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Design Languages >> Verilog-AMS >> Rise and Fall time
https://designers-guide.org/forum/YaBB.pl?num=1317176074

Message started by zahrein on Sep 27^th, 2011, 7:14pm

Title: Rise and Fall time
Post by zahrein on Sep 27^th, 2011, 7:14pm

HI All,
Copied my checkers. I don’t understand, why my time_vhi &time_vlo is fluctuating at different signals. Another thing is that my rise and fall time also is not consistant.

module measure_riset_fallt (drive_en, pad);

input pad;
input drive_en;

// need to declare variables so that the equation of the Rise/fall time can be calculated

parameter real padh =1.05;
real time_vhi;
real time_vlo;
real rise_time;
real fall_time;

analog begin

@(cross(V(pad) - ((padh)*0.3), 1)) begin
time_vlo= $abstime;

end
@(cross (V(pad) -((padh)*0.7),1)) begin //3
time_vhi=$abstime;

// For rising time . Calculate between 70% and 30% of the peak Voltage
rise_time=time_vhi-time_vlo;
`AMS_INFORM($display("Rise Time= %e",rise_time))

end //3

if (drive_en==1'b1) begin //1

@(cross(V(pad) - ((padh)*0.7), -1)) begin
time_vhi= $abstime;

end

@ (cross (V(pad) -((padh)*0.3),-1)) begin
time_vlo=$abstime;

fall_time = time_vhi - time_vlo;
`AMS_INFORM($display("Fall Time= %e",fall_time))

end

end
end
endmodule

Title: Re: Rise and Fall time
Post by Ken Kundert on Sep 27^th, 2011, 11:43pm

Your code was unreadable and I know it is unlikely that you will get many responses if people cannot easily understand your code, so I took the liberty of reformatting it to make it easier to understand:

Code:

module measure_riset_fallt (drive_en, pad);
input pad;
input drive_en;
parameter real padh = 1.05;
real time_vhi, time_vlo, rise_time, fall_time;

analog begin
@(cross(V(pad) - padh*0.3, 1)) begin
time_vlo = $abstime;
end
@(cross(V(pad) - padh*0.7, 1)) begin
time_vhi =$abstime;
// For rise time . Calculate between 70% and 30% of the peak Voltage
rise_time=time_vhi-time_vlo;
$display("Rise Time= %e",rise_time);
end

if (drive_en==1'b1) begin
@(cross(V(pad) - padh*0.7, -1)) begin
time_vhi= $abstime;
end
@(cross(V(pad) - padh*0.3, -1)) begin
time_vlo=$abstime;
fall_time = time_vhi - time_vlo;
$display("Fall Time= %e",fall_time);
end
end
end
endmodule

First a few generic comments:

One problem with this code is that you put cross functions inside an if statement. That is a no-no. And it seems unnecessary. After all, you did not put your rise time code inside an if statement.
You did not specify tolerances on your cross functions. If your signals are small, your variation may simply be due to loose tolerances.
To get better accuracy, you might want to use last_crossing (see "Time interval measurement" on verilogams.com).

None of this explains the problem you are having, which seems to be due to the fact that your vlo rising edge threshold detection simply fails to operate on every fourth edge. The code looks okay to me, so I suspect the problem is elsewhere.

What simulator is this?

-Ken

Title: Re: Rise and Fall time
Post by zahrein on Sep 28^th, 2011, 6:06pm

Hi Ken,
Thanks for your feedback and rearranging my code so that it is readable. The simulator i used is using the DVE. I believe most of the Logic Guyz use this.

I will try modify base on your recomendations but i dont think tolerance play an issue as we cross on voltages and not the timing.

Will let you know once its successfull. Thanks.

Title: Re: Rise and Fall time
Post by zahrein on Sep 28^th, 2011, 10:51pm

Manage to fix it. The "begin" and "end" have some disorganize. Here are the Codes and the waveform pics.

module measure_riset_fallt (drive_en, pad);

input pad;
input drive_en;

// need to declare variables so that the equation of the Rise/fall time can be calculated

parameter real padh =1.05;
real time_vhi;
real time_vlo;
real rise_time;
real fall_time;

analog begin

// For rising time . Calculate between 70% and 30% of the peak Voltage

@(cross(V(pad) - (padh*0.3), 1)) begin
if (drive_en==1'b1) begin
time_vlo= $abstime;
end
end

@(cross (V(pad) -(padh*0.7),1)) begin //3
if (drive_en==1'b1) begin
time_vhi=$abstime;

rise_time=time_vhi-time_vlo;
`AMS_INFORM($display("Rise Time= %e",rise_time))
end
end //3

// For falling time . Calculate between 70% and 30% of the peak Voltage

@(cross(V(pad) - (padh*0.7), -1)) begin
if (drive_en==1'b1) begin
time_vhi= $abstime;
end
end

@ (cross (V(pad) -(padh*0.3),-1)) begin
if (drive_en==1'b1) begin
time_vlo=$abstime;

fall_time = time_vhi - time_vlo;
`AMS_INFORM($display("Fall Time= %e",fall_time))
end
end

end

endmodule

Title: Re: Rise and Fall time
Post by Geoffrey_Coram on Oct 7^th, 2011, 7:45am

zahrein wrote on Sep 28^th, 2011, 10:51pm:

Manage to fix it. The "begin" and "end" have some disorganize.

It always helps me keep begin/end straight if I use proper indentation or formatting, as Ken showed you. (You need the "code" tag in the forum, so that spaces aren't combined.)