基于stm32f103rc控制板，实现freemodbus的从机移植，PWM精确脉冲个数发送；用户代码调用BootLoader；I2C扩展IO等； 正在学习这方面知识的同学可以参考。(Based on stm32f103rc control board, the slave of freemodbus was transplanted and the PWM precise pulse number was sent.BootLoader is called by user code;I2C