:
   if(ck) q <= d;
   The always_latch procedure determines its sensitivity and executes identically to the always_comb procedure.

1.4. always_ff
   This block is defined only in System Verilog and is a procedure which can be used to model synthesizable sequential logic behavior. For example:
   always_ff @(posedge clock or posedge reset) begin
   r1 <= reset ? 0 : r2 + 1;

2. Block Statements

System Verilog Block statements provide a mechanism to group sets of statements together. A named block is used to identify the entire statement block. A named block creates a new hierarchy scope. The block can declare its own variables, and those variables are specific to that block. SystemVerilog allows a matching block name to be specified after the block's end statement preceded by a colon. This can help document the "end" associated with the corresponding begin or fork when there are nested blocks.
Vivado Synthesis supports usage of Block Statements.

seq_block ::=
begin [ : block_identifier ] { block_item_declaration } { statement_or_null }
end [ : block_identifier ]

begin : my_block
logic temp;
temp = in1 & in2;
out1 = temp;
end : my_block

Note: The block name after the end statement is not mandatory but it improves code readability.
Vivado Synthesis Unsupported: System Verilog Parallel Blocks (or Fork..Join blocks) are not supported in Vivado Synthesis.

3. Procedural Timing Controls.

Vivado Synthesis supports procedural timing control defined in system Verilog standard:

3.1. Delay Control
    delay_control ::=
    # delay_value

    # 10 q = p;

This construct specifies the amount of time between the statement and its execution. This is not useful for synthesis, and Vivado synthesis ignores the time statement while still creating logic for the assignment.
3.2. Event Control
    event_control ::=
    @ ( event_expression )
    | @*
    | @ (*)

   always@(posedge clk)

This construct makes the assignment occur with a specific event. This is same as Verilog and Vivado Synthesis supports the System Verilog @* event control which eliminates simulation mismatch due to incorrect sensitivity list. 

 Logic always@* begin

Also, the usage of logical "or" operator or a comma to separate the sensitivity list is supported in Vivado Synthesis.

 always@(a or b or c)
Coding Examples for Processes

Table 1-1

Coding example name Process Constructs 


  •  always procedural block
  •  always_ff procedural block
  •  operator: &
  •  procedural timing control


  • always_comb procedural block
  • always_ff procedural block
  • operator: &


  • always_comb procedural block
  • operator: ==
  • block statement
  • always_ff procedural block
  • always_ff procedural block
  • operators : ++
  • always_latch procedural block
  • block statement
  • operator : &
  • Event Control: @(*), @(a or b)
  • Delay Control : #delay
  • operator : &



Associated Attachments

Name File Size File Type
processes_example1.zip 934 Bytes ZIP
processes_example2.zip 960 Bytes ZIP
processes_example3.zip 956 Bytes ZIP
processes_example4.zip 898 Bytes ZIP
processes_example5.zip 921 Bytes ZIP
processes_example6.zip 912 Bytes ZIP
processes_example7.zip 924 Bytes ZIP
processes_example8.zip 1 KB ZIP

Linked Answer Records

Master Answer Records

Answer Number Answer Title Version Found Version Resolved
51360 Design Assistant for Vivado Synthesis - Help with SystemVerilog Support N/A N/A
AR# 51835
Date Created 09/15/2012
Last Updated 04/03/2013
Status Active
Type Solution Center
  • Vivado Design Suite