在使用SignalTAP II的过程中，我经常发现一些用于调试的逻辑（比如调试用的计数器）会被优化掉，不能出现在调试波形中。在Altera的一篇文档中，发现了以下关键信 息：

In the logic synthesis stage, the Quartus II software may optimize away signals that you are trying to analyze with the SignalTap II Embedded Logic Analyzer. If this occurs, you will see a compilation error. You can force the Quartus II software to preserve these signals by adding the keep or preserve attribute in the source HDL to the signals you want to monitor.

Thekeepattribute is used for a wire or net node. For example:
In Verilog:
wire my_wire /* synthesis keep = 1 */:
In VHDL:
signal my_signal: bit;
attribute syn_keep : boolean;
attribute syn_keep of my_signal: signal is true;

Thepreserveattribute is used for a register. For example:
In Verilog:
reg my_reg /* synthesis preserve = 1 */:
In VHDL:
signal my_reg: stdlogic;
attribute preserve : boolean;
attribute preserve of my_signal: signal is true;

背景资料：Using SignalTap II Embedded Logic Analyzers in SOPC Builder Systems