"Warning: NUMERIC_STD, "=": metavalue detected, returning FALSE". ISim, Xilinx

Introduction 

Whenever it’s possible I avoid using ISIM (the ISE embedded simulator). Among other things, it has poor support, and also very encrypted Warnings and Errors messages. In the last project I worked, I had no choice but using ISim, and when I was finishing the top level simulation I got the famous warning "Warning: NUMERIC_STD," = "metavalue detected, returning FALSE". And I say famous, because I goggled it in the web and found many entries, but almost none gave me an idea of what happened and even less how to fix it. Well, that is the reason I’m writing this post, to tell you what this means and at least one concrete way to find a clue where or how this Warning is generated.

What is it "Metavalue Detected" ? 

This message means that the simulator encountered as a result of an expression a non-numeric value, which can be for example "UUUU" or "XXXX" or some other value.

Commonly this result is obtained during the initialization of the system under test, and depending on the type of test it can repeat this message periodically or just once. As an example,  in the following figure you can read the messages I got during my simulation.

How to find the source of the warning message? 

As it can be seen from the figure shown above, there is nothing that looks "rare/strange" in the waveform of the various signals shown, so that, the solution is not at simple sight.

One way to find the source of this warning message, at least in this particular case that I’m showing you, it is to try to see the different internal signals of the whole system described in VHDL. To do this, go to the panel called "Instance and Processes" and click on the arrow that is next to the test bench, show in the figure above, click on the arrow next to top_tb. The name of the instance of the component under test in the test bench appears, in this case I called 'uut'. Again click on the arrow that is next to 'uut' and the various sub-components of the system are now listed. The following figure details what it was explained.

The next step is to select each sub-component and adding the different E/S and internal signals to the waveforms window (Wave Window). Select a sub-component by simply click on its name, then in the 'Objects' panel all the E / S and internal signals  of this sub-component appear. Then select all signals (because we do not know which one is malfunctioning), and add then to the Wave Window.

The following figure details the steps explained.

In the figure above, I have added to the Wave Window all the signals of the cont_4bits_1 sub-component. I can also add the signals of the other sub-components, freq_div_1 and bcd_7seg_1, but it happened that just adding the signals of cont_4bits_1, I found part of the problem. How? ... Well, once I added the signals of this sub-component, I run the simulation again, and immediately I visualized something that caught my attention .... what was that? .. well, try to find it out by yourself in the following figure.:

What do you see weird ???? .......

Yes ! ( I guess you found it out J )…  the last two signals, count and i_count, are not defined ('U' = undefined) .... We found something! ....

Possible Solutions

Now I come to this point, the steps to follow are two:

1) review the VHDL / Verilog subcomponent cont_4bits_1, and check especially initializing the count and i_count vectors.

2) review the bench test and verify the value given to the signal that initializes the system, commonly called reset.

In one of the two sides is the origin of Warning, and the solution to work with .....! ... (In my particular case it was a no well defined reset value in the test bench....).

Finally a note for those using ModelSim, you can read in various forums that ModelSim has an option for this type of Warnings (based on 'metavalues') that allow you to ignore them. Of course I DO NOT share that view, because it is always good to know the origin of the Warning. In my particular case if I had used ModelSim and had used the option to ignore these Warning, I had never encountered the problem that I found with the reset signal. . .

Well, I hope you find it useful ... reminded me if so!

See you soon ...