The Cortex-M4 and Cortex-M7 are both ARM processor cores designed for embedded and IoT applications. The key differences between them come down to performance, features, and intended use cases.
In a nutshell, the Cortex-M7 is more powerful and has more advanced features than the Cortex-M4. It is geared towards more computationally demanding applications like automotive ADAS, industrial control, and home automation. The Cortex-M4 strikes a balance between performance and power efficiency for cost-sensitive embedded applications.
Architecture and Performance
The Cortex-M4 core has a 3-stage pipeline and uses the ARMv7-M architecture. It can execute most instructions in a single cycle, delivering 1.25 DMIPS/MHz. With a max clock speed of 300 MHz, the peak performance is 300 DMIPS.
The Cortex-M7 implements the ARMv7E-M architecture and has a dual-issue superscalar pipeline. It can dispatch two instructions per cycle for higher throughput. The M7 delivers 2.14 DMIPS/MHz and can go up to 1 GHz clock speed for a peak of 1000+ DMIPS.
In essence, the M7 architecture allows greater instruction-level parallelism and frequency scaling for around 3-4X higher performance than the M4. It includes a microcontroller optimized floating point unit that supports single and double precision operations.
Memory System
Both cores support Harvard architecture with separate instruction and data buses. This allows simultaneous access to program memory and data memory.
The M4 has a 3-stage pipeline, so it needs an instruction buffer of 3 words during execution. The M7’s deeper 5-stage dual-issue pipeline requires an instruction buffer size of 10 words.
For data access, the M4 has a single load/store unit while the M7 has two. The M7 can do two data transfers per cycle to keep the pipelines filled. This necessitates inclusion of a data prefetch buffer and tightly coupled memory subsystem.
As such, the M7 has larger instruction and data caches than the M4 to feed its higher bandwidth pipelines. The M7 also includes memory protection units and hardware page table walking not found on the M4.
Peripheral Set
Both the Cortex-M4 and M7 provide an extensive set of peripherals for interfacing with on-chip and external devices. This includes timers, PWM, ADC, DAC, external bus interface, and various serial interfaces like UART, SPI, I2C, CAN, USB.
The M7 differentiates with a higher number of peripherals and interfaces. For example, it has up to 32 serial communications interfaces versus 12 on M4. Other additional capabilities include true random number generator, hash functions, cyclic redundancy check (CRC), cryptographic extensions, and described instruction support.
Power Management
The Cortex-M4 implements basic power management techniques like an optional sleep mode, dynamic voltage scaling, and power gating of unused blocks.
The M7 core has more advanced low-power features like independent clock gating of all the pipeline stages. This allows finer-grained control to gate off unused parts of the pipeline. The M7 also adds a data retention sleep mode allowing fast wakeup while retaining registers and SRAM state.
In general, the M4 is more power efficient for a given performance level owing to its simpler pipeline. But the M7 implements the latest low-power techniques to maximize efficiency.
Supported Toolchain
For software development, both the Cortex-M4 and M7 are supported by ARM’s Keil MDK toolkit featuring μVision IDE and debugger, compiler, linker, libraries, and code analysis tools.
They can be programmed using C/C++ or assembly language. The M7’s architectural enhancements like dual-issue pipelines require advanced code generation optimizations to maximize performance. So an M7-enhanced compiler is needed compared to a standard one for M4.
Use Cases
The Cortex-M4 hits the sweet spot between cost, power efficiency, and performance for mainstream embedded applications like IoT edge nodes, wearables, home audio products, robotics controllers, industrial automation, etc.
With its higher CPU throughput, memory bandwidth, advanced peripherals, and functional safety features, the Cortex-M7 is geared for more demanding workloads in applications like:
- Automotive ADAS, infotainment
- Industrial HMIs, servo control
- Medical imaging, diagnostics
- Communications infrastructure
- High-end IoT products
The Cortex-M7 is commonly found in microcontroller products ranging from 100 MHz to 300 MHz max clock frequency. The M4 product range spans 50 MHz to 300 MHz devices.
Conclusion
In summary, the Cortex-M7 is the higher performance and more advanced core with dual-issue superscalar pipeline, floating point unit, larger caches, memory protection, and extensive peripherals. It is suited for applications requiring substantial computing capability.
The Cortex-M4 offers a robust feature set focused on power efficiency and cost effectiveness rather than peak performance. With its solid versatility, the M4 remains a popular choice for mainstream embedded systems.
