DZone
Thanks for visiting DZone today,
Edit Profile
  • Manage Email Subscriptions
  • How to Post to DZone
  • Article Submission Guidelines
Sign Out View Profile
  • Post an Article
  • Manage My Drafts
Over 2 million developers have joined DZone.
Log In / Join
Refcards Trend Reports Events Over 2 million developers have joined DZone. Join Today! Thanks for visiting DZone today,
Edit Profile Manage Email Subscriptions Moderation Admin Console How to Post to DZone Article Submission Guidelines
View Profile
Sign Out
Refcards
Trend Reports
Events
Zones
Culture and Methodologies Agile Career Development Methodologies Team Management
Data Engineering AI/ML Big Data Data Databases IoT
Software Design and Architecture Cloud Architecture Containers Integration Microservices Performance Security
Coding Frameworks Java JavaScript Languages Tools
Testing, Deployment, and Maintenance Deployment DevOps and CI/CD Maintenance Monitoring and Observability Testing, Tools, and Frameworks
Partner Zones AWS Cloud
by AWS Developer Relations
Culture and Methodologies
Agile Career Development Methodologies Team Management
Data Engineering
AI/ML Big Data Data Databases IoT
Software Design and Architecture
Cloud Architecture Containers Integration Microservices Performance Security
Coding
Frameworks Java JavaScript Languages Tools
Testing, Deployment, and Maintenance
Deployment DevOps and CI/CD Maintenance Monitoring and Observability Testing, Tools, and Frameworks
Partner Zones
AWS Cloud
by AWS Developer Relations
Building Scalable Real-Time Apps with AstraDB and Vaadin
Register Now

Trending

  • Best Practices for Securing Infrastructure as Code (Iac) In the DevOps SDLC
  • Auto-Scaling Kinesis Data Streams Applications on Kubernetes
  • Azure Virtual Machines
  • Tech Hiring: Trends, Predictions, and Strategies for Success

Trending

  • Best Practices for Securing Infrastructure as Code (Iac) In the DevOps SDLC
  • Auto-Scaling Kinesis Data Streams Applications on Kubernetes
  • Azure Virtual Machines
  • Tech Hiring: Trends, Predictions, and Strategies for Success
  1. DZone
  2. Coding
  3. Languages
  4. Using Python, Gatttool, and BLE With Hexiwear

Using Python, Gatttool, and BLE With Hexiwear

The combination of Python scripts, gatttool, BLE, and Hexiwear lets you send and access data over your devices with a few handy scripts.

Erich Styger user avatar by
Erich Styger
·
Jan. 03, 17 · Tutorial
Like (3)
Save
Tweet
Share
17.04K Views

Join the DZone community and get the full member experience.

Join For Free

If you've been following along with this series, now I can use the data on the Hexiwear over BLE with the gatttool (see Tutorial: Hexiwear Bluetooth Low Energy Packet Sniffing with Wireshark and Tutorial: BLE Pairing the Raspberry Pi 3 Model B with Hexiwear). This article is taking things a step further and uses a Python script on Linux to access the sensor data on the BLE device:

Accessing Hexiwear Sensor Data with Python

Accessing Hexiwear sensor data with Python

This article is about accessing the Mikroelektronika Bluetooth Low Energy Hexiwear device with Python scripting from a Raspberry Pi. That way, it is possible to send and receive data over BLE and do whatever I want. The precondition is to have a working BLE connection and pairing with the Hexiwear using Bluez (see Tutorial: BLE Pairing the Raspberry Pi 3 Model B with Hexiwear). Python is a powerful scripting language and can be used for all kind of automation.

Installation

I’m using Python with the ‘pexpect’ package. First, make sure that latest packages are used:

sudo apt-get update


Next, make sure the PIP (Python Package Package) is installed:

sudo apt-get install git build-essential python-dev python-pip


Install the pexpect, a package to control other applications from Python:

sudo pip install pexpect


I’m using here the 4.2 version of the pexpect package.

Pexpect

With 'pexpect', I can spawn a process from Python and then control it like I would type in the commands manually. With pexpect , I can run the gatttool as I would type the commands on a console/terminal.

Here are the basic building blocks to access the data on the Hexiwear with it.

The following Python variable is used to store the address of the BLE device used:

DEVICE = "00:32:40:08:00:12"


I run the gatttool with the -I (interactive) option:

child = pexpect.spawn("gatttool -I")


This returns the child process handle I can use. With...

child.sendline("connect {0}".format(DEVICE))


...I send the string to the gatttool (spawned process). The format member function is used to build the formatted string: “connect 00:32:40:08:00:12” as I would have it typed in.

Next, I need to wait for the connection. For this, I use

child.expect("Connection successful", timeout=5)


Which waits for the “Connection successful” string from the gatttool. I have specified that it should timeout after 5 seconds.

To read BLE characteristics, I use

child.sendline("char-read-hnd 0x30")


The handle 0x30 is for reading the accelerometer values. The gatttool would print something like this:

Characteristic value/descriptor: 02 00 00 00 a1 ff


I’m waiting for the first part of the output:

child.expect("Characteristic value/descriptor: ", timeout=10)


Then I wait for the end of the line:

child.expect("\r\n", timeout=10)


With child.before, I get the string just before the line end. For example...

child.before[0:5]


...will return “02 00” as substring (first 4 characters) from “02 00 00 00 a1 ff” string. That way, I get substrings of x, y and z accelerometer values.

To transform the hex string (in little endian) into a signed 16-bit number, I use the following Python sub-function:

# function to transform hex string like "0a cd" into signed integer
def hexStrToInt(hexstr):
    val = int(hexstr[0:2],16) + (int(hexstr[3:5],16)<<8)
    if ((val&0x8000)==0x8000): # treat signed 16bits
        val = -((val^0xffff)+1)
    return val


The Hexiwear's accelerometer values are ‘centi-float’ values. For example, the numerical value 123 would be 1.23. With this, I can print the x, y, and z values:

print(float(hexStrToInt(child.before[0:5]))/100),
print(float(hexStrToInt(child.before[6:11]))/100),
print(float(hexStrToInt(child.before[12:17]))/100)


Python Code to Read Accelerometer, Gyro, and Magnetometer

Here is the full source code of the code discussed above:

# Using Hexiwear with Python
import pexpect
import time
 
DEVICE = "00:32:40:08:00:12"
 
print("Hexiwear address:"),
print(DEVICE)
 
# Run gatttool interactively.
print("Run gatttool...")
child = pexpect.spawn("gatttool -I")
 
# Connect to the device.
print("Connecting to "),
print(DEVICE),
child.sendline("connect {0}".format(DEVICE))
child.expect("Connection successful", timeout=5)
print(" Connected!")
 
# function to transform hex string like "0a cd" into signed integer
def hexStrToInt(hexstr):
 val = int(hexstr[0:2],16) + (int(hexstr[3:5],16)<<8)
 if ((val&0x8000)==0x8000): # treat signed 16bits
 val = -((val^0xffff)+1)
 return val
 
#while True:
# Accelerometer
child.sendline("char-read-hnd 0x30")
child.expect("Characteristic value/descriptor: ", timeout=10)
child.expect("\r\n", timeout=10)
print("Accel: "),
print(child.before),
print(float(hexStrToInt(child.before[0:5]))/100),
print(float(hexStrToInt(child.before[6:11]))/100),
print(float(hexStrToInt(child.before[12:17]))/100)
 
# Accelerometer
child.sendline("char-read-hnd 0x34")
child.expect("Characteristic value/descriptor: ", timeout=10)
child.expect("\r\n", timeout=10)
print("Gyro: "),
print(child.before),
print(float(hexStrToInt(child.before[0:5]))/100),
print(float(hexStrToInt(child.before[6:11]))/100),
print(float(hexStrToInt(child.before[12:17]))/100)
 
# Magnetometer
child.sendline("char-read-hnd 0x38")
child.expect("Characteristic value/descriptor: ", timeout=10)
child.expect("\r\n", timeout=10)
print("Magneto:"),
print(child.before),
print(hexStrToInt(child.before[0:5])),
print(hexStrToInt(child.before[6:11])),
print(hexStrToInt(child.before[12:17]))


Save the script to a file (e.g. gatttool.py) and run it with:

python gatttool.py


This produces something like this:

Hexiwear address: 00:32:40:08:00:12
Run gatttool...
Connecting to 00:32:40:08:00:12 Connected!
Accel: 03 00 ff ff a1 ff 0.03 -0.01 -0.95
Gyro: 00 00 02 00 00 00 0.0 0.02 0.0
Magneto: b8 fc e2 04 c8 28 -840 1250 10440


Instead of printing the values, I can store them to a file or whatever I would like to do.

Python Script to Update Date/Time

Below is a Python script similar to the one above, which sets the Hexiwear's current date and time using the Unix time (seconds after 1970) from the Raspberry Pi:

# Python script to set the time on the Hexiwear
import pexpect
import time
from time import gmtime, strftime
 
print("---------------------")
print("Setting linux time")
print("local time: "),
print(time.ctime())
unixTime = int(time.time())
print("secs since 1970: "),
print(int(unixTime))
print("---------------------")
 
DEVICE = "00:32:40:08:00:12"
 
# Run gatttool interactively.
print("Running gatttool...")
child = pexpect.spawn("gatttool -I")
 
# Connect to the device.
print("Connecting to"),
print(DEVICE),
child.sendline("connect {0}".format(DEVICE))
child.expect("Connection successful", timeout=5)
print("Connected!")
 
# Write local time
command = "char-write-req 61 0304{0:02x}{1:02x}{2:02x}{3:02x}0000000000000000000000000000".format(unixTime&0xff, (unixTime>>8)&0xff, (unixTime>>16)&0xff, (unixTime>>24)&0xff)
print(command)
child.sendline(command)
child.expect("Characteristic value was written successfully", timeout=10)
 
print("done!")


And this is how it looks on the terminal:

Writing Unix Time to Hexiwear

Writing Unix time to Hexiwear

Summary

Python is great for scripting things. With the pexpect Python class, I can spawn a process and then send and receive strings. I’m using this in my article to read sensor values from a BLE device connected to my Raspberry Pi.

Happy Pythoning!

Python (language)

Published at DZone with permission of Erich Styger, DZone MVB. See the original article here.

Opinions expressed by DZone contributors are their own.

Trending

  • Best Practices for Securing Infrastructure as Code (Iac) In the DevOps SDLC
  • Auto-Scaling Kinesis Data Streams Applications on Kubernetes
  • Azure Virtual Machines
  • Tech Hiring: Trends, Predictions, and Strategies for Success

Comments

Partner Resources

X

ABOUT US

  • About DZone
  • Send feedback
  • Careers
  • Sitemap

ADVERTISE

  • Advertise with DZone

CONTRIBUTE ON DZONE

  • Article Submission Guidelines
  • Become a Contributor
  • Visit the Writers' Zone

LEGAL

  • Terms of Service
  • Privacy Policy

CONTACT US

  • 600 Park Offices Drive
  • Suite 300
  • Durham, NC 27709
  • support@dzone.com

Let's be friends: