The key difference between the ARM7 and Cortex-M3 microcontrollers is that the ARM7 is an older generation 32-bit RISC processor core, while the Cortex-M3 is a newer generation 32-bit ARM processor core specifically designed for microcontroller applications. The Cortex-M3 offers higher performance, more features, and better power efficiency compared to the older ARM7.
Overview of ARM7
The ARM7 is a 32-bit RISC processor core designed by ARM Holdings. It was one of the first ARM processor cores launched in the early 1990s. The ARM7 core has a 32-bit architecture, which means it can process data and instructions in 32-bit chunks. This provides better performance compared to earlier 8-bit and 16-bit processors.
The ARM7 is based on the Von Neumann architecture and has a single bus interface for both instructions and data. It has a 3-stage pipeline to increase instruction throughput. The pipeline allows multiple instructions to be processed simultaneously in different stages of execution.
The ARM7 core has a 32-bit ALU, 32 general purpose registers, and a barrel shifter for efficient bit manipulation. It supports both ARM and Thumb instruction sets. The Thumb instruction set provides higher code density compared to regular 32-bit ARM instructions.
The ARM7TDMI is the most popular variant of ARM7. The ‘T’ stands for Thumb instruction set, ‘D’ for on-chip debug support, ‘M’ for enhanced multiplier, and ‘I’ for embedded ICE hardware debugging. The multiplier enhancement speeds up DSP algorithms.
The ARM7 core is capable of up to 130 DMIPS performance at 100 MHz clock speed. The power consumption depends on specific implementation, but typically ranges from 0.1 to 2 mW per MHz.
The ARM7 has been used in a variety of embedded applications such as microcontrollers, CPUs for cellphones, disk drives, handheld devices, digital TVs, robots, network switches, laser printers and other embedded systems.
Overview of Cortex-M3
The Cortex-M3 is a 32-bit processor core designed specifically for microcontroller applications. It was introduced by ARM Holdings in 2004 as part of the Cortex-M series of microcontroller focused cores. The Cortex-M3 architecture builds upon the earlier ARM7TDMI core with a number of enhancements:
- Higher CPU performance – Up to 1.25 DMIPS/MHz, 1.5x faster than ARM7TDMI
- Superscalar architecture – Dual issue pipeline can execute multiple instructions per clock cycle
- Microcontroller focused features – Low latency interrupt handling, optional memory protection unit (MPU)
- THUMB-2 instruction set – Improved Thumb encoding, interworks with ARM instructions
- Single cycle digital signal processing – MAC operations execute in a single cycle
- Low power design – Operates in active or sleep mode,Wakeup interrupt controller (WIC)
- Memory architecture – Von Neumann, Harvard and modified Harvard bus architectures
The Cortex-M3 is a 3-stage 32-bit RISC processor with superscalar dual-issue pipeline. It achieves up to 1.25 DMIPS/MHz, which is approximately 1.5 times faster than the ARM7TDMI core. The Thumb-2 instruction set used in Cortex-M3 provides improved performance and code density compared to older Thumb-1 encoding.
The processor supports low latency interrupt handling and optional memory protection unit for enhanced application security. A single cycle hardware multiplier improves performance of DSP applications. It includes features like watchdog timers, sleep modes, and wakeup interrupt controller to facilitate low power designs.
The Cortex-M3 can be implemented with a Von Neumann, Harvard or modified Harvard bus architecture. Von Neumann uses a unified memory interface for both instructions and data. Harvard uses separate bus interfaces for code and data memory. Modified Harvard provides a unified interface with separate caches for instructions and data.
The Cortex-M3 is implemented in various microcontrollers from vendors like STMicroelectronics, NXP, Microchip, Texas Instruments etc. It is widely used in applications like automotive body systems, industrial automation, robotics, IoT sensors, home appliances and consumer devices. The core is optimized to provide high performance and low power capabilities in embedded real-time applications.
Key Differences
Here are some of the key differences between the ARM7 and Cortex-M3 cores:
1. Performance
The Cortex-M3 provides significantly higher performance compared to the older ARM7 core. Cortex-M3 can achieve up to 1.25 DMIPS/MHz, while ARM7TDMI can do up to 0.9 DMIPS/MHz. This is because Cortex-M3 uses a superscalar dual-issue pipeline that allows two instructions to be issued simultaneously. In comparison, ARM7 uses a simpler non-superscalar pipeline.
2. Power Efficiency
Cortex-M3 offers better power efficiency than ARM7. At 90 nm process node, Cortex-M3 can operate down to 9 μA/MHz, while ARM7TDMI consumes 22 μA/MHz. Cortex-M3 includes features like Wakeup Interrupt Controller, sleep modes, and clock gating that enable energy efficient operation required for battery powered devices.
3. Instruction Set
Both cores support Thumb instruction set to improve code density. But Cortex-M3 implements Thumb-2, which is an enhancement over ARM7’s Thumb-1 set. Thumb-2 provides significant performance improvements while retaining the code size advantage of Thumb encoding.
4. Interrupt Handling
Cortex-M3 has lower interrupt latency of 12 clock cycles compared to 27 clock cycles for ARM7. This enables Cortex-M3 to respond faster to external and internal events that require real-time attention in embedded applications.
5. Debugging Support
Both cores provide on-chip debugging capability. ARM7TDMI supports JTAG based embedded ICE while Cortex-M3 supports the more advanced Serial Wire Debug protocol with higher data transfer rates.
6. Fabrication Process
ARM7 cores were fabricated on older 180 nm or 130 nm processes. Cortex-M3 is fabricated on more advanced 90 nm or smaller geometry fabrication technologies. The smaller process nodes enable integration of larger flash memory capacities and peripherals.
7. Licensing Model
ARM7 cores use licensing model where ARM Holdings licenses the core designs to semiconductor companies. Cortex-M3 uses a different licensing model where the cores are directly licensed to end manufacturers of microcontroller chips.
Summary
In summary, the Cortex-M3 is a newer generation, high performance microcontroller core designed for low power embedded applications. It enhances the earlier ARM7 core with features like higher CPU performance, Thumb-2 instruction set, single cycle DSP capabilities, lower interrupt latency, debug enhancements, and advanced power saving options. The Cortex-M3 forms the processing backbone of many modern 32-bit embedded microcontrollers used in IoT, industrial, consumer and medical devices.
ARM7 Microarchitecture
The ARM7 microarchitecture is based on the Von Neumann model which uses a single shared bus interface for both instruction and data transfers. It has a three stage instruction pipeline that enables pipelined execution of instructions. The three pipeline stages are Fetch, Decode and Execute. This enables processing multiple instructions simultaneously in different stages.
The CPU core contains a 32-bit Arithmetic Logic Unit (ALU), 32 general purpose 32-bit registers, a dedicated Program Counter register, and a barrel shifter that can shift or rotate operand contents efficiently. The registers are visible to user programs. R15 is the Program Counter, R14 is the Link Register to hold return instruction addresses, and R13 is the Stack Pointer.
The ARM architecture uses load/store architecture where data processing operates only on register contents, not directly on memory contents. All instruction operands are registers reads and instruction results are stored in registers. Memory access is done only through explicit load/store instructions.
The ARM instruction set uses fixed 32-bit length instructions. A key feature is that ARM instructions are conditional, which means each instruction specifies a condition under which it executes. This allows efficient if-then-else structures without explicit branch instructions.
The ARM7TDMI core implements three different instruction sets – 32-bit ARM, 16-bit Thumb, and Jazelle. The Thumb instruction set provides improved code density while retaining most ARM instructions. Jazelle is designed for execution of Java bytecode.
For memory access, the ARM7 incorporates MMU functionality to provide virtual to physical address translation and memory protection. Caches are optional and depend on specific implementation. ARM7TDMI does not contain caches, but external caches can be added.
Pipeline Stages
The ARM7 uses a three stage pipeline to improve instruction throughput. The three stages are:
- Fetch – Instruction is fetched from memory
- Decode – Instruction is decoded into control signals
- Execute – Instruction is executed
Pipelining reduces the average execution time per instruction by allowing next instructions to start before current instruction is finished. So while one instruction is being executed, the next one can be decoded and another fetched from memory.
ALU
The Integer ALU performs arithmetic and logical operations on 32-bit operands. It consists of circuits like adders, shifters, logic gates to enable operations like addition, subtraction, bitwise AND/OR/NOT etc. It has a 32-bit result output.
Shifter
The barrel shifter performs shift and rotate operations on operand contents like left/right logical shift, arithmetic shift, rotation etc. Having a dedicated hardware shifter improves performance of bit manipulation intensive tasks.
Branches
The ARM architecture uses PC relative branching. Branch instructions contain signed 8-bit/11-bit immediate offsets which get shifted and added to the PC to compute target address. Long branches are achieved through loading the target address into a register and doing a branch-register instruction.
Interrupts
The ARM7 core receives interrupt signals from various internal peripheral modules and external sources. These pass through prioritization and masking logic before reaching the core. Up to 32 unique interrupt sources are supported. Low interrupt latency is achieved by retaining core pipeline state during exception processing.
Cortex-M3 Microarchitecture
The Cortex-M3 microarchitecture is designed as a high performance processor core optimized specifically for microcontroller applications. It integrates a 3-stage dual-issue superscalar pipeline, Thumb-2 instruction set, single cycle DSP capabilities, low latency interrupts, memory protection unit and other features required for embedded real-time processing.
Pipeline
The Cortex-M3 implements a 3 stage pipeline consisting of Fetch, Decode and Execute stages. A key enhancement over ARM7 is the superscalar dual-issue pipeline which allows two instructions to be issued simultaneously to the Decode stage under certain conditions. The processor can therefore execute two instructions in parallel during a single clock cycle, improving performance.
The Fetch stage fetches two Thumb-2 instructions every cycle from the memory system. The instruction fetch unit contains branch prediction logic for efficient changes in control flow. Prefetch buffers are also used to minimize stalls during instruction fetch.
In the Decode stage, two instructions are decoded and checked for data dependencies or resource conflicts. If no hazard conditions exist, two instructions may be issued in parallel to the Execute stage. Single cycle DSP extensions like multiply and accumulate (MAC) are also decoded here.
The Execute stage either executes instructions in parallel or serially depending on issue rate. Load/store instructions access the data memory system through a 64-bit AHB-lite bus interface. Completion of load/stores may take more than 1 cycle depending on wait states configured.
Registers
The Cortex-M3 core contains 37 32-bit general purpose registers, of which 15 are available for user programs. R0-R12 are core registers, while R13-R15 have special functions – SP, LR, and PC respectively. The non-user registers are used by exception handlers and to store special data like exception return information.
Buses
Cortex-M3 supports Von Neumann, Harvard and modified Harvard memory architectures through its flexible bus interface. This allows instruction and data transfers via unified or separate address/data buses depending on requirements.
Von Neumann uses a unified memory interface for both code and data. Harvard architecture uses separate buses, enabling simultaneous access. Modified Harvard uses unified bus but with separate instruction and data caches for higher performance.
Instruction Set
The Cortex-M3 implements the Thumb-2 instruction set which is a variable length encoding that provides a balance of high code density and good performance. It improves upon the older Thumb-1 instruction set. Thumb-2 instruction lengths can be either 16-bit or 32-bit.
16-bit Thumb-2 instructions retain the space saving advantage of traditional Thumb instructions. 32-bit instructions provide additional encodings to improve performance for complex functions like interrupts, branches and loads/stores. The instruction set remains backward compatible with previous Thumb-1 implementations.
Interrupts
The Nested Vectored Interrupt Controller (NVIC) integrated with the core provides low latency exception and interrupt handling required for real-time embedded applications. It supports up to 240 distinct interrupt sources with configurable priority levels for each source.
The NVIC allows all exception and interrupt priorities to be split into preemption priority and sub priority bits for enhanced flexibility in priority assignment. Interrupt latency is reduced to a minimum of 12 clock cycles from interrupt assertion to the start of the interrupt handler.
DSP Extensions
The Cortex-M3 incorporates DSP capabilities through its single cycle hardware multiplier block and related MAC instructions. The 32×32 bit multiplier can execute multiply and multiply-accumulate operations in a single cycle, greatly improving DSP performance.
DSP algorithms involve extensive use of repeated MAC operations. By reducing the multiply and MAC operations to a single cycle, significant performance gains are achieved in Cortex-M3 based microcontrollers targeting DSP-heavy workloads.
Memory Protection Unit
The optional MPU integrated with the core provides user configurable memory protection across different memory regions. Memory protection improves application reliability and security by preventing unauthorized or unintentional accesses to restricted memory regions.
The MPU contains 8 individually configurable regions for defining memory attributes like cacheability, executability, read/write permissions etc. MPU configurations can be defined based on privilege levels to enforce protection for user and privileged modes.
Debug Support
The Cortex-M3 integrates debug components like Breakpoint Units, Embedded Trace Macrocell (ETM) and Debug Access Port (DAP) to facilitate software debugging via JTAG/SWD interfaces. This provides detailed visibility into program execution, memory access, interrupt handling and real-time task scheduling.
Debugging features supported include breakpoint debugging, instruction trace, data trace, profiling counters and watchpoint exception generation. These capabilities lower the effort and time required for optimizing and troubleshooting MCU software.
Comparative Analysis
Here is a detailed feature by feature comparison between the ARM7 and Cortex-M3 cores:
Architecture
ARM7 uses 32-bit RISC load/store architecture while Cortex-M3 extends it further by integrating microcontroller specific capabilities like single cycle DSP instructions, MPU etc.
Pipeline Depth
Both utilize a 3-stage pipeline. But ARM7 pipeline is linear while Cortex-M3 adds parallel dual-issue capabilities to the Decode and Execute stages.
ALU Configuration
ARM7 has a 32-bit datapath and ALU while Cortex-M3 increases it to 64-bit for improved performance. Both contain a hardware barrel shifter.
Clock Speed
ARM7 implementations typically range from 60-200 MHz. Cortex-M3 implementations reach up to 300 MHz.
Instruction Set
ARM7 uses 32-bit ARM instructions or 16-bit Thumb-1 instructions. Cortex-M3 implements the enhanced Thumb-2 instruction set which has 16/32-bit variable length encoding.
Addressing Modes
Both ARM7 and Cortex-M3 support a variety of addressing modes for memory access – immediate, register direct, register indirect, indexed etc. Cortex-M3 adds additional register based addressing modes to enhance code density.
Interrupt Support
ARM7 supports up to 32 external interrupt sources while Cortex-M3 supports up to 240 interrupts with configurable priority levels for each source.
Exception Support
Both processors handle exceptions like resets, interrupts, hard faults, etc. Cortex-M3 additionally supports usage faults, debug faults and other system protection exceptions.
Memory Protection
ARM7 does not contain memory protection features. Cortex-M3 adds an optional MPU to define up to 8 memory regions with configurable access permissions.
Power Management
ARM7 relies on system level design for power optimization. Cortex-M3 incorporates additional power control features like sleep modes, wakeup interrupt controller, clock gating etc.
Fabrication Process
ARM7 uses legacy 130 nm to 180 nm processes. Cortex-M3 leverages newer 90 nm or smaller geometry fabrication processes.
Software Development
ARM7 software can be developed using ARM Assembly, C or C++. Cortex-M3 benefits from code developed in C and C++ due to Thumb-2 enhancements.
Debugging Support
Both provide JTAG/SWD debug capabilities. Cortex-M3 adds more advanced real-time trace and profiling features for software debugging.
Licensing Model
ARM7 uses licensing model while Cortex-M3 shifted to direct licensing model.
Conclusion
In conclusion, the Cortex-M3 builds upon the ARM7 foundation with significant enhancements in microarchitecture, instruction set, debugging features, power management and interrupt handling. This enabled Cortex-M3 based microcontrollers to deliver much higher performance, better power efficiency and real-time responsiveness required in modern embedded applications.
