SignalTap does not show all signals

Thanks ted for the reply!!

Wow, didn't think it was that simple. I guess the compiler removes the nodes that are redundant during its optimization process.

Keep:

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You cannot use this synthesis attribute for nodes that have no fan-out.

To use the keep synthesis attribute, you can specify the keep synthesis attribute in a comment that is on the same line as the combinational node you want Analysis & Synthesis to keep. In the comment, precede the synthesis attribute with the synthesis keyword.

For example, in the following code, the comment /* synthesis keep */ directs Analysis & Synthesis to not minimize the keep_wire combinational node:

wire keep_wire /* synthesis keep */;

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Preserve:

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There are two important limitations of the preserve synthesis attribute:

It prevents a register from being inferred as a state machine.

It does not preserve fanout-free registers. Use the noprune synthesis attribute to prevent Analysis & Synthesis from removing fanout-free registers.

You can use Verilog 2001 attribute syntax to preserve a register, as shown in the following code:

(*preserve*) reg reg1;

You can also embed the attribute in a block comment that follows the variable declaration for the register you wish to preserve. You can also set the attribute on a module, which directs Analysis & Synthesis to preserve all registers in the module, except for those registers that infer state machines.

For example, in the following code, the comment /* synthesis preserve */ directs Analysis & Synthesis to preserve the reg1 register:

reg reg1 /* synthesis preserve */;

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