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Developing ZigBee driver with SynthOS

Posted: 01 Jul 2015 ?? ?Print Version ?Bookmark and Share

Keywords:application specific operating system? ASOS? robot? ZigBee? SynthOS?

We had a project to take an off-the-shelf robot kit, develop control algorithms, and use SynthOS to create a real-time application specific operating system (ASOS) to schedule and coordinate the various robot tasks. The requirement was to build a robot that can move around an obstacle course while avoiding walls and other objects and not getting trapped in narrow places. We also wanted it to be able to adjust its speed for the left and right tracks independently, and if anything fails, such as one of the tracks getting stuck, it should give an indication (a beep) and shut down its own power. We were happy with the results.

Figure 1: Our robot.

The second phase of the project involved adding wireless communication to the robot to enable remote communication for debug, monitoring, and control. In this article, we talk about the unique issues in the development of a ZigBee device driver in particular and with the development of drivers for communication devices in general using SynthOS.

What is ZigBee?
We chose ZigBee because it is an open standard that is used worldwide and has found significant use in the Internet of Things. ZigBee is particularly focused on the requirements of sensors for use in consumer, commercial, and industrial applications. It is supported by the ZigBee Alliance, an open, non-profit association with about 400 members driving the development of the ZigBee standards.

ZigBee is a low power, low cost wireless mesh network standard, operating in the 2.4MHz unlicensed spectrum. The raw, over the air data rate is 250 Kbit/s per channel in the 2.4GHz band, and the indoor effective transition distance is 10-20 m depending on the environment contraction materials. ZigBee devices are required to conform to IEEE 802.15.4-2003 Low-Rate Personal Area Network (LP-WPAN). This standard specifies the physical layer Media Access Control and the data link layer.

Figure 2: ZigBee OSI Model (courtesy of The ZigBee Alliance).

The particular implementation of ZigBee that we chose is XBee, which is the brand name from Digi International for a family of form-factor -ompatible radio modules based on the 802.15.4-2003 standard designed for point-to-point and star communications at over-the-air baud rates of 250 Kbit/s. We chose XBee because of the low cost and ease of ordering from Digi. The particular module that we chose was the XBee ZB low power ZigBee Module with integrated wire antenna, product #XB24-Z7WIT-004, shown in figure 3.

Figure 3: XBee XB24-Z7WIT-004 from Digi.

The hardware platform
The hardware platform for the project was an off-the-shelf DFRobot Rover v2 kit from RobotShop Distribution Inc. The Rover uses a Tamiya twin motor gearbox and the Tamiya track and wheel set. An on-board buzzer that can generate a variety of tones is used for simple notifications such as failure modes. A rechargeable 3.7V LiPo battery is included and can be recharged using the onboard charger.

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