Is CMSIS a library?

Yes, CMSIS stands for Cortex Microcontroller Software Interface Standard and it is a vendor-independent hardware abstraction layer for microcontrollers based on Arm Cortex processors. So CMSIS is essentially a library that provides a standard way to access the hardware features of Cortex-M processor-based microcontrollers from various Arm chip vendors.

What is CMSIS?

CMSIS defines interfaces to peripheral devices, real-time operating systems, and middleware components. It enables software re-use across different microcontroller products from various silicon vendors. The key goals of CMSIS include:

• Provide a common way to access processor features and capabilities
• Enable code re-use across devices from different vendors
• Simplify software development process for embedded applications
• Allow middle-ware components to work across various hardware

At its core, CMSIS provides C language headers with common interfaces to peripherals, registers, interrupts, and core features. It also includes optimized interrupt service routines, DSP libraries, and other utility components. The CMSIS specifications are developed and maintained by Arm in collaboration with silicon and software vendors.

CMSIS Components

The main components provided by CMSIS include:

• CMSIS-Core: Defines core registers, intrinsic functions, data types, and common core peripherals like NVIC, SysTick, and debug access.
• CMSIS-RTOS: Specifies interfaces for real-time OS (RTOS) support.
• CMSIS-DSP: Provides DSP algorithms optimized for Cortex-M processors.
• CMSIS-Driver: Defines common software driver interfaces for peripherals.
• CMSIS-SVD: System view description to auto-generate peripheral register headers.
• CMSIS-Pack: Package manager for software components.

How does CMSIS work?

CMSIS provides a hardware abstraction layer that hides the differences between various Arm Cortex-M microcontrollers. This allows software developers to access the underlying hardware using a consistent and standard interface. Here is how it works:

• Chip vendors provide CMSIS compliant device header files that define peripheral registers, interrupts, capabilities for their microcontrollers.
• Software projects include the CMSIS headers for the target device to access its features.
• The application code uses CMSIS APIs instead of vendor specific register access.
• CMSIS takes care of low-level hardware interactions. So software is shielded from device differences.

This approach simplifies software development and makes the code portable across devices from different vendors. Developers don’t need to change their application code when migrating to new microcontroller hardware.

Key Benefits of using CMSIS

Here are some of the major benefits provided by CMSIS:

• Hardware abstraction: Hides hardware differences between Cortex-M devices from multiple vendors.
• Code reuse: Software can be reused across compatible devices from different silicon vendors.
• Portability: Simplifies migration to new microcontroller products.
• Vendor independence: Avoids vendor lock-in, code can be reused across vendor devices.
• Speeds development: Reduces software development time by using standardized interfaces.
• Common tools: Enables using common development tools across vendor devices.
• Middleware support: Middleware and RTOSes can work across compliant devices.
• Ecosystem: Broad ecosystem of CMSIS compliant components from multiple vendors.

Who uses CMSIS?

CMSIS is widely used by various stakeholders in the Cortex-M ecosystem:

• Chip vendors – Provide CMSIS compliant device support for their microcontrollers.
• Software developers – Use it to abstract hardware differences and achieve portability.
• Middleware vendors – Enable their middleware stacks to work across CMSIS devices.
• Tool vendors – Integrate CMSIS support into their development tools.
• OEMs and Product companies – Simplify software development and maintenance.

Many leading microcontroller companies including STMicroelectronics, NXP, Microchip, Renesas, Cypress Semiconductor, Infineon, etc. provide CMSIS support for their Cortex-M based products. It is widely used by embedded system designers across various industries.

How to use CMSIS in a project?

Using CMSIS in an embedded software project is straightforward:

1. Select a CMSIS compliant target microcontroller.
2. Get the vendor provided CMSIS device headers for that target device.
3. Include the CMSIS headers in your project.
4. Use CMSIS defined interfaces to access hardware features.
5. Link necessary CMSIS libraries like Startup code, System files, Device Peripheral Access Layer.

Many IDEs and toolchains like IAR, Keil, GCC have inbuilt support for integrating CMSIS into projects. It takes care of including the right headers and libraries during build. Refer to toolchain documentation for details.

Evolution of CMSIS

The CMSIS specifications have evolved over the years with ARM adding more features and components:

• CMSIS 1.0 – Initial release in 2004. Focused on Cortex-M3.
• CMSIS 2.0 – Added support for Cortex-M0/M1, RTOS, DSP in 2010.
• CMSIS 3.0 – Introduced SVD for automatic peripheral register access in 2012.
• CMSIS 4.0 – Added Pack for software components, MISRA-C support in 2014.
• CMSIS 5.0 – Renewed focus on security, safety, and online resources in 2017.
• CMSIS 5.7 – Latest release in 2020. Includes Crypto, RTOS2, and compliance test suite.

With each release, CMSIS has expanded its scope with new components, broader device support, and alignment with latest standards. The evolution continues to meet the needs of the growing Arm Cortex-M ecosystem.

Limitations of CMSIS

While CMSIS offers several benefits, it also has some limitations to be aware of:

• Cannot completely abstract away hardware differences between devices.
• Uneven vendor support and compliance quality for CMSIS components.
• Header complexity increases with more configurable peripherals.
• May not optimize for specialized application performance needs.
• Requires chip vendors to provide and maintain CMSIS compliant device support.

So CMSIS may not be suitable for very low level hardware dependent firmware, or extremely optimized applications. For most embedded projects using Cortex-M microcontrollers, the pros far outweigh the cons.

Conclusion

CMSIS provides a vendor-independent hardware abstraction layer for Cortex-M devices that promotes software portability and reuse. By using consistent interfaces to processor and peripherals, it insulates software from underlying hardware differences. This simplifies development, reduces vendor lock-in, and enables middleware components to work across devices. CMSIS adoption continues to grow with support from leading silicon and toolchain vendors in the Arm ecosystem.