The Arm Cortex-M1 and the Xilinx Microblaze are both 32-bit reduced instruction set computer (RISC) processors commonly used in embedded systems. Choosing between them often comes down to the design goals and constraints of a particular project.
Overview
The Cortex-M1 was designed by Arm as a power-efficient 32-bit processor for microcontroller applications. It has a 3-stage pipeline, optional MPU, and support for Thumb code compression. The M1 is licensed to chip manufacturers as IP to be fabricated on their process technology.
The Microblaze is a soft processor core designed by Xilinx for their FPGAs. As it is synthesized from HDL code, it can be customized to trade off performance, area, and power consumption. The Microblaze has a 5-stage pipeline and features like branch prediction. It runs code compiled for the 32-bit ISA.
Architecture
The Cortex-M1 has a 32-bit RISC architecture optimized for low-cost and low-power embedded applications. It has a 3-stage pipeline consisting of Fetch, Decode, and Execute stages. The processor includes a subset of the Thumb instruction set and supports mixed 16-bit and 32-bit instructions.
The Microblaze has a 32-bit load-store RISC architecture with 5 pipeline stages: Fetch, Decode, Execute, Memory Access, and Writeback. It utilizes a Harvard architecture with separate 32-bit instruction and data buses. The Microblaze ISA includes instructions for DSP and embedded processing like hardware loops and barrel shifting.
Memory and Addressing Modes
The Cortex-M1 supports a 32-bit linear address space and can address up to 4GB of memory. Memory access instructions in Thumb state are 16-bit with limited addressing modes. The M1 has multi-cycle 32-bit instructions that enable full addressing capabilities.
Microblaze provides 32-bit instruction and data address buses, supporting up to 4GB of memory. Its load/store instructions have several addressing modes like register indirect, immediate offsets, and absolute addressing. Microblaze also uses a 32-bit arithmetic logic unit (ALU).
Pipelining and Performance
The 3-stage pipeline of the Cortex-M1 allows for single-cycle execution of most Thumb instructions. With a clock frequency up to 200 MHz, the M1 can deliver 0.9 DMIPS/MHz. The short pipeline limits clock speed but also reduces power usage.
The Microblaze 5-stage pipeline enables higher clock frequencies up to 286 MHz while maintaining single-cycle execution for most instructions. Pipelining increases throughput, with Microblaze reaching 1.97 DMIPS/MHz. The barrel shifter also boosts performance on shifts and multiply-accumulate operations.
On-Chip Peripherals
The Cortex-M1 only contains the CPU logic, so on-chip peripherals depend on the implementation. Typical M1 microcontrollers integrate basic peripherals like timers, serial communication, GPIO, ADC, and external bus interfaces.
In an FPGA, designers can customize Microblaze with different peripherals through IP cores. Common additions include UART, SPI, Ethernet, DDR memory controllers, and custom accelerators. Peripherals connect to the processor via AXI interfaces.
Physical Implementation
Since Arm licenses the Cortex-M1 as IP, its physical implementation can vary across manufacturers. The M1 core is synthesized by chip vendors and added to microcontroller designs in a specific process technology.
With Microblaze, designers specify the core configuration and microarchitecture via HDL code. This allows Microblaze to be optimized for Xilinx FPGAs in terms of speed, area, and power. The soft core is then mapped, placed, and routed like other FPGA logic.
Process Technologies
The Cortex-M1 has been fabricated in processes ranging from 180 nm down to 40 nm. More advanced nodes like 28nm further reduce power consumption. Migration to new processes requires re-implementing the physical design of the M1 core.
As a soft core, Microblaze is technology independent. It can be mapped to any Xilinx FPGA family from Spartan to Virtex UltraScale+. This span ranges from 28 nm to 5 nm FinFET processes while maintaining software compatibility.
Footprint and Clock Speed
Typical Cortex-M1 implementations range from 18k to 30k gates. At 65nm, its core power consumption is about 4.5 mW/MHz. With a small die area, M1 microcontrollers can run from 32 kHz up to 200 MHz.
Microblaze can be configured for area or speed. A minimal area-optimized core uses 900 LUTs on Xilinx FPGAs, while the high performance version needs 3200 LUTs. Maximum clock frequencies range from 80-286 MHz depending on the FPGA.
Development Tools
The Cortex-M1 can be programmed using the Arm RealView Microcontroller Development Kit (MDK). This Eclipse-based IDE supports C/C++ with debuggers, compilers, and software packs for device peripherals.
Microblaze development leverages Xilinx’s Soft-IP ecosystem. This includes Vitis IDE for writing, debugging, and optimizing software in C/C++ and OpenCL. IP Integrator allows integrating hardware blocks, while SDKs add support libraries, drivers, and RTOSes.
Instruction Set Simulators
Arm offers the RealView Instruction Set Simulator to model Cortex-M1 CPU execution, memory, and peripherals. This enables testing software without needing the physical microcontroller hardware.
For Microblaze, Xilinx provides a functional instruction set simulator for early software development. It models the processor pipeline, memory, and interfaces to confirm program correctness before FPGA implementation.
Debug and Profiling
Debugging options for Cortex-M1 chips include JTAG, SWD, and proprietary OnCE interfaces. Vendor-specific IDEs connect to on-chip debug modules to set breakpoints, trace execution, and access memory and registers.
Microblaze supports Xilinx’s ChipScope for analyzing and debugging soft processor systems. This provides JTAG-based debug cores for breakpoint triggering, program trace, and visibility into internal processor signals.
Use Cases
The Cortex-M1 sees extensive use in microcontroller units (MCUs) for cost-sensitive embedded applications like home appliances, toys, and office equipment. MCU vendors often pair it with peripheral IP to create system-on-chips (SoCs).
Microblaze powers many soft processor systems across Xilinx FPGAs. Use cases involve software control of hardware accelerators and pre/post processing with flexibility to customize datapaths and peripherals.
Real-Time Control
With deterministic execution timings, the Cortex-M1 works well for real-time control in embedded systems. Microcontrollers with the M1 core drive applications like motor control, industrial automation, and IoT edge nodes.
Microblaze also suits closed-loop control tasks that demand low latency responses. FPGA parallelism allows for custom hardware that Microblaze software can interact with in real-time without operating system overhead.
Signal Processing
The compact Thumb ISA limits signal processing capabilities on the Cortex-M1. For DSP-centric applications, microcontroller vendors typically integrate application-specific accelerators and math coprocessors.
In contrast, Microblaze has dedicated DSP instructions for arithmetic and multiply-accumulate operations. Coupled with FPGA-based math functions, it can handle high throughput signal processing like Fourier transforms and filters.
Summary
In summary, the Cortex-M1 and Microblaze offer two different approaches for implementing 32-bit embedded processors. The M1 provides a fixed optimized CPU for low-cost microcontrollers and SoCs. Microblaze serves as a customizable soft core for building flexible FPGA-based systems.
The Cortex-M1 favors simplicity, low power operation, and compactness for cost-driven applications. Microblaze emphasizes configurability, higher performance, and tight hardware acceleration integration. Ultimately, the distinguishing use cases and design constraints help determine which architecture fits a particular project.
