3.4AP_HAL::RCInput and AP_HAL::RCOutput

前言

 前面梳理了AP_HAL的硬件抽象层，包括串口、调度器、I2C等等，这次涉及到增加电机的输出口，所以有必要RC的输入输出的框架梳理一遍

AP_HAL::RCInput

 这里是对RCinput进行了抽象，具体的实现我们参看AP_HAL_PX4下的RCInput.cpp的实现

#define RC_INPUT_MIN_PULSEWIDTH 900
#define RC_INPUT_MAX_PULSEWIDTH 2100

class AP_HAL::RCInput {
public:
  /**
  * Call init from the platform hal instance init, so that both the type of
  * the RCInput implementation and init argument (e.g. ISRRegistry) are
  * known to the programmer. (It's too difficult to describe this dependency
  * in the C++ type system.)
  */
  virtual void init() = 0;// 初始化订阅 orb_subscribe(ORB_ID(input_rc))消息
  virtual void teardown() {};

  /**
  * Return true if there has been new input since the last call to new_input()
  * 通过_rcin.timestamp_last_signal来判断是否有新的数值更新上来
  */
  virtual bool new_input(void) = 0;

  /**
  * Return the number of valid channels in the last read
  * 返回_rcin.channel_count的通道数目
  */
  virtual uint8_t num_channels() = 0;

  /* Read a single channel at a time */优先返回 _override[ch]的值，没有的情况下返回_rcin.values[ch]的值
  virtual uint16_t read(uint8_t ch) = 0;

  /* Read an array of channels, return the valid count */从_rcin.values[ch]中读取len个channel的值
  virtual uint8_t read(uint16_t* periods, uint8_t len) = 0;

  /**
  * Overrides: these are really grody and don't