下面是完整的警告信息，选的芯片是altera的EP4CE15F17C8N，手册上不是说最高可以得到400多MHz的频率吗，为什么会有这个警告啊，该怎么修改
Critical Warning (176584): Output pin "pwm_clk" (external output clock of PLL "pll:pll_inst|altpll:altpll_component|pll_altpll:auto_generated|pll1") uses I/O standard 3.3-V LVTTL, has current strength 4mA, output load 0pF, and output clock frequency of 246 MHz, but target device can support only maximum output clock frequency of 85 MHz for this combination of I/O standard, current strength and load
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