What is delivered in a Cortex-M processor?

Cortex-M processors from ARM are 32-bit reduced instruction set computing (RISC) processors optimized for embedded applications. They provide a balance of performance, power efficiency, and cost effectiveness. The key components delivered in a Cortex-M processor are the CPU core, memory interfaces, peripherals, debug/trace capabilities, and power management.

CPU Core

The CPU core implements the ARMv6-M, ARMv7-M, or ARMv8-M architecture instruction sets. It contains a register bank, arithmetic logic unit (ALU), multiplier, barrel shifter, instruction pipeline, and memory management unit. Key features include:

• 3-stage pipeline for efficient execution of instructions
• Thumb-2 instruction set with both 16-bit and 32-bit instructions
• Hardware multiply and divide support
• Single-cycle fast integer math support
• Low interrupt latency
• Deterministic interrupt processing times
• Banked stack pointers for fast, low overhead context switches

The CPU core executes the program code and instructions to carry out the desired functionality and applications. Cortex-M cores range from the ultra-low power Cortex-M0/M0+ to the higher performance Cortex-M4/M7 cores.

Memory Interfaces

Cortex-M processors include various memory interfaces to connect with different types of memory and peripherals:

• Bus Matrix: Connects the CPU core to memory and peripherals using an AHB-Lite bus.
• Flash Memory Interface: Provides interface to external flash memory for code storage.
• SRAM Interface: Interface to fast on-chip or off-chip SRAM for data storage.
• ROM Interface: Interface to mask ROM for boot code.
• BIT Interface: Interface to execute code directly from flash memory.
• ETB Interface: Interface to trace memory buffer.
• SWD/JTAG Interface: Interface for debug access and flash programming.

These interfaces enable the processor to access necessary code and data storage as well as connect with peripherals and debugging tools.

Peripherals

Cortex-M processors integrate various peripherals to enable connectivity and I/O operations:

• Timer/Counters: General purpose timer modules with capture, compare, and PWM capabilities.
• Real-Time Clock (RTC): Clock module to maintain time and date.
• Watchdog Timer: Resets the system if software fails to periodically service the timer.
• Nested Vectored Interrupt Controller (NVIC): Handles prioritized interrupt requests.
• SysTick Timer: 24-bit system tick timer and interrupt.
• Serial Wire (SW): Serial wire viewer and debugger interface.
• Serial Peripheral Interface (SPI): Synchronous serial interface for communicating with peripherals.
• I2C Interface: Two-wire serial interface for sensors and slow peripherals.
• Universal Synchronous/Asynchronous Receiver/Transmitter (USART): Asynchronous serial interface for communications.
• Analog-to-Digital Converter (ADC): Converts analog inputs to digital values.

These integrated peripherals enable connectivity, timing, debugging, communications, and analog interfacing without needing external components.

Debug and Trace

Cortex-M processors provide debugging and tracing capabilities to assist with testing and validating software:

• Debug Access Port (DAP): Debug port that enables halt, stepping, breakpoint, and watchpoint capabilities.
• Instrumentation Trace Macrocell (ITM): Outputs trace data and printf logging information.
• Embedded Trace Macrocell (ETM): Instruction and program flow tracing to off-chip trace buffer.
• Micro Trace Buffer (MTB): On-chip trace buffer that captures trace and profiling data.
• Data Watchpoint and Trace (DWT): Watchpoint and logging capabilities.
• Flash Patch and Breakpoint (FPB): In-place code patch and breakpoint setting.

These features enable non-intrusive debugging of software without impacting real-time behavior. Developers can track down bugs, analyze code flow, and optimize performance.

Power Management

Cortex-M processors provide various power saving capabilities:

• Wait for Interrupt (WFI): Places processor into low power idle state.
• Wait for Event (WFE): Places processor into low power idle state until event occurs.
• Sleep and Deep Sleep: Stops CPU clock while retaining main clock and peripherals.
• Voltage Scaling: Operates CPU and peripherals at lower voltages to save power.
• Clock Gating: Disables clocks to unused modules to reduce power.

Using these features allows Cortex-M processors to minimize power consumption in embedded battery-powered applications.

Conclusion

In summary, Cortex-M processors deliver a RISC CPU core, flexible memory interfaces, common peripherals, advanced debugging capabilities, and power management. This combination enables Cortex-M processors to meet the performance, power, cost, and ease-of-use requirements of a wide range of embedded applications in the automotive, industrial, consumer electronics, and IoT markets.