Procedural Continuous Assignments
Formal Definition
Procedural continuous assignments provide a means to continuously
drive a value into a register or a net.
Simplified Syntax
assign register_assignment ;
deassign register_left_value ;
force register_assignment |
net_assignment ;
release register_left_value
| net_left_value ;
register_left_value - left hand-side operand ;
net_left_value - left hand-side operand ;
Description
In the assign statement, the
left hand-side operand can be a register or a concatenation of
registers in contrast to continuous assignments where the left
hand-side operands are nets. However, the memory word, bit-select or
part-select cannot be used on the left hand-side. The priority of
procedural continuous assignment is higher than that of the
procedural assignments. The deassign
keyword informs the end of the driving of the corresponding register.
After a deassign, register values remain the same until driven by
another procedural or procedural continuous assignment. If the assign
keyword is used a second time for the same register, the first
register will be deassigned and a new procedural continuous
assignment statement takes place.
The keywords force and release
have the same effect as assign
and deassign; however the
left hand-side operand should be a net or a bit-select or a
part-select of vector net or concatenation of both. It cannot be
part-select or bit-select of a vector register or a memory word.
An assign declared as a force
overrides all procedural assignments to the register until the release
statement is executed. In the same way, if a net is driven by a
procedural continuous assignment all previous assignments are
overridden until the release statement is executed. In the register
case, the value will be maintained to the next procedural continuous assignment.
Examples
Example 1
module example1(clk, d,
reset, set, q) ;
input clk, d, reset, set;
output q;
reg q;
always @(posedge
clk) q = d;
always @(reset or
set) begin
if (reset) assign
q = 1'b0;
else if (set) assign
q= 1'b1;
else deassign q;
end
endmodule
Important Notes
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