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18.10 时间单位与精度</div>
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SystemVerilog has a time unit and precision declaration which has the equivalent functionality of the ‘timescale compiler directives in Verilog-2001. Use of these declarations removes the file order dependencies problems with compiler directives. The time unit and precision can be declared by the timeunit and timeprecision keywords, respectively, and set to a time literal which must be a power of 10 units. For example: </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00816"><pre class="Element12"><span style="color: #0000FF">timeunit</span> <span style="color: #FF0000">100</span><span style="color: #0000FF">ps</span><span style="color: #800080">;</span>
<span style="color: #0000FF">timeprecision</span> <span style="color: #FF0000">10</span><span style="color: #0000FF">fs</span><span style="color: #800080">;</span></pre></div></div>
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There shall be at most one time unit and one time precision for any module, program, package or interface definition, or in any compilation-unit scope. This shall define a time scope. If specified, the timeunit and timeprecision declarations shall precede any other items in the current time scope. The timeunit and timeprecision declarations can be repeated as later items, but must match the previous declaration within the current time scope. </p>
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If a timeunit is not specified in the module, program, package or interface definition, then the time unit is shall be determined using the following rules of precedence:
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<li value="1" class="Element604">If the module or interface definition is nested, then the time unit shall be inherited from the enclosing module or interface (programs and packages cannot be nested).</li>
<li value="2" class="Element604">Else, if a ‘timescale directive has been previously specified (within the compilation unit), then the time unit shall be set to the units of the last ‘timescale directive.</li>
<li value="3" class="Element604">Else, if the compilation-unit scope specifies a time unit (outside all other declarations), then the time unit shall be set to the time units of the compilation unit.</li>
<li value="4" class="Element604">Else, the default time unit shall be used.</li>
</ol>The time unit of the compilation-unit scope can only be set by a timeunit declaration, not a ‘timescale directive. If it is not specified then the default time unit shall be used. </p>
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If a timeprecision is not specified in the current time scope, then the time precision shall be determined following the same precedence as with time units. </p>
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The global time precision is the minimum of all the timeprecision statements and the smallest time precision argument of all the `timescale compiler directives (known as the precision of the time unit of the simulation in Section 19.8 of the IEEE 1364-2001 standard) in the design. The step time unit is equal to the global time precision.</p></div>
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