基于stm32f4的水下超声波测距：包括PWM输出到LED控制LED的亮度，KEY接外部中断，调节PWM占空比，定时器3中断，调节LED2闪烁，LCD显示AD采样值。-Based on the underwater ultrasonic ranging stm32f4: including PWM output to the LED control LED brightness, KEY external interrupt, adjust the PWM duty cycle, timer i