The Cortex-M and Cortex-M4 are both ARM processor cores designed for embedded and IoT applications. The main difference is that Cortex-M4 has more advanced features like digital signal processing (DSP) instructions, floating point unit (FPU), and memory protection unit (MPU).
Overview of Cortex-M
The Cortex-M processor cores are 32-bit RISC CPUs optimized for low-power embedded applications. Cortex-M cores are designed to be very energy efficient while still providing good performance for embedded workloads.
Some key attributes of Cortex-M cores:
- In-order execution pipeline for simple logic and low power
- Thumb-2 instruction set combines high code density with 32-bit performance
- NVIC for flexible interrupt handling
- Microcontroller features like SYSTICK timer, debug access port
- Memory Protection Unit (MPU) in some variants
- Wake up from sleep in less than 10us
Cortex-M cores are extensively used in various microcontroller units (MCUs) targeting home automation, industrial control, medical devices, consumer electronics and IoT edge nodes.
Cortex-M4 Enhancements
The Cortex-M4 builds on the strengths of Cortex-M3 and adds several enhancements:
- Floating Point Unit (FPU) – Hardware FPU improves floating point performance and efficiency. Allows use of DSP algorithms.
- Digital Signal Processing (DSP) Instructions – Special instructions to accelerate DSP workloads like filters, transforms, etc.
- Memory Protection Unit (MPU) – Hardware MPU allows creating isolated protected memory regions to implement privilege separation.
- ETM Trace – Embedded trace macrocell provides instruction trace capabilities for debugging.
- Low Latency – Cortex-M4 includes latency reducing techniques to improve real-time performance.
DSP Capabilities
A key difference between Cortex-M and Cortex-M4 is the dedicated DSP capabilities in M4. This allows substantial performance improvements on signal processing and math intensive code.
The DSP extension in Cortex-M4 provides:
- SIMD Instructions – Single instruction multiple data instructions can perform dual 16-bit operations in one cycle.
- Saturating Arithmetic – Saturating add/sub instructions clamp results instead of overflowing.
- Hardware Divide – Low latency divide instructions improve efficiency.
- Accelerated Multiply – Multiply accumulates and parallel multiply instructions.
By utilizing the DSP instructions, applications like digital filters, transforms, PID controls, and math functions can have much better performance.
Floating Point Unit
The FPU in Cortex-M4 provides hardware acceleration for floating point arithmetic. The FPU implements the ARMv7E-M floating point architecture.
Key features of the FPU:
- IEEE-754 compliant for precision.
- Supports 32-bit single precision and 64-bit double precision.
- Low latency floating point instructions.
- Accumulator based design avoids register spillage.
- Uses little energy per operation.
With the FPU, algorithms using trigonometric functions, matrix math, filters, PID controls, etc can leverage floating point capability.
Memory Protection Unit
The Memory Protection Unit (MPU) in Cortex-M4 enables creation of isolated memory regions for privilege separation.
Key capabilities of MPU:
- Up to 8 protected memory regions.
- configurable as code or data regions.
- Programmable access permissions (read/write/execute) for each region.
- Enforce privilege separation between privileged and user code.
MPU allows safe execution of less trusted user code in RTOS and real-time applications. This improves robustness and security.
Development Tools
Cortex-M4 supports the same development tools as Cortex-M cores. This includes:
- Compilers – GCC, Arm Compiler, IAR
- Debuggers – JTAG, SWD
- IDEs – Keil MDK, IAR EWARM, Arm MCU DesignStart
- RTOS – FreeRTOS, Mbed OS, Amazon FreeRTOS
- Emulators – Segger J-Link, ST-LINK
The consistency in tools makes it easy to migrate from Cortex-M to Cortex-M4.
Use Cases
Some common use cases where Cortex-M4 microcontrollers are used:
- Industrial – Motor controls, power conversion, servo controls.
- Automotive – ADAS, infotainment, diagnostics.
- Consumer – Digital cameras, wearables, smart home.
- Medical – Patient monitoring, medical imaging, prosthetics.
- IoT – Predictive maintenance, asset tracking, smart city.
Cortex-M4 is well suited for applications that need digital signal processing along with time critical response and low power operation.
Summary
In summary, Cortex-M4 builds on Cortex-M3 and provides DSP, FPU and MPU enhancements. This enables higher performance on signal processing, math intensive and real-time safety critical applications. The consistent toolchain makes it easy to leverage Cortex-M4 capabilities.
