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发表于 2011-1-27 23:42:16
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可以实现的,使用Keep约束。参考Xilinx的Constrains Guide(Xilinx\doc\usenglish\books\docs\cgd\cgd.pdf)
Before using KEEP, declare it with the following syntax:
attribute keep : string;
After KEEP has been declared, specify the VHDL constraint as follows:
attribute keep of signal_name: signal is “true”;
For a detailed discussion of the basic VHDL syntax, see “VHDL”.
Example:
attribute keep : string;
attribute keep of keep1: signal is "true";
attribute keep of keep2: signal is "true";
keep1 <= not sig_delay;
keep2 <= not keep1;
我也觉得这方法不大可靠,不好随便用。另外FPGA的倍频不是随便就能用的,有最小频率要求,还要很多个周期的稳定时间,在这里是肯定不适用的。 |
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