一、开发笔记
常用的地磁传感器主要有FreeScale(飞思卡尔)的MAG 系列和 Honeywell(霍尼韦尔)的HMC 系列.
HMC5883 的工作电压在2.16V~3.6V 之间,典型为3.3V.可通过VDDIO 口来指定IO的工作电压(1.71v-VDD).
设备默认地址0x3C for write operations, or 0x3D for read operations.
磁场范围广(+/-8Oe),测量误差1-2度,最大输出频率可达160Hz ,默认15hz
HMC5883L仅有13个寄存器,其中03-08为数据输出寄存器(只读),09为状态寄存器(只读),标示是否有新数据等.00-02为配置和模式选择寄存器(读写).10-12为识别寄存器,对应的ASCII字符分别是‘H’,‘4’,‘3’(见下表寄存器列表)
支持自动地址指针:例如访问过最后一个数据输出寄存器08后,地址指针自动指向03,不必再写入03地址,可以再次进行数据读取.
If (address pointer = 08) then address pointer = 03
Else if (address pointer >= 12) then address pointer = 0
Else (address pointer) = (address pointer) + 1
二、初始化与工作流程
1. 设置测量模式
默认为单次测量模式,可以通过更改02模式选择寄存器切换成连续测量模式
通电时间后传送三个字节:0x3C 0x02 0x00
2. 设置采样率(默认15Hz)
修改00-配置寄存器,配置数据输出寄存器的写入频率,和每次写入的采样次数.以下展示一种防止出错的写法:
#define HMC5883L_RA_CONFIG_A 0x00 //配置寄存器A 地址00
#define HMC5883L_AVERAGING_1 0x00 //4种采样次数宏定义,CRA6-CRA5
#define HMC5883L_AVERAGING_2 0x01
#define HMC5883L_AVERAGING_4 0x02
#define HMC5883L_AVERAGING_8 0x03
#define HMC5883L_RATE_0P75 0x00 //7种采样频率的宏定义 CRA4-CRA2
#define HMC5883L_RATE_1P5 0x01
#define HMC5883L_RATE_3 0x02
#define HMC5883L_RATE_7P5 0x03
#define HMC5883L_RATE_15 0x04
#define HMC5883L_RATE_30 0x05
#define HMC5883L_RATE_75 0x06
#define HMC5883L_BIAS_NORMAL 0x00 //校准选项 CRA1-CRA0
#define HMC5883L_BIAS_POSITIVE 0x01
#define HMC5883L_BIAS_NEGATIVE 0x02
寄存器拼接:将以上宏定义设置的位,移动到对应的位置
#define HMC5883L_CRA_AVERAGE_BIT 6
#define HMC5883L_CRA_AVERAGE_LENGTH 2
#define HMC5883L_CRA_RATE_BIT 4
#define HMC5883L_CRA_RATE_LENGTH 3
#define HMC5883L_CRA_BIAS_BIT 1
#define HMC5883L_CRA_BIAS_LENGTH 2
(HMC5883L_AVERAGING_8 << (HMC5883L_CRA_AVERAGE_BIT - HMC5883L_CRA_AVERAGE_LENGTH + 1)) |
(HMC5883L_RATE_15 << (HMC5883L_CRA_RATE_BIT - HMC5883L_CRA_RATE_LENGTH + 1)) |
(HMC5883L_BIAS_NORMAL << (HMC5883L_CRA_BIAS_BIT - HMC5883L_CRA_BIAS_LENGTH + 1))
//配置信息:采样率15hz,每次结果来自8次采样,不进行校准
解读:采样次数选择为HMC5883L_AVERAGING_8,需要尾部对齐到 CRA5,已知头部的位置为CRA6(HMC5883L_CRA_AVERAGE_BIT),该选项暂居2bit(HMC5883L_CRA_AVERAGE_LENGTH)空间,可以推导出需要尾部移动6+1-2位 (HMC5883L_CRA_AVERAGE_BIT - HMC5883L_CRA_AVERAGE_LENGTH + 1)
3. 设置测量范围
默认增益1090,范围+-1.3Ga.无人机项目选择增益为660,测量范围为+-2.5Ga
4. 校准
应用层校准,保存初始值.
5. 开始测量
如果是连续测量模式,直接读取寄存器03-08值即可,需要进行拼接操作.如果选择单次模式,读取之后,需要将模式寄存器再次置位.
i2cdevReadReg8(I2Cx, devAddr, HMC5883L_RA_DATAX_H, 6, buffer);
if (mode == HMC5883L_MODE_SINGLE) i2cdevWriteByte(I2Cx, devAddr, HMC5883L_RA_MODE, HMC5883L_MODE_SINGLE << (HMC5883L_MODEREG_BIT - HMC5883L_MODEREG_LENGTH + 1));
*x = (((int16_t)buffer[0]) << 8) | buffer[1];
*y = (((int16_t)buffer[4]) << 8) | buffer[5];
*z = (((int16_t)buffer[2]) << 8) | buffer[3];
6. 自测试操作(可选)
设置增益为440 (HMC5883L_GAIN_440 // Gain value during self-test)
设置寄存器A,进行正向测试 HMC5883L_BIAS_POSITIVE
设置寄存器B,进行负向测试 HMC5883L_BIAS_NEGATIVE
#define HMC5883L_ST_GAIN HMC5883L_GAIN_440 // Gain value during self-test
#define HMC5883L_ST_GAIN_NBR 440
#define HMC5883L_ST_ERROR 0.2 // Max error
#define HMC5883L_ST_DELAY_MS 250 // delay in millisec during self test */
#define HMC5883L_ST_X_NORM (int32_t)(1.16 * HMC5883L_ST_GAIN_NBR)
#define HMC5883L_ST_X_MIN (int32_t)(HMC5883L_ST_X_NORM - (HMC5883L_ST_X_NORM * HMC5883L_ST_ERROR))
#define HMC5883L_ST_X_MAX (int32_t)(HMC5883L_ST_X_NORM + (HMC5883L_ST_X_NORM * HMC5883L_ST_ERROR))
#define HMC5883L_ST_Y_NORM (int32_t)(1.16 * HMC5883L_ST_GAIN_NBR)
#define HMC5883L_ST_Y_MIN (int32_t)(HMC5883L_ST_Y_NORM - (HMC5883L_ST_Y_NORM * HMC5883L_ST_ERROR))
#define HMC5883L_ST_Y_MAX (int32_t)(HMC5883L_ST_Y_NORM + (HMC5883L_ST_Y_NORM * HMC5883L_ST_ERROR))
#define HMC5883L_ST_Z_NORM (int32_t)(1.08 * HMC5883L_ST_GAIN_NBR)
#define HMC5883L_ST_Z_MIN (int32_t)(HMC5883L_ST_Z_NORM - (HMC5883L_ST_Z_NORM * HMC5883L_ST_ERROR))
#define HMC5883L_ST_Z_MAX (int32_t)(HMC5883L_ST_Z_NORM + (HMC5883L_ST_Z_NORM * HMC5883L_ST_ERROR))
三、寄存器表
1. 00-配置寄存器A
主要用于配置数据输出速率,每次输出结果的采样数.
2. 01-配置寄存器B
3. 02-模式寄存器
配置不同的工作模式:连续测量模式、单一测量模式、闲置模式.
4. 09-状态寄存器
LOCK:放置输出寄存器读取期间写入数据,在数据读取期间置位,数据不会在更新,读取完成后自动解锁. RDY:Ready Bit,与DRDY引脚电平绑定,当数据写入数据寄存器完成,该位置位.开始写入时状态清零并保持250us.
5. 10-12 Identification Register A/B/C
三个寄存器的数值,对应成ASCII码分别是‘H’ ‘4’ ‘3’
6. 03-08 Data Output Register
每个通道的输出对应两个8位寄存器A和B,大数端存放.
寄存器A存储MSB(高位数据),寄存器B存储LSB(低位数据).
两个寄存器保存数据为补码形式,范围0xF800–0x07FF(-2048–2047 )
