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Design Languages >> Verilog-AMS >> wreal: reaction to a signal change is not working properly
https://designers-guide.org/forum/YaBB.pl?num=1442493664

Message started by Zorro on Sep 17^th, 2015, 5:41am

Title: wreal: reaction to a signal change is not working properly
Post by Zorro on Sep 17^th, 2015, 5:41am

Hi Community,

I am having a problem that I cannot understand.

This simulation uses wreal only.

I have 2 internal signals which should react to a variation in the signal iload.

If iload > value the response signal should be high, otherwise the response signal should be low.

Example 1 (working as expected)
I am varying iload in steps of 1m.
Let's say value=100m
If iload is 100m or higher, then the response signal should be high
If iload is 99m or lower then the response signal should be low
-> this is working fine

The problem occurs if I use another "value", let's say value=50m

Example 2 (not working as expected)
I am varying iload in steps of 1m.
Let's say value=50m
If iload is 50m or higher, then the response signal should be high
If iload is 49m or lower then the response signal should be low
-> this is not working fine. When iload decreases the response signal should become low when iload=49m, but this is ocurring when iload=50m and this is not correct

This is the code that I am using:

real high_current_overload;
real low_current_overload;

always @(iload) begin

if (iload >= 100m) begin
high_current_overload = 1.0;
end else begin
high_current_overload = 0.0;
end

end

always @(iload) begin

if (iload >= 50m) begin
low_current_overload = 1.0;
end else begin
low_current_overload = 0.0;
end

end

See Picture 1.

Please any suggestion to understand what is causing this problem?

Thank you!

Zorro.

Title: Re: wreal: reaction to a signal change is not working properly
Post by Zorro on Sep 17^th, 2015, 5:41am

Picture 1.

Title: Re: wreal: reaction to a signal change is not working properly
Post by boe on Sep 17^th, 2015, 8:57am

Zorro wrote on Sep 17^th, 2015, 5:41am:

Hi Community,
...
Example 2 (not working as expected)
I am varying iload in steps of 1m.
Let's say value=50m
If iload is 50m or higher, then the response signal should be high
If iload is 49m or lower then the response signal should be low
-> this is not working fine. When iload decreases the response signal should become low when iload=49m, but this is ocurring when iload=50m and this is not correct
...
Please any suggestion to understand what is causing this problem?

Accumulation of numerical errors. I am sure your code will behave as expected if you use 49.995m as threshold.
- B O E

Title: Re: wreal: reaction to a signal change is not working properly
Post by Zorro on Sep 17^th, 2015, 10:59am

Hi boe,

thanks for your reply.

Your suggestions partly solves the problem. To understand it properly could you please tell me what you mean when you say "Accumulation of numerical errors" ???

If I use value=49.995m as you suggests it works fine, as expected. See Picture 2.

But the question is: how do I know when I need to subtract that small delta from my threshold value???

As an example, if I use value=60m it works fine, without needing to subtract any delta. See Picture 3.

Thank you.

Zorro.

Title: Re: wreal: reaction to a signal change is not working properly
Post by Zorro on Sep 17^th, 2015, 11:00am

Picture 2.

Title: Re: wreal: reaction to a signal change is not working properly
Post by Zorro on Sep 17^th, 2015, 11:00am

Picture 3.

Title: Re: wreal: reaction to a signal change is not working properly
Post by boe on Sep 21^st, 2015, 2:16am

Zorro wrote on Sep 17^th, 2015, 10:59am:

Zorro,
the simulator (usually) uses a binary representation for numbers, and the binary representation of 1m is infinite. Therefore, numerical errors occur and propagate through your calculation.
You may want to try the following (pseudo-)code

Code:

sum = 0
for i = 1 to 10000
sum = sum + 1m
print i, ": ", sum - 0.001*i

- B O E