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Modeling >> Behavioral Models >> a basic verilogA question
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Message started by LRC on Sep 29^th, 2006, 7:01pm

Title: a basic verilogA question
Post by LRC on Sep 29^th, 2006, 7:01pm

Hi,

I am trying to model the following actions. At the rising clock edge, A is assigned a value. After tdelay, B is assigned.
How should I create the delay in assignment in verilogA? Is the following approach reasonable?

@(cross(V(clkSig)-vtran, +1, slack, clkSig.potential.abstol)) begin
A = xxx.
currenttime = $abstime
@(timer(currenttime + tdelay))
B = xxx.

I am a beginner verilogA. I will appreciate your help.

thanks,

LRC

Title: Re: a basic verilogA question
Post by Ken Kundert on Sep 29^th, 2006, 10:15pm

You can not place an event statement within an event statement. Conceptually this does not make sense. An event is something that occurs at an instance and has zero duration. Thus the second event statement could never get called. Verilog-A is different than Verilog in this regard. In Verilog, an @event statement would cause execution of the thread to block or hold at that point until the event occurs. In Verilog-A statements do not block. The statements are evaluated for all time, and each statement has zero duration. So in Verilog-A, an @event statement is simply bypassed except at the instant when the event occurs.

To implement what you want, try something like the following

Code:

@(cross(V(clk)-thresh,+1)) begin
A = ...
next = $abstime + tdelay;
end
@(timer(next))
B = ...

-Ken

Title: Re: a basic verilogA question
Post by jbdavid on Sep 30^th, 2006, 2:20am

probably some of the confusion for digital guys is that in the Discrete time domain we have the possibility of
BLOCKING events and delays at the beginning of the line..

Code:

always @(A) begin
#4 B = !A;
#4 C = !B;
end

which has NO equivalent in the analog side
instead you should think about your model as if you could only use something like:

Code:

always @(A)
B <= #4 !A;
always @(B)
C <= #4 !B;

kens example is like this..
jbd

Title: Re: a basic verilogA question
Post by LRC on Oct 2^nd, 2006, 7:03am

Thanks very much for Ken and jbDavid's help. I understand better now.