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to: team@leaflabs.com
subj: paper review: "Nonblocking Assignments in Verilog Synthesis..."
TL;DR: this is something like a "goto considered harmful" w/r/t using confusing
blocking assignment in (non-sythesizable?) Verilog.
# Context
This paper was written in 2000 and seems to target Verilog programmers who
write non-synthesizable simulation code. Despite the word "Synthesis" in the
title. After reading, the implication that this paper might have any new
insights for an engineer whose failure might "kill" stikes fear in my gut.
Apparently this won a "Best Paper" award at a conference back when it was
published.
On page 15 there is a note about about synthesis performance: "The latter would
be inefficient from a simulation time perspective"; perhaps this was the
historical temptation of these bad practices?
# Judgement
There's really nothing new here (for jess/aj/bryan at least): for sequential
logic use nonblocking assignment in always@ blocks, and for combinatoral logic
use 'assign' statements outside of a block, unless you have something really
tight and complicated going on, in which case use an always block with a
carefully selected sensitivity list and all blocking assignments inside.
# Nuggets
From page 20: "Nonblocking assignments are updated after all $display
commands". I did not know this! The example given is pretty good; $strobe is
recommended as the alternative:
module display_cmds;
reg a;
initial $monitor("\$monitor: a = %b", a);
initial begin
$strobe ("\$strobe : a = %b", a);
a = 0;
a <= 1;
$display ("\$display: a = %b", a);
#1 $finish;
end
endmodule
gives:
$display: a = 0
$monitor: a = 1
$strobe : a = 1
# Appendix: Verilog Coding Guidelines
Verbatim from paper:
1: When modeling sequential logic, use nonblocking assignments.
2: When modeling latches, use nonblocking assignments.
3: When modeling combinational logic with an always block, use blocking
.
4: When modeling both sequential and combinational logic within the same always
nonblocking assignments.
5: Do not mix blocking and nonblocking assignments in the same always block.
6: Do not make assignments to the same variable from more than one always block.
7: Use $strobe to display values that have been assigned using nonblocking
.
8: Do not make assignments using #0 delays.
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