Write a program to generate a 25-Hz digital waveform with a 50 percent duty cycle on the PT0 pin as long as the voltage level on the PP7 pin is high. Your program should consist of two parts:
1. Entry test. As long as the PP7 pin is low, it stays in this loop.
2. Waveform generation body. This part generates a pulse with 20 ms high time and 20 ms low time, and at the end of a period tests the PP7 signal. If PP7 is still high, it generates the next pulse. Otherwise, it jumps to entry test.