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 Python Using Linux Drivers

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 I/O Control

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 ***NEW!!

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

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