The Cortex-M4 is a high-performance microcontroller from ARM that is part of their Cortex-M series. It features a 32-bit ARM Cortex-M4 CPU core with single-cycle digital signal processing (DSP) capabilities and floating point unit (FPU).
The Cortex-M4 CPU core can be run at a wide range of frequencies depending on the specific implementation. The maximum clock frequency of the Cortex-M4 core itself is unlimited in theory, but in practice it depends on the silicon process, voltage, temperature, and other design factors.
Typical Frequency Range
For commercial Cortex-M4 microcontrollers, the typical frequency range is from 50 MHz to 300 MHz. However, some lower and higher frequency parts exist:
- Lower end around 30-50 MHz for ultra low power or cost optimized applications.
- Higher end reaching 400-500 MHz for performance demanding applications.
The most common frequencies tend to be in the 100-216 MHz range for mainstream applications requiring good performance but not extremely high clock speeds.
Factors Affecting Maximum Frequency
There are several factors that affect the maximum achievable frequency in a Cortex-M4 implementation:
- Semiconductor process – Smaller transistor geometries allow higher frequencies. E.g. 40nm vs 180nm.
- Operating voltage – Higher voltage levels enable higher frequencies.
- Temperature range – Wider operating temperatures require lower frequencies.
- Packaging – Smaller, lower pin count packages reduce max frequencies.
- Features – Additional features like DSP instructions reduce max speed.
- Design complexity – More complex systems with more logic slow down max speed.
Chip designers optimize the Cortex-M4 implementation’s frequency according to the target application requirements and constraints.
Typical Frequency Ranges of Popular Cortex-M4 Microcontrollers
Here are some examples of typical frequencies for popular Cortex-M4 microcontroller variants from major vendors:
- STM32F4xx – Up to 180 MHz
- STM32L4xx – Up to 80 MHz
- Kinetis K2x – Up to 120 MHz
- TI Tiva TM4C12x – Up to 120 MHz
- NXP LPC40xx – Up to 120 MHz
- Microchip SAM4L – Up to 96 MHz
- Cypress PSoC 4 – Up to 100 MHz
Higher end Cortex-M4 micros designed for performance applications include:
- STM32F7xx – Up to 216 MHz
- NXP i.MX RT10xx – Up to 600 MHz
Adjusting Frequency
The clock frequency of a Cortex-M4 MCU is typically configurable and adjustable. Lower frequencies are used to reduce power consumption, while higher frequencies enable higher performance.
Frequency scaling allows dynamic adjustment of the clock speed during runtime according to the performance needs and power constraints of the application. This is done using the microcontroller’s clock generation and distribution system.
Developers need to balance frequency against factors like power usage, heat generation, and system performance when selecting a clock speed for their Cortex-M4 application.
Oscillator Sources
The reference clock for the Cortex-M4 CPU is provided by an oscillator source like a crystal or ceramic resonator. Common oscillator frequencies include:
- 8 MHz
- 12 MHz
- 16 MHz
- 24 MHz
The oscillator frequency is fed into a phase-locked loop (PLL) circuit to generate higher frequencies for the CPU core clock. Many Cortex-M4 microcontrollers allow selecting between multiple PLL output frequencies.
Instruction Frequency vs Bus Frequency
For Cortex-M4 MCUs, it is common for the instruction core frequency to run faster than the bus frequency. For example, STM32F407 MCUs can run the core at 168 MHz while the bus frequency is limited to 50 MHz.
This allows the CPU to execute instructions rapidly without being limited by the speed of external memory accesses. The bus clock (AHB/APB) is derived separately using prescalers to lower the bus frequency relative to the system clock.
DSP and FPU Effects
The Cortex-M4 instruction set includes DSP and floating point (FPU) instructions that require multi-cycle execution and thus reduce the maximum achievable clock frequency compared to simpler cores like Cortex-M0/M3.
For example, the Cortex-M4F, which includes the FPU, has a lower max frequency compared to the Cortex-M4 core without FPU. So engineers balance frequency vs features when selecting the M4 variant.
Tools for Measuring Frequency
Frequencies can be measured on physical Cortex-M4 boards using techniques like:
- Oscilloscope – Probe clock signals
- Logic analyzer – Capture clock waveforms
- Microcontroller debug module – Software profiling
- Frequency counter – Direct frequency measurement
This allows verifying that the microcontroller is running at the expected CPU and bus frequencies during operation.
Summary
In summary, the Cortex-M4 CPU frequency range spans 30 MHz to over 500 MHz depending on the implementation, with 100-216 MHz being most common. Factors like semiconductor process, voltage, temperature range, packaging, and included features constrain the max frequency. Developers can tune frequency to balance performance and power for the target application.
The Cortex-M4 provides a powerful CPU architecture whose speed can be optimized for a wide variety of embedded systems and IoT applications.
