Co-processor instructions in Arm Cortex-M series microcontrollers provide an interface to optional co-processors that can be added to the core system to enhance functionality. They allow the Cortex-M CPU to offload specialized tasks like digital signal processing, graphics rendering or cryptography to dedicated hardware accelerators. This frees up the main CPU and results in significant performance improvements for workloads utilizing the co-processor.
Introduction to Co-processors
A co-processor is a specialized processing unit that operates alongside the main CPU to handle specific computationally intensive tasks. For example, in a smartphone, a graphics processing unit (GPU) serves as a co-processor to render 3D graphics and process visual data without overloading the main application processor. Co-processors are useful when the target applications require extensive mathematical computations or other repetitive numeric calculations that would significantly slow down the main CPU. The co-processor executes these operations instead, freeing up the main CPU for other tasks.
In the Arm Cortex-M series of microcontrollers, co-processors provide an optional extension to the CPU core system. They connect to the CPU via the Advanced Peripheral Bus (APB) interface. The Cortex-M CPUs include co-processor instructions in their Thumb-2 instruction set that provide access to up to 16 co-processors labeled CP0 to CP15. This allows application developers to leverage co-processors transparently by invoking these special instructions.
Benefits of Using Co-processors
Here are some of the major benefits of using co-processors in Arm Cortex-M based microcontroller systems:
- Improved Performance – Offloading tasks to dedicated hardware improves execution speed, throughput and overall system performance.
- Power Efficiency – Co-processors reduce the power consumption of the main CPU for specialized workloads.
- Flexibility – The CPU instruction set provides transparent access to optional co-processors only if they are present.
- Domain-specific Optimization – Each co-processor can be custom designed to accelerate tasks in a particular domain like signal processing, machine learning, graphics, etc.
- Scalability – Multiple co-processors can be added to create a heterogeneous processing system.
- Easy Development – Co-processors behave like functional extensions to the CPU from a programming perspective.
Types of Co-processors
Some common co-processors used with Cortex-M series microcontrollers include:
Digital Signal Processors (DSPs)
DSP co-processors have specialized data paths and MAC (multiply–accumulate) hardware to quickly execute signal processing algorithms like filters, transforms, speech recognition, etc. They handle extensive parallel data computations without utilizing the main CPU.
Graphics Processing Units (GPUs)
GPU co-processors designed for embedded systems can accelerate 2D and 3D graphics rendering. They are useful in applications like automotive dashboards, industrial HMIs, etc. that render complex graphical user interfaces.
Cryptographic Co-processors
These perform cryptographic operations like encryption, decryption, hashing, authentication, etc. in hardware at high speed. Security is a concern in many embedded applications today.
Machine Learning Co-processors
ML accelerators have specialized architectures to speed up neural network inferencing using trained models. They enable advanced embedded AI applications.
Other Domain-Specific Co-processors
Additional co-processors for video encoding/decoding, raster image processing, bioinformatics, computer vision etc. can be integrated based on application requirements.
Co-processor Instructions
The Arm Thumb-2 instruction set used in Cortex-M series contains the following instructions to access co-processors:
MCR and MRC
MCR (Move to Co-processor from ARM Register) and MRC (Move to ARM Register from Co-processor) instructions are used to move data between ARM core registers and co-processor registers. MCR p#, op1, Rt, CRn, CRm, op2 MRC p#, op1, Rt, CRn, CRm, op2
Here p# identifies the co-processor number 0 to 15, Rt is the ARM register, CRn and CRm encode the co-processor register number. op1 and op2 provide additional operational options.
MCRR and MRRC
MCRR and MRRC are used to transfer data between a pair of ARM registers and a pair of co-processor registers. MCRR p#, op, Rt, Rt2, CRm MRRC p#, op, Rt, Rt2, CRm
Rt and Rt2 indicate the two ARM source or destination registers. CRm identifies the coprocessor register pair.
CDP, CDP2
CDP and CDP2 instructions perform general arithmetic and data processing operations on the co-processor registers. CDP p#, op1, CRd, CRn, CRm, op2 CDP2 p#, op1, CRd, CRn, CRm, op2
CRd, CRn, CRm identify the co-processor registers to act as source and destination operands. op1 and op2 determine the operation.
LDC and STC
LDC (Load to Co-processor from memory) and STC (Store to memory from Co-processor) are used to transfer data between co-processor registers and main memory. LDC{L} p#, CRd, [Rn], imm STC{L} p#, CRd, [Rn], imm
The square brackets indicate memory access using the base ARM register Rn. LDC2 and STC2 variants allow transfer of register pairs.
Using Co-processors in Software
The workflow for utilizing a co-processor in a Cortex-M based system is as follows:
- Check availability – The CPACR register indicates which co-processors are present. Software can check for a co-processor before invoking any instructions.
- Initialization – Perform any required setup of co-processor internal state using MCR, MRC, CDP instructions.
- Enable co-processor – Set the appropriate bit in CPACR to enable a co-processor.
- Execute main workload – Use co-processor instructions from the application code to offload tasks.
- Disable co-processor – Clear the CPACR bit to disable it if no longer required.
- Optional – Save/restore context during task switches if using a multitasking OS.
This allows transparent migration of specialized computations from the Cortex-M CPU to the co-processor hardware with minimal changes to application software.
Co-processor Use Cases
Here are some examples of how co-processors are utilized in real-world Arm Cortex-M based microcontroller applications:
Wearable Health Trackers
A low-power DSP co-processor is used to process data from biosensors for heart rate detection and other health metrics while the main MCU handles system tasks.
Smart Home Devices
For audio processing in smart speakers, a dedicated audio DSP co-processor performs noise cancellation, speech recognition and encoding/decoding in hardware.
Industrial Motor Drives
High-speed control loops for motor control are offloaded to a co-processor, allowing precise real-time control using the main Cortex-M CPU for supervisory tasks.
Surveillance Systems
A cryptographic co-processor handles encryption/decryption for secure video transmission, authentication and other security operations.
Automotive Systems
A GPU co-processor displays high-resolution graphical console, instrumentation panel, navigation maps etc., freeing up CPU bandwidth for other automotive applications.
Conclusion
Co-processors enhance the capabilities of Arm Cortex-M series MCUs while reducing power consumption by offloading specialized processing tasks. The co-processor interface enables transparent acceleration for workloads involving extensive mathematical computations, multimedia processing, cryptography or other niche domains. Overall, co-processors help developers optimize complex embedded systems using Cortex-M CPUs.
