The ARM Cortex-M1 processor is a 32-bit reduced instruction set computing (RISC) processor designed by ARM Holdings. It is part of the Cortex-M series of microcontrollers and is intended for deeply embedded applications requiring high efficiency and low power consumption.
The ARM Cortex-M1 is a very simple processor with minimal hardware features compared to other Cortex-M processors. It is designed to be low-cost and enable highly efficient implementations for basic embedded applications that do not require advanced features.
Key Features of Cortex-M1
- 32-bit RISC architecture
- 3-stage pipeline
- Up to 50 MHz clock frequency
- Harvard architecture with separate instruction and data buses
- 26 interrupts with configurable priorities
- Memory Protection Unit (MPU) with 8 regions
- Wake-up Interrupt Controller (WIC)
- Advanced power management capabilities
Some of the key features of the Cortex-M1 processor are:
Simple 3-stage Pipeline
The Cortex-M1 has a simple 3-stage instruction pipeline consisting of fetch, decode, and execute stages. This allows efficient execution of instructions with minimal power consumption. The simple pipeline avoids complex branch prediction or speculative execution logic.
Harvard Architecture
Cortex-M1 uses separate instruction and data buses (Harvard architecture). This allows concurrent access to program memory and data memory, improving overall throughput.
Wake-up Interrupt Controller
The Wake-up Interrupt Controller (WIC) allows the processor to wake up from sleep/low power modes on interrupt events. This enables low power operation critical for embedded applications.
Memory Protection Unit
An 8-region Memory Protection Unit (MPU) provides basic memory access control and protects privileged areas of memory from access by less privileged software. This improves robustness and security.
Advanced Power Management
The Cortex-M1 supports multiple low power modes including sleep, deep sleep and shutdown. Clock gating and power gating techniques reduce dynamic and static power consumption respectively when not in use.
Intended Use Cases
The Cortex-M1 targets cost-sensitive and power-constrained deeply embedded applications such as:
- Simple industrial controllers
- Home appliances
- Consumer electronics
- Toys
- Low-end wearable devices
It prioritizes extreme power efficiency and smaller silicon area over performance or features. The use of Cortex-M1 allows cheaper and simpler implementations than higher-end Cortex-M processors.
Architecture Overview
The Cortex-M1 processor has a streamlined architecture optimized for lower cost and power efficient implementation.
Instruction Set
Cortex-M1 implements the Thumb instruction set which is a condensed 16-bit and 32-bit instruction set designed for better code density compared to regular 32-bit ARM instructions. The streamlined Thumb ISA requires fewer transistors to implement, reducing silicon area.
Simple Pipeline
As mentioned earlier, a simple 3-stage pipeline fetches instructions, decodes them and then executes in an assembly-line manner. This avoids complex pipeline control logic for speculative execution or branch prediction.
Minimal Unit Count
The core contains only approximately 12k logic gates, very small compared to other microcontroller architectures. There are no cache memories. Fewer logic gates and transistors results in smaller silicon area and lower power.
Positioning vs Cortex-M0
The Cortex-M0 is ARM’s smallest and lowest cost processor aimed at simple microcontroller applications. But Cortex-M1 is even smaller and simpler than M0 with the following distinctions:
- The M0 core has a 2-stage pipeline while M1 has 3 simpler stages
- M0 supports hardware multiply instructions unlike M1
- M0 includes more advanced debugging capabilities
- M1 is approximately 25% smaller than M0 in terms of silicon area
Thus Cortex-M1 targets an even lower hardware cost point at the expense of performance and features.
Performance
The Cortex-M1 delivers very modest performance of approximately 0.9 DMIPS/MHz. This reflects its intended use only for the simplest embedded applications with basic processing needs. The 3-stage pipeline allows clock frequencies of up to 50 MHz. However, typical operating frequencies are 20-35 MHz range.
There is no instruction or data cache. All code executes out of internal or external flash memory. External flash often limits maximum clock speed.
The lack of a hardware multiplier block means multiply operations have to be implemented as software routines taking multiple cycles. This also impacts DSP performance adversely.
Overall, performance takes a backseat to silicon size, hardware simplicity and power efficiency in Cortex-M1’s design.
Development Tools
The Cortex-M1 can be programmed using ARM’s Keil MDK-ARM toolkit which includes the μVision IDE and debugger or using gcc-based open source tools. Documentation and software development tools are provided by processor IP vendors.
Debugging support is limited compared to Cortex-M0/M3 class processors. There is no Embedded Trace Macrocell (ETM) for instruction trace. However, breakpoints, watchpoints and basic profiling features are supported.
Licensing and Manufacturing
As a proprietary processor architecture, the Cortex-M1 requires a per-device royalty license from ARM Holdings. It is available as synthesizable RTL IP to chip manufacturers and SoC designers who can integrate it with their own logic and memories.
The Cortex-M1 processor is manufactured in various nanometer scale processes offered by foundry companies like TSMC, Samsung, GlobalFoundries etc. Implementations by IP vendors are available supporting processes ranging from 180nm all the way down to 28nm and below.
Summary
In summary, the ARM Cortex-M1 is an ultra low-cost 32-bit RISC processor microarchitecture designed for simple, deeply embedded applications. It trades off performance and advanced features for a very small silicon footprint and powr efficiency. With its minimalist in-order pipeline, stripped down Thumb ISA and lack of cache, the Cortex-M1 focuses solely on embedded control applications requiring the utmost hardware efficiency.
