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1. 源由

鉴于ArduPilot开源飞控之AP_Baro中涉及Sensor Driver有以下总线类型：

1. I2C
2. Serial UART
3. CAN
4. SITL //模拟传感器(暂时并列放在这里)

ArduPilot之开源代码Sensor Drivers设计的front-end / back-end分层设计思路，AP_Baro主要描述的是front-end。

为了更好的从整体理解气压计这个传感器的嵌入式应用，这里深入到back-end驱动层，针对基于MSP协议的气压计设备，进行一个研读和理解。

2. back-end抽象类

AP_Baro_Backend驱动层需实现方法：

• void update()
• static AP_Baro_Backend *probe(AP_Baro &baro, AP_HAL::OwnPtr<AP_HAL::Device> dev)

注：通常来说使用ChibiOS的都有定时器，如果没有定时器，可以使用void accumulate(void)来实现传感器的数据定时获取。

class AP_Baro_Backend
{
public:AP_Baro_Backend(AP_Baro &baro);virtual ~AP_Baro_Backend(void) {};// each driver must provide an update method to copy accumulated// data to the frontendvirtual void update() = 0;// accumulate function. This is used for backends that don't use a// timer, and need to be called regularly by the main code to// trigger them to read the sensorvirtual void accumulate(void) {}void backend_update(uint8_t instance);//  Check that the baro valid by using a mean filter.// If the value further that filtrer_range from mean value, it is rejected.bool pressure_ok(float press);uint32_t get_error_count() const { return _error_count; }#if AP_BARO_MSP_ENABLEDvirtual void handle_msp(const MSP::msp_baro_data_message_t &pkt) {}
#endif#if AP_BARO_EXTERNALAHRS_ENABLEDvirtual void handle_external(const AP_ExternalAHRS::baro_data_message_t &pkt) {}
#endif/*device driver IDs. These are used to fill in the devtype fieldof the device ID, which shows up as BARO_DEVID* parameters tousers.*/enum DevTypes {DEVTYPE_BARO_SITL     = 0x01,DEVTYPE_BARO_BMP085   = 0x02,DEVTYPE_BARO_BMP280   = 0x03,DEVTYPE_BARO_BMP388   = 0x04,DEVTYPE_BARO_DPS280   = 0x05,DEVTYPE_BARO_DPS310   = 0x06,DEVTYPE_BARO_FBM320   = 0x07,DEVTYPE_BARO_ICM20789 = 0x08,DEVTYPE_BARO_KELLERLD = 0x09,DEVTYPE_BARO_LPS2XH   = 0x0A,DEVTYPE_BARO_MS5611   = 0x0B,DEVTYPE_BARO_SPL06    = 0x0C,DEVTYPE_BARO_UAVCAN   = 0x0D,DEVTYPE_BARO_MSP      = 0x0E,DEVTYPE_BARO_ICP101XX = 0x0F,DEVTYPE_BARO_ICP201XX = 0x10,DEVTYPE_BARO_MS5607   = 0x11,DEVTYPE_BARO_MS5837   = 0x12,DEVTYPE_BARO_MS5637   = 0x13,DEVTYPE_BARO_BMP390   = 0x14,};protected:// reference to frontend objectAP_Baro &_frontend;void _copy_to_frontend(uint8_t instance, float pressure, float temperature);// semaphore for access to shared frontend dataHAL_Semaphore _sem;virtual void update_healthy_flag(uint8_t instance);// mean pressure for range filterfloat _mean_pressure; // number of dropped samples. Not used for now, but can be usable to choose more reliable sensoruint32_t _error_count;// set bus ID of this instance, for BARO_DEVID parametersvoid set_bus_id(uint8_t instance, uint32_t id) {_frontend.sensors[instance].bus_id.set(int32_t(id));}
};

3. 方法实现

由于气压数据来自串口，因此，其逻辑相对简单，没有校准等复杂物理公式。

3.1 AP_Baro_MSP

实例初始化。

AP_Baro_MSP::AP_Baro_MSP├──> msp_instance = _msp_instance;├──> instance = _frontend.register_sensor();└──> set_bus_id(instance, AP_HAL::Device::make_bus_id(AP_HAL::Device::BUS_TYPE_MSP,0,msp_instance,0));

3.2 update

front-end / back-end数据更新。

AP_Baro_MSP::update└──> <count>├──> WITH_SEMAPHORE(_sem);├──> _copy_to_frontend(instance, sum_pressure/count, sum_temp/count);├──> sum_pressure = sum_temp = 0;└──> count = 0;

3.3 handle_msp

处理MSP协议中气压数据。

AP_Baro_MSP::handle_msp├──> <pkt.instance != msp_instance> // not for us│   └──> return;├──> WITH_SEMAPHORE(_sem);├──> sum_pressure += pkt.pressure_pa;├──> sum_temp += pkt.temp*0.01;└──> count++;typedef struct PACKED {uint8_t instance;uint32_t time_ms;float pressure_pa;int16_t temp; // centi-degrees C
} msp_baro_data_message_t;

3.4 MSP UART port

满足MSP协议格式，参考：BetaFlight模块设计之三十二：MSP协议模块分析

AP_Vehicle::setup└──> AP_MSP::init└──> AP_MSP::loop  //thread_create└──> AP_MSP_Telem_Backend::process_incoming_data└──> AP_MSP_Telem_Backend::msp_process_received_command└──> AP_MSP_Telem_Backend::msp_process_command└──> AP_MSP_Telem_Backend::msp_process_sensor_command└──> AP_MSP_Telem_Backend::msp_handle_baro└──> AP_Baro::handle_msp└──> AP_Baro_MSP::handle_msp

4. 参考资料

【1】ArduPilot开源飞控系统之简单介绍
【2】ArduPilot之开源代码Task介绍
【3】ArduPilot飞控启动&运行过程简介
【4】ArduPilot之开源代码Library&Sketches设计
【5】ArduPilot之开源代码Sensor Drivers设计