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Cellular Interface

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Introduction

The Cellular Interface library exposes the capability of a few popular cellular modems through a uniform API. Currently, this project contains libraries for these three cellular modems:

The current version of the Cellular Interface library encapsulates the TCP stack offered by those cellular modems. They all implement the same uniform Cellular Interface library API. That API hides the complexity of AT commands, and exposes a socket-like interface to C programmers.

Even though applications can choose to use the Cellular Interface library API directly, the API is not designed for such a purpose. In a typical FreeRTOS system, applications use higher level libraries, such as the coreMQTT library and the coreHTTP library, to communicate with other end points. These higher level libraries use an abstract interface, the Transport Interface, to send and receive data. A Transport Interface can be implemented on top of the Cellular Interface library. The FreeRTOS Cellular Demo project uses such an implementation.

Most cellular modems implement more or less of the AT commands defined by the 3GPP TS v27.007 standard. This project provides an implementation of such standard AT commands in a reusable common component. The three Cellular Interface libraries in this project all take advantage of that common code. The library for each modem only implements the vendor-specific AT commands, then exposes the complete Cellular Interface API.

The common component that implements the 3GPP TS v27.007 standard has been written in compliance with the following code quality criteria:

  • GNU Complexity scores are not over 8
  • MISRA C:2012 coding standard. Any deviations from the standard are documented in source code comments marked by "coverity".

Getting Started

Download the source code

The source code can be downloaded from the FreeRTOS libraries or by itself.

To clone from Github using HTTPS:


git clone https://github.com/FreeRTOS/FreeRTOS-Cellular-Interface.git
Using SSH:

git clone git@github.com:FreeRTOS/FreeRTOS-Cellular-Interface.git

Folder structure

At the root of this repository are these folders:

  • source: reusable common code that implements the standard AT commands defined by 3GPP TS v27.007
  • modules : vendor-specific code that implements non-3GPP AT commands for each cellular modem
  • doc : documentation
  • test: unit test and cbmc
  • tools: tools for Coverity static analysis and CMock

Configure and build the Library

The Cellular Interface library should be built as part of an application. In order to do so, certain configurations must be provided. The FreeRTOS Cellular Demo project provides an example of how to configure the build. More information can be found in the Cellular Interface API Reference.

Please refer to the Cellular Interface Demo (Mutual Authentication) for more information.

Integrate the Cellular Interface Library with MCU platforms

The Cellular Interface library runs on MCUs that use an abstracted interface, the Comm Interface, to communicate with cellular modems. A Comm Interface must be implemented on the MCU platform as well. The most common implementations of the Comm Interface work over UART hardware, but they can be implemented over other physical interfaces, such as SPI, as well. The documentation for the Comm Interface can be found in the Cellular API Reference. These are example implementations of the Comm Interface:

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