实验目的:
　　学习定时器的PWM功能
　　
硬件资源:
　　1,DS0(连接在PB1)　
　　2,定时器3(TIM3),使用TIM3通道4(CH4),将TIM3_CH4输出到PB1.从而实现PWM输
　　出控制DS0亮度. 　　
　　
实验现象:
　　本实验,DS0由暗到亮,再由亮到暗,再由暗到亮,依次循环.(stm32f7 pwm output
Objective:
Learning the PWM function of the timer

Hardware resources:
1, DS0 (connected to PB1)
2, the timer 3 (TIM3), using the TIM3 channel 4 (CH4), output TIM3_CH4 to PB1. to achieve PWM transmission
Out of control DS0 brightness.

Experimental phenomena:
In this experiment, DS0 from dark to bright, then bright to dark, and then dark to bright, circulated in turn.)