Matrix - Pressure and Temperature Sensor

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1 Introduction

• The BMP180 is a high precision, ultra-low power small digital pressure sensor for consumer applications in mobile phones, PDAs, GPS navigation devices and outdoor equipments. With a low altitude noise of merely 0.25m at fast conversion time, the BMP180 offers superior performance. The I2C interface allows for easy system integration with a microcontroller.
• The BMP180 is based on piezo-resistive technology for EMC robustness, high accuracy and linearity as well as long term stability.
• The BMP180 consists of a piezo-resistive sensor, an analog to digital converter and a control unit with E2PROM and a serial I2C interface. The E2PROM has stored 176 bit of individual calibration data. This is used to compensate offset, temperature dependence and other parameters of the sensor.

2 Features

• I2C，3.3V
• Pressure data(16 to 19 bit)
• Temperature data(16 bit)
• PCB Dimension(mm): 16 x 16

• Pin Description:

Pin	Description
SDA	I2C SDA
SCL	I2C SCL
5V	Supply Voltage 5V
GND	Ground

3 Basic Device Operation

• The mode (ultra low power, standard, high, ultra high resolution) can be selected by the variable oversampling_setting (0, 1, 2, 3) in the C code.
• Calculation of true temperature and pressure in steps of 1Pa (= 0.01hPa = 0.01mbar) and temperature in steps of 0.1°C.
• With the measured pressure p and the pressure at sea level p0 e.g. 1013.25hPa, the altitude in meters can be calculated with the international barometric formula:
• With the measured pressure p and the absolute altitude the pressure at sea level can be calculated:

Thus, a difference in altitude of ∆altitude = 10m corresponds to 1.2hPa pressure change at sea level.

4 Download Matrix Source Code

All the matrix modules' code samples are open source. They are maintained on GitHub - https://github.com/friendlyarm/matrix.git
Each branch in this hub contains the matrix modules' code samples for a board that the matrix modules can work with.

• The nanopi branch contains the matrix modules' code samples for the NanoPi
• The nanopi 2 branch contains the matrix modules' code samples for the NanoPi 2
• The tiny4412 branch contains the matrix modules' code samples for the Tiny4412
• The raspberrypi branch contains the matrix modules' code samples for the RaspberryPi

Please follow the steps below to get the source code:
Install the git utility on a PC running Ubuntu14.04

$ sudo apt-get install git

Clone the matrix code from GitHub

$ git clone https://github.com/friendlyarm/matrix.git

If this is successful a "matrix" directory will be generated, which will contain all the matrix modules' code samples.

5 Connect to NanoPi 2

5.1 Hardware Connection

Please refer to the following connection diagram to connect the Matrix-Pressure_and_Temperature_Sensor to the NanoPi 2:

Connection Details:

Matrix-Pressure_and_Temperature_Sensor_nanopi	NanoPi 2
SDA	Pin3
SCL	Pin5
5V	Pin4
GND	Pin6

5.2 Compile Test Program

Please login the matrix hub and enter the nanopi2 branch

$ cd matrix
$ git checkout nanopi2

Compile the matrix code

$ make CROSS_COMPILE=arm-linux- clean
$ make CROSS_COMPILE=arm-linux-
$ make CROSS_COMPILE=arm-linux- install

Note: please make sure to install the cross compiler "arm-linux-gcc-4.9.3" on your PC, which is used to compile files for the NanoPi 2.
Generated library files are under the "install/lib" directory. The test program is under the "install/usr/bin" directory.
The modules are under the "modules" directory. The driver's source code is in github: https://github.com/friendlyarm/linux-3.4.y.git

5.3 Run Test Program

Please insert a TF card which is flashed with Debian to a Linux host and mount its boot and rootfs sections.
We assume the rootfs is mounted to /media/rootfs then please run the following commands to copy the module, library and test program to the card.

$ cp modules /media/rootfs/ -r
$ cp install/lib/* /media/rootfs/lib/ -d
$ cp install/usr/bin/* /media/rootfs/usr/bin/

Insert this TF card to your NanoPi 2, power on and run the following command to load the driver

$ cd /modules
$ insmod bmp085.ko

Start the matrix-pressure_temp program

$ matrix-pressure_temp

Note: this module is not plug and play therefore before running the module please make sure it is connected to a NanoPi 2.
Here is what you expect to observe:

5.4 Code Sample

int main(int argc, char ** argv) 
{ 
    int ret = -1;
    int bmpTemp = 0;
    int bmpPressure = 0;
    float altitude = 0;
 
    if ((ret = bmp180Read(BMP180_TEMP, &bmpTemp)) != -1) {
        printf("Get temperature : %.1f C\n", (float)bmpTemp / 10);
    } else {
        printf("Faided to get humidity\n");
    }
 
    if ((ret = bmp180Read(BMP180_PRESSURE, &bmpPressure)) != -1) {
        printf("Get pressure : %.2f hPa\n", (float)bmpPressure / 100);
    } else {
        printf("Faided to get pressure\n");
    }
 
    altitude = 44330 * ( 1 - pow( ((float)bmpPressure / 100 / 1013.25), (1/5.255) ) );
    printf("Get altitude : %.2f m\n", altitude);
    return 0;
}

6 与NanoPi连接使用

6.1 准备工作

在NanoPi上运行Debian系统，然后在主机PC上安装并使用相应的编译器，参考wiki: NanoPi & How to Build the Compiling Environment。
注意: 只有使用nanopi-v4.1.y-matrix分支编译出来的内核才能配合Matrix配件正常工作。
下载NanoPi内核源代码并编译:

$ git clone https://github.com/friendlyarm/linux-4.x.y.git
$ cd linux-4.x.y
$ git checkout nanopi-v4.1.y-matrix
$ make nanopi_defconfig
$ touch .scmversion
$ make

编译好后的zImage位于内核源码arch/arm/boot/目录下，把该zImage替换掉NanoPi烧写文件sd-fuse_nanopi/prebuilt下的zImage，重新制作SD卡即可。

6.2 硬件连接

参考下图连接模块Matrix-Pressure_and_Temperature_Sensor和NanoPi：
Matrix-Pressure_and_Temperature_Sensor_nanopi

连接说明：

Matrix-Pressure_and_Temperature_Sensor_nanopi	NanoPi
SDA	Pin3
SCL	Pin5
5V	Pin4
GND	Pin6

6.3 编译测试程序

进入Matrix代码仓库，切换到nanopi分支

$ cd matrix
$ git checkout nanopi

编译Matrix配件代码

$ make CROSS_COMPILE=arm-linux- clean
$ make CROSS_COMPILE=arm-linux-
$ make CROSS_COMPILE=arm-linux- install

注意：请确保你的主机PC当前使用的交叉编译器为NanoPi-Debian配套的arm-linux-gcc-4.4.3。
编译出来的库文件位于install/lib目录下，而测试程序则位于install/usr/bin目录下，模块Matrix-Pressure_and_Temperature_Sensor对应的测试程序为matrix-pressure_temp。

6.4 运行测试程序

将带有Debian系统的SD卡插入一台运行Linux的电脑，可以挂载SD卡上的boot和rootfs分区。
假设rootfs分区的挂载路径为/media/rootfs，执行以下命令将Matrix的硬件驱动、库文件和测试程序拷贝到NanoPi的文件系统上。

$ cp modules /media/rootfs/ -r
$ cp install/lib/* /media/rootfs/lib/ -d
$ cp install/usr/bin/* /media/rootfs/usr/bin/

将SD卡重新插入NanoPi 2，上电启动，在Debian的shell终端中执行以下命令加载硬件驱动。

$ cd /modules
$ insmod bmp085-i2c.ko

运行模块Matrix-Pressure_and_Temperature_Sensor的测试程序。

$ matrix-pressure_temp

注意：此模块并不支持热插拔，启动系统前需要确保硬件连接正确。

6.5 代码展示

int main(int argc, char ** argv) 
{ 
    int ret = -1;
    int bmpTemp = 0;
    int bmpPressure = 0;
    float altitude = 0;
 
    if ((ret = bmp180Read(BMP180_TEMP, &bmpTemp)) != -1) {
        printf("Get temperature : %.1f C\n", (float)bmpTemp / 10);
    } else {
        printf("Faided to get humidity\n");
    }
 
    if ((ret = bmp180Read(BMP180_PRESSURE, &bmpPressure)) != -1) {
        printf("Get pressure : %.2f hPa\n", (float)bmpPressure / 100);
    } else {
        printf("Faided to get pressure\n");
    }
 
    altitude = 44330 * ( 1 - pow( ((float)bmpPressure / 100 / 1013.25), (1/5.255) ) );
    printf("Get altitude : %.2f m\n", altitude);
    return 0;
}

10 相关资料

BMP180.pdf

11 Update Log

11.1 Feb-24-2016

• Added the whole English version