The key differences between the ARM Cortex-M3 and Cortex-M4 microcontrollers are in their core designs, with the M4 having additional features for digital signal processing (DSP) and floating point arithmetic. The Cortex-M4 also supports more interrupts and has a higher clock speed.
Overview of ARM Cortex-M3
The ARM Cortex-M3 is a 32-bit RISC processor core licensed by ARM Holdings. It is part of the Cortex-M series microcontrollers designed for embedded and IoT applications where high efficiency and low power consumption are required. The M3 core has a 3-stage pipeline and uses the ARMv7-M architecture.
Key features of the Cortex-M3 core include:
- 32-bit RISC architecture with Thumb-2 instruction set
- 3-stage pipeline to achieve high performance at low power
- Operates at frequencies up to 150 MHz
- Memory Protection Unit (MPU) for system protection
- Nested Vectored Interrupt Controller (NVIC) with up to 240 interrupts
- Wake-up Interrupt Controller (WIC) for low power operation
- Single-cycle digital signal processing (DSP) instructions
- Embedded Trace Macrocell (ETM) for program tracing
The Cortex-M3 is widely used in various embedded systems including automotive, industrial, home automation, and consumer devices. It achieves high efficiency through its Thumb-2 instruction set while minimizing power consumption.
Overview of ARM Cortex-M4
The ARM Cortex-M4 is also a 32-bit RISC core with the Thumb-2 instruction set. It is an enhanced version of the Cortex-M3 with additional features for digital signal processing and floating point arithmetic. The M4 core has a 3-stage pipeline and uses the ARMv7E-M architecture.
Key features of the Cortex-M4 core include:
- DSP instructions for improved digital signal processing
- Single-precision floating point unit (FPU)
- Optional Double-precision FPU
- Memory Protection Unit (MPU)
- Wake-up Interrupt Controller (WIC)
- Nested Vectored Interrupt Controller (NVIC) with up to 496 interrupts
- Embedded Trace Macrocell (ETM)
- Operates at frequencies up to 150 MHz
The addition of the FPU and enhanced DSP instructions allow the Cortex-M4 to take on more complex math-intensive tasks compared to the M3. It is commonly used for applications such as motor control, industrial automation, and audio processing.
Key Differences Between Cortex-M3 and Cortex-M4
While the M3 and M4 cores share some common features, here are some of the key differences between the two:
DSP Instructions
The Cortex-M4 contains enhanced digital signal processing (DSP) instructions to handle more complex math-intensive code. This includes saturated arithmetic, SIMD arithmetic, and enhanced bit manipulation capabilities. The M3 has basic DSP instructions for MAC operations.
Floating Point Unit
The M4 incorporates either a single-precision or double-precision floating point unit (FPU) to handle non-integer math. The M3 does not have a dedicated FPU, requiring software routines for floating point math.
Interrupts
The NVIC on the M4 supports up to 496 external interrupts compared to 240 on the M3. This allows the M4 to handle more real-time events.
Core Clock Speed
Both cores can run at up to 150 MHz. However, the M4 achieves this while executing more instructions per cycle than the M3 due to its dual-issue superscalar pipeline.
Power Consumption
The additional features of the M4 generally require more power compared to the simpler M3. However, the M4 includes some power-saving options such as clock gating and Wake-up Interrupt Controller.
Core Architecture
The M3 uses the ARMv7-M architecture while the M4 uses the enhanced ARMv7E-M architecture with additional DSP instructions.
Cost
The M4 costs more per unit than the M3 given its more advanced core design with the FPU and extra interrupts.
Performance Comparison
Here is an overview performance comparison between the Cortex-M3 and Cortex-M4:
| Feature | Cortex-M3 | Cortex-M4 |
| Max Clock Speed | 150 MHz | 150 MHz |
| DSP Instructions | Basic | Enhanced |
| Floating Point Unit | No | Single or Double Precision |
| Max Interrupts | 240 | 496 |
| Core Architecture | ARMv7-M | ARMv7E-M |
| Power Consumption | Lower | Higher |
| Manufacturing Cost | Lower | Higher |
Based on the table, the Cortex-M4 outperforms the M3 when it comes to advanced DSP and math processing thanks to its floating point unit and expanded instruction set. The M4 can also handle about twice as many real-time interrupts as the M3. However, the M3 maintains an advantage in simpler low-power applications where floating point and intensive signal processing are not required.
Sample Applications
Here are some example applications where each core excels:
Cortex-M3
- Low-power sensor devices
- Basic motor control
- Home appliances
- IoT edge nodes
- Wearable health trackers
Cortex-M4
- Advanced motor control
- Industrial automation
- Smart grid and AMI systems
- Audio processing
- Computer vision
For simple embedded devices like sensors and basic actuators, the Cortex-M3 provides enough performance at very low power. For systems requiring complex signal analysis, math processing, and real-time control, the Cortex-M4 is better equipped.
Conclusion
The ARM Cortex-M3 and Cortex-M4 are both highly efficient 32-bit RISC cores designed for embedded systems. The M4 builds on the compact M3 design by adding DSP capabilities, single and double precision floating point, and more interrupts.
For minimal power consumption, the M3 is hard to beat. It excels at basic tasks like reading sensors, handling I/O, and monitoring devices. When advanced math performance and signal processing are critical, the M4’s DSP and FPU give it a significant performance boost.
Both offer excellent performance per watt in the microcontroller space. The choice between M3 and M4 depends on the specific requirements of the target application and how much cost can be tolerated.
