Timing Controls - Verilog HDL

TIMING CONTROLS

A timing control is either a delay control or an event control.

1. Delay control:

A delay control delays an assignment by a specified amount of time. A timescale compiler directive is used to specify the units of time followed by the precision used to calculate time expressions, `timescale 1ns/10ps // Units of time are ns. Round times to 10 ps.

Time units may only be s , ns , ps , or fs and the multiplier must be 1, 10, or 100. We can delay an assignment in two different ways:

·           Sample the RHS immediately and then delay the assignment to the LHS.

·           Wait for a specified time and then assign the value of the LHS to the RHS.

Here is an example of the first alternative (an intra-assignment delay):

a = #1 b; // intra-assignment delay

The second alternative is delayed assignment:

#1 a = b; // delayed assignment

These two alternatives are not the same. The intra-assignment delay is equivalent to the following code:

begin // Equivalent to intra-assignment delay. hold = b; // Sample and hold y immediately.

#1; // Delay.

a = hold; // Assignment to a. Overall same as a = #1 b. end

In contrast, the delayed assignment is equivalent to a delay followed by an assignment as follows:

begin // Equivalent to delayed assignment.

#1; // Delay.

a = b; // Assign y to x. Overall same as #1 a = b. end

2. Event control:

An event control, delays an assignment until a specified event occurs. Here is the formal definition:

event_control ::= @ event_identifier | @ (event_expression)

event_expression ::= expression | event_identifier

| posedge expression | negedge expression

| event_expression or event_expression

A positive edge (denoted by the keyword posedge ) is a transition from '0' to '1' or 'x' , or a transition from 'x' to '1 '. A negative edge ( negedge ) is a transition from '1' to '0' or 'x' , or a transition from 'x' to '0'. Transitions to or from 'z' do not count. Here are examples of event controls:

moduledelay_controls; reg X, Y, Clk, Dummy;

always #1 Dummy=!Dummy; // Dummy clock, just for graphics.

// Examples of delay controls:

always begin #25 X=1;#10 X=0;#5; end

// An event control:

always @(posedgeClk) Y=X; // Wait for +ve clock edge. always #10 Clk = !Clk; // The real clock.

initial begin Clk = 0;

$display("T Clk X Y");

$monitor("%2g",$time,,,Clk,,,,X,,Y);

$dumpvars;#100 $finish; end endmodule

T    Clk X Y

The dummy clock in delay_controls helps in the graphical waveform display of the results (it provides a one-time-tick timing grid when we zoom in, for example). Figure shows the graphical output (white is used to represent the initial unknown values). The assignment statements to 'X' in the always statement repeat (every 25 + 10 + 5 = 40 time ticks).

Events can be declared (as named events), triggered, and detected as follows:

moduleshow_event;

reg clock;

event event_1, event_2; // Declare two named events.always @(posedge clock) -> event_1; // Trigger event_1. always @ event_1

begin $display("Strike 1!!"); -> event_2; end // Trigger event_2. always @ event_2 begin $display("Strike 2!!");

$finish; end // Stop on detection of event_2. always #10 clock = ~ clock; // We need a clock. initial clock = 0;

end module