The Complete Program

import subprocess 
import io
import fcntl
from time import sleep
def crcCheck(msb, lsb, check):
    data32 = (msb << 16)|(lsb <<8)| check
    divisor = 0x988000 
    for i in range(16):
        if( data32 & 1<<(23 - i) ):
            data32 ^= divisor
        divisor>>= 1
    return  data32
def checkI2CBus():
    temp = subprocess.Popen(["sudo", "dtparam", "-l"],
                            stdout = subprocess.PIPE) 
    output = str(temp.communicate())
    print(output)
    lasti2c=output.rfind("i2c_arm")
    if lasti2c!=-1:
        lasti2c=output.find("i2c_arm=on",lasti2c)
    if lasti2c==-1:
        temp = subprocess.Popen(["sudo", "dtparam",
                       "i2c_arm=on"],
                            stdout = subprocess.PIPE)
        output = str(temp.communicate())
    return
checkI2CBus()
I2C_SLAVE=0x0703
fdr = io.open("/dev/i2c-1", "rb", buffering=0)
fdw = io.open("/dev/i2c-1", "wb", buffering=0)
fcntl.ioctl(fdr, I2C_SLAVE, 0x40)
fcntl.ioctl(fdw, I2C_SLAVE, 0x40)
fdw.write( bytearray([0xF3]))
while(True):
    try:
        data=fdr.read(3)
        break
    except:
        sleep(0.01)
msb=data[0]
lsb=data[1]
crc=data[2]
data16= (msb << 8) |(lsb & 0xFC)
temp = -46.85 +(175.72 * data16 /65536)
print("Temperature=",temp,"C")
fdw.write( bytearray([0xF5]))
while(True):
    try:
        data=fdr.read(3)        
        break
    except:
        sleep(0.01)
msb=data[0]
lsb=data[1]
crc=data[2]
data16= (msb << 8) |(lsb & 0xFC)
hum = -6 + 125.0 * data16 / 65536
print("humidity=",hum,"%")
print(crcCheck(msb, lsb, crc))
fdw.close()
fdr.close()

Not included in this extract but in chapter

• I2C Tools

Summary

• The I2C driver can be loaded dynamically and it provides the basic facilities to interface with any I2C device.

• The I2C driver creates a number of new folders and it also accepts ioctl commands.

• As an example of using the driver, the HTU21D is easy to set up and read. It also has a dedicated Linux driver which is discussed in Chapter 14.

• Without clock stretching support, all we can do is to poll for data to be ready to read.

• Computing a CRC is something every IoT programmer needs to know how to do in the general case.

• There are a number of command line tools that let you work with I2C, but they need to be used with caution.

Raspberry Pi IoT In PythonUsing Linux Drivers
Second Edition

By Harry Fairhead & Mike James

Buy from Amazon.

Contents

1.  Choosing A Pi For IoT
2.  Getting Started With Python
3.   Drivers: A First Program
4.  The GPIO Character Driver 
5.  GPIO Using Ioct ***NEW!!
6.  GPIO Events
7.  The Device Tree
      Extract: The DHT22 * 
8.  Some Electronics
9.  Pulse Width Modulation
      Extract: PWM *
10. SPI Devices
11. I2C Basics
       Extract: I2C *
12. The I2C Linux Driver
13. Advanced I2C
14. Sensor Drivers
15. 1-Wire Bus
       Extract 1-Wire And The DS18B20 *
16. Going Further With Drivers
17. Appendix I

*From the first edition waiting for update.

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