利用定时器TMR2与周期寄存器PR2配合工作，从引脚RD0上输出一个占空比为50 的对称方波，该方波信号的周期为0.5ms。可以利用示波器观察和验证该波形，单片机初次加电或者复位之后，发光二极管LED7不停地闪烁，LED0亦被点亮（实际上，LED0在快速闪烁，只不过由于其频率过高人眼不易察觉），表明方波发生器正常工作。-Timer TMR2 Period register, PR2 work with, from the pin RD0, output a 50　duty cycle symmetric square wave, the cycle of the square wave signal of 0.5ms. Can use the oscilloscope to observe and verify the waveform, the microcontroller power for the first time or after a reset, the light-emitting diode LED7 kept flashing, LED0 were also lit (In fact, LED0 in quick flashes, just because its frequency is too high the human eye imperceptible ), indicating that the square wave generator is working properly.