The Cortex Microcontroller Software Interface Standard (CMSIS) is a vendor-independent hardware abstraction layer for the Cortex-M processor series and defines generic tool interfaces. The CMSIS enables consistent device support and simple software interfaces to the processor and the peripherals, simplifying software re-use, reducing the learning curve for microcontroller developers, and reducing the time to market for new devices.

The CMSIS is defined in close cooperation with various silicon and software vendors and provides a common approach to interface to peripherals, real-time operating systems, and middleware components. The CMSIS is intended to enable the combination of software components from multiple middleware vendors.

The CMSIS components are:

CMSIS-CORE: API for the Cortex-M processor core and peripherals. It provides at standardized interface for Cortex-M0, Cortex-M3, Cortex-M4, Cortex-M7, SC000, and SC300. Included are also SIMD intrinsic functions for Cortex-M4 and Cortex-M7 SIMD instructions.

CMSIS-Driver: defines generic peripheral driver interfaces for middleware making it reusable across supported devices. The API is RTOS independent and connects microcontroller peripherals with middleware that implements for example communication stacks, file systems, or graphic user interfaces.

CMSIS-DSP: DSP Library Collection with over 60 Functions for various data types: fix-point (fractional q7, q15, q31) and single precision floating-point (32-bit). The library is available for all Cortex-M cores. The Cortex-M4 and Cortex-M7 implementations are optimized for the SIMD instruction set.

CMSIS-RTOS API: Common API for Real-Time operating systems. It provides a standardized programming interface that is portable to many RTOS and enables therefore software templates, middleware, libraries, and other components that can work across supported the RTOS systems.

CMSIS-Pack: describes with a XML based package description (PDSC) file the user and device relevant parts of a file collection (called software pack) that includes source, header, and library files, documentation, Flash programming algorithms, source code templates, and example projects. Development tools and web infrastructures use the PDSC file to extract device parameters, software components, and evaluation board configurations.

CMSIS-SVD: System View Description for Peripherals. Describes the peripherals of a device in an XML file and can be used to create peripheral awareness in debuggers or header files with peripheral register and interrupt definitions.

CMSIS-DAP: Debug Access Port. Standardized firmware for a Debug Unit that connects to the CoreSight Debug Access Port. CMSIS-DAP is distributed as separate package and well suited for integration on evaluation boards. This component is provided as separate download.

Note: Refer to ARM::CMSIS Pack for more information on the content of the Software Pack.

CMSIS Structure

Motivation

CMSIS has been created to help the industry in standardization. It enables consistent software layers and device support across a wide range of development tools and microcontrollers. CMSIS is not a huge software layer that introduces overhead and does not define standard peripherals. The silicon industry can therefore support the wide variations of Cortex-M processor-based devices with this common standard.

In detail the benefits of the CMSIS are:

Overall CMSIS reduces the learning curve, development costs, and time-to-market. Developers can write software quicker through a variety of easy-to-use, standardized software interfaces.
Consistent software interfaces improve the software portability and re-usability. Generic software libraries and interfaces provide consistent software framework.
Provides interfaces for debug connectivity, debug peripheral views, software delivery, and device support to reduce time-to-market for new microcontroller deployment.
Provides a compiler independent layer that allows using different compilers. CMSIS is supported by all mainstream compilers (ARMCC, IAR, and GNU).
Enhances program debugging with peripheral information for debuggers and ITM channels for printf-style output and RTOS kernel awareness.
CMSIS is delivered in CMSIS-Pack format which enables fast software delivery, simplifies updates, and enables consistent integration into development tools.

Coding Rules

The CMSIS uses the following essential coding rules and conventions:

Compliant with ANSI C and C++.
Uses ANSI C standard data types defined in <stdint.h>.
Variables and parameters have a complete data type.
Expressions for #define constants are enclosed in parenthesis.
Conforms to MISRA 2004. MISRA rule violations are documented.

In addition, the CMSIS recommends the following conventions for identifiers:

CAPITAL names to identify Core Registers, Peripheral Registers, and CPU Instructions.
CamelCase names to identify function names and interrupt functions.
Namespace_ prefixes avoid clashes with user identifiers and provide functional groups (i.e. for peripherals, RTOS, or DSP Library).

The CMSIS is documented within the source files with:

Comments that use the C or C++ style.
Doxygen compliant function comments that provide:
- brief function overview.
- detailed description of the function.
- detailed parameter explanation.
- detailed information about return values.

Doxygen comment example:

/** 
 * @brief  Enable Interrupt in NVIC Interrupt Controller
 * @param  IRQn  interrupt number that specifies the interrupt
 * @return none.
 * Enable the specified interrupt in the NVIC Interrupt Controller.
 * Other settings of the interrupt such as priority are not affected.
 */

Licence

The CMSIS is provided free of charge by ARM and can be used for all Cortex-M based devices.

The software portions that are deployed in the application program are under a BSD license which allows usage of CMSIS in any commercial or open source projects.

View the LICENCE AGREEMENT for CMSIS in detail.

ARM::CMSIS Pack

The ARM::CMSIS Pack contains the following:

File/Directory	Content
ARM.CMSIS.pdsc	Package description file
CMSIS	CMSIS components (see below)
CMSIS_RTX	Keil RTX implementation of CMSIS-RTOS
Device	ARM reference implementations of Cortex-M devices

CMSIS Directory

The directory CMSIS contains the "CMSIS End User License Agreement" as PDF and RTF, a README text file, the index.html file for this documentation, and the following sub-directories:

Directory	Content
Documentation	This documentation
DAP	CMSIS-DAP Debug Access Port source code and reference implementations
Driver	Header files for the CMSIS-Driver peripheral interface API
DSP_Lib	CMSIS-DSP software library source code
Include	Include files for CMSIS-CORE and CMSIS-DSP
Lib	CMSIS-DSP generated libraries for ARMCC and GCC
Pack	CMSIS-Pack example
RTOS	CMSIS-RTOS API header file
SVD	CMSIS-SVD example
UserCodeTemplates\ARM	ITM_Retarget.c, CMSIS retarget output to ITM Channel 0 template file
Utilities	PACK.xsd (CMSIS-Pack schema file), PackChk.exe (checking tool for software packs), CMSIS-SVD.xsd (CMSIS-SVD schema file), SVDConv.exe (conversion tool for SVD files)