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.
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.
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.
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.
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.
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.