DCM_ADV的Fout超过dcm的范围

在我的项目中已经使用了virtex-5中的dcm_adv，但是有些东西。我不明白。
我可以设置M = FF，D = 00到DRP。根据Fout = Fin×（M + 1）/（D + 1），当Fin = 50MHz时我可以得到Fout，显然，Fout超过dcm的范围。但是超出
我的期望，ISim不要警告我，clkfx周期是波形78ps。为什么？
即使dfs_freq_mode很高，clkfx_out也超出范围。任何建议将不胜感激



以下为原文

a dcm_adv in virtex-5 has been used in my project,but there is.something.wrong i dont understand. i can set  M=FF,D=00 through DRP .according to Fout=Fin×(M+1)/(D+1),i can get Fout when Fin=50MHz,obviously,Fout exceed range of dcm.
but beyond my expection,ISim dont warn me and clkfx period is 78ps from waveform.
why? even though dfs_freq_mode is high,clkfx_out also exceeds range.
any suggestion will be appreciated