作为最常见的传感器，本篇（来自于知乎某大神的文章）实现气压传感的交互：
1.先说明BMP180的特点
• 压力范围：300~1100hPa（海拔 9000 米~-500 米）
• 电源电压：1.8V~3.6V（VDDA）， 1.62V~3.6V（VDDD）
• 尺寸：3.6mmx3.8x0.93mm
• 低功耗：5μA，在标准模式
• 高精度：低功耗模式下，分辨率为 0.06hPa（0.5 米）
• 高线性模式下，分辨率为 0.03hPa（0.25 米）
• 含温度输出
• I2C 接口
• 温度补偿
• MSL 1 反应时间：7.5ms
• 待机电流：0.1μA
注意了该传感器是I2C接口的，因此在后续步骤要稍加注意。
2. 接线
I2C 有两个总线：SCL 为时钟信号，和 SDA 为双向数据传输。 每个 I2C 器件采用独特的 7 位地址，这意味着你可以有超过 120 个独特的 I2C 器件共享总线， 并且可以同时控制这些器件一起工作。
将 BMP180 的 VCC 引脚用红色跳线连接树莓派的 3V3 （ 3.3 伏电源）。将 BMP180 的 SDA 引脚 接入树莓派的 SDA 引脚并将 BMP180 SCL 引脚接入树莓派的 SCL 引脚，该引脚提供一个规律的 时钟信号。SDA 传递数据信号。BMP180 的 GND 引脚通过黑色跳线连接树莓派的接地（ GND） 引脚。 在上电前，一定多检查两次接线的准确性。
3. 环境和程序部分
配置文件/boot/config.txt：
dtparam=i2c_arm=on
重启树莓派，用如下命令查看传感器是否接上：
i2cdetect -y 1
如果接上了的话会显示下图：
编写 BMP180.py:
#!/usr/bin/env python
import time
import smbus
# BMP085 default address.
BMP180_I2CADDR = 0x77
# Operating Modes
BMP180_ULTRALOWPOWER = 0
BMP180_STANDARD = 1
BMP180_HIGHRES = 2
BMP180_ULTRAHIGHRES = 3
# BMP085 Registers
BMP180_CAL_AC1 = 0xAA # R Calibration data (16 bits)
BMP180_CAL_AC2 = 0xAC # R Calibration data (16 bits)
BMP180_CAL_AC3 = 0xAE # R Calibration data (16 bits)
BMP180_CAL_AC4 = 0xB0 # R Calibration data (16 bits)
BMP180_CAL_AC5 = 0xB2 # R Calibration data (16 bits)
BMP180_CAL_AC6 = 0xB4 # R Calibration data (16 bits)
BMP180_CAL_B1 = 0xB6 # R Calibration data (16 bits)
BMP180_CAL_B2 = 0xB8 # R Calibration data (16 bits)
BMP180_CAL_MB = 0xBA # R Calibration data (16 bits)
BMP180_CAL_MC = 0xBC # R Calibration data (16 bits)
BMP180_CAL_MD = 0xBE # R Calibration data (16 bits)
BMP180_CONTROL = 0xF4
BMP180_TEMPDATA = 0xF6
BMP180_PRESSUREDATA = 0xF6
# Commands
BMP180_READTEMPCMD = 0x2E
BMP180_READPRESSURECMD = 0x34
class BMP180(object):
def __init__(self, address=BMP180_I2CADDR, mode=BMP180_STANDARD):
self._mode = mode
self._address = address
self._bus = smbus.SMBus(1)
# Load calibration values.
self._load_calibration()
def _read_byte(self,cmd):
return self._bus.read_byte_data(self._address,cmd)
def _read_u16(self,cmd):
MSB = self._bus.read_byte_data(self._address,cmd)
LSB = self._bus.read_byte_data(self._address,cmd+1)
return (MSB << 8) + LSB
def _read_s16(self,cmd):
result = self._read_u16(cmd)
if result > 32767:result -= 65536
return result
def _write_byte(self,cmd,val):
self._bus.write_byte_data(self._address,cmd,val)
def _load_calibration(self):
"load calibration"
self.cal_AC1 = self._read_s16(BMP180_CAL_AC1) # INT16
self.cal_AC2 = self._read_s16(BMP180_CAL_AC2) # INT16
self.cal_AC3 = self._read_s16(BMP180_CAL_AC3) # INT16
self.cal_AC4 = self._read_u16(BMP180_CAL_AC4) # UINT16
self.cal_AC5 = self._read_u16(BMP180_CAL_AC5) # UINT16
self.cal_AC6 = self._read_u16(BMP180_CAL_AC6) # UINT16
self.cal_B1 = self._read_s16(BMP180_CAL_B1) # INT16
self.cal_B2 = self._read_s16(BMP180_CAL_B2) # INT16
self.cal_MB = self._read_s16(BMP180_CAL_MB) # INT16
self.cal_MC = self._read_s16(BMP180_CAL_MC) # INT16
self.cal_MD = self._read_s16(BMP180_CAL_MD) # INT16
def read_raw_temp(self):
"""Reads the raw (uncompensated) temperature from the sensor."""
self._write_byte(BMP180_CONTROL, BMP180_READTEMPCMD)
time.sleep(0.005) # Wait 5ms
MSB = self._read_byte(BMP180_TEMPDATA)
LSB = self._read_byte(BMP180_TEMPDATA+1)
raw = (MSB << 8) + LSB
return raw
def read_raw_pressure(self):
"""Reads the raw (uncompensated) pressure level from the sensor."""
self._write_byte(BMP180_CONTROL, BMP180_READPRESSURECMD + (self._mode << 6))
if self._mode == BMP180_ULTRALOWPOWER:
time.sleep(0.005)
elif self._mode == BMP180_HIGHRES:
time.sleep(0.014)
elif self._mode == BMP180_ULTRAHIGHRES:
time.sleep(0.026)
else:
time.sleep(0.008)
MSB = self._read_byte(BMP180_PRESSUREDATA)
LSB = self._read_byte(BMP180_PRESSUREDATA+1)
XLSB = self._read_byte(BMP180_PRESSUREDATA+2)
raw = ((MSB << 16) + (LSB << 8) + XLSB) >> (8 - self._mode)
return raw
def read_temperature(self):
"""Gets the compensated temperature in degrees celsius."""
UT = self.read_raw_temp()
X1 = ((UT - self.cal_AC6) * self.cal_AC5) >> 15
X2 = (self.cal_MC << 11) / (X1 + self.cal_MD)
B5 = X1 + X2
temp = ((B5 + 8) >> 4) / 10.0
return temp
def read_pressure(self):
"""Gets the compensated pressure in Pascals."""
UT = self.read_raw_temp()
UP = self.read_raw_pressure()
X1 = ((UT - self.cal_AC6) * self.cal_AC5) >> 15
X2 = (self.cal_MC << 11) / (X1 + self.cal_MD)
B5 = X1 + X2
# Pressure Calculations
B6 = B5 - 4000
X1 = (self.cal_B2 * (B6 * B6) >> 12) >> 11
X2 = (self.cal_AC2 * B6) >> 11
X3 = X1 + X2
B3 = (((self.cal_AC1 * 4 + X3) << self._mode) + 2) / 4
X1 = (self.cal_AC3 * B6) >> 13
X2 = (self.cal_B1 * ((B6 * B6) >> 12)) >> 16
X3 = ((X1 + X2) + 2) >> 2
B4 = (self.cal_AC4 * (X3 + 32768)) >> 15
B7 = (UP - B3) * (50000 >> self._mode)
if B7 < 0x80000000:
p = (B7 * 2) / B4
else:
p = (B7 / B4) * 2
X1 = (p >> 8) * (p >> 8)
X1 = (X1 * 3038) >> 16
X2 = (-7357 * p) >> 16
p = p + ((X1 + X2 + 3791) >> 4)
return p
def read_altitude(self, sealevel_pa=101325.0):
"""Calculates the altitude in meters."""
# Calculation taken straight from section 3.6 of the datasheet.
pressure = float(self.read_pressure())
altitude = 44330.0 * (1.0 - pow(pressure / sealevel_pa, (1.0/5.255)))
return altitude
def read_sealevel_pressure(self, altitude_m=0.0):
"""Calculates the pressure at sealevel when given a known altitude in
meters. Returns a value in Pascals."""
pressure = float(self.read_pressure())
p0 = pressure / pow(1.0 - altitude_m/44330.0, 5.255)
return p0
然后编写调用函数bmp180_example.py:
#!/usr/bin/python
import time
from BMP180 import BMP180
# Initialise the BMP085 and use STANDARD mode (default value)
# bmp = BMP085(0x77, debug=True)
bmp = BMP180()
# To specify a different operating mode, uncomment one of the following:
# bmp = BMP085(0x77, 0) # ULTRALOWPOWER Mode
# bmp = BMP085(0x77, 1) # STANDARD Mode
# bmp = BMP085(0x77, 2) # HIRES Mode
# bmp = BMP085(0x77, 3) # ULTRAHIRES Mode
while True:
temp = bmp.read_temperature()
# Read the current barometric pressure level
pressure = bmp.read_pressure()
# To calculate altitude based on an estimated mean sea level pressure
# (1013.25 hPa) call the function as follows, but this won't be very accurate
altitude = bmp.read_altitude()
# To specify a more accurate altitude, enter the correct mean sea level
# pressure level. For example, if the current pressure level is 1023.50 hPa
# enter 102350 since we include two decimal places in the integer value
# altitude = bmp.readAltitude(102350)
print "Temperature: %.2f C" % temp
print "Pressure: %.2f hPa" % (pressure / 100.0)
#print "Altitude: %.2f\n" % altitude
time.sleep(1)
执行./bmp180_example.py就可以在终端看到实时的温度/气压输出。