用PWM产生锯齿波，应使PWM寄存器中的值线性增加，那么在PWM输出端将得到宽度线性增加的脉冲波，经滤波后，便可得到锯齿波。-have used PWM ramp, it should PWM register with the value of linear increase, then the PWM output will be increased linearly width of the pulse wave, filter, which will be ramp.