DDS直接数字频率合成器，能产生正弦波，方波，锯齿波，三角波四种波形，同时能在12864上显示波形类型和频率，用FPGA verilog实现的-DDS direct digital frequency synthesizer can produce sine, square wave, sawtooth wave, triangle wave four waveform, while in the 12864 on display the waveform type and frequency