Communication Between Dual Cortex – Cortex-A & Cortex-M Cores

Homepage Communication Between Dual Cortex – Cortex-A & Cortex-M Cores

Communication is key, in embedded development and all other walks of life. Getting the most from your hardware often comes down to speed and synchronicity of communication, and never is this truer than with dual coretex processors.

By including both A and M-cortexes in a single MPU, such as the NXP i.MX93 and STM32MP1, you can combine high-performance processing capabilities with real-time control and efficiency.

Utilizing ARM architecture this way is great for Linux-based embedded systems such as those found in industrial automation, medical devices, and IoT gateways. However, getting those components to communicate is no easy task.

In this article, we delve into the pros and cons of five of the best communication methods available and some of the challenges facing those looking to take advantage of dual-core processing in embedded development.

Cortex Software Challenges

Let’s get the scary part out of the way first.

Getting the kind of functionality that multicore devices can offer does not come easily. Synchronization is paramount, and timing disparities can lead to race conditions and data corruption.  

Maintaining data integrity and consistency can also be tricky when sharing resources, demanding the utmost precision to prevent conflicts.

Certain communication methods may also inflate overhead, leading to latency and reduced system efficiency. 

Balancing power efficiency and system coherency is another challenge, such as when cores operate at different power states, requiring intelligent power management and optimization strategies.  

The same can be said of debugging. Tracing the flow of execution between both cores can be especially intricate and time-consuming. Linux BSP and kernel optimization can help you overcome many of these challenges, but this requires a keen eye and plenty of expertise.

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Cortex Communication Methods

Anyone looking to avoid these pitfalls will need a clear direction. Deciding on the right communication method will influence the direction and success of your entire project.

With that in mind, let’s have a look at a few of the main contenders.  

1. Shared Memory Communication 

Shared memory communication is a method used in multicore systems where multiple processor cores, such as Cortex-A and Cortex-M cores, share a common region of memory.

This shared memory serves as a communication medium, allowing cores to exchange data and information with each other.

The fundamental idea is to have a portion of memory that is accessible by all participating cores, enabling them to read from and write to shared variables. 

2. RPMSG Multipoint Communication

RPMSG, or Remote Processor Messaging, is a framework that enables communication between a master processor (e.g., Cortex-A) and multiple slave processors (e.g., Cortex-M) in a multicore system.

RPMSG offers a scalable many-to-one and one-to-many communication model. Although it can be particularly helpful in heterogeneous architectures, such as in industrial or automotive systems, it must be very carefully designed to maintain synchronicity and avoid latency issues. 

While RPMSG uses shared memory as part of its process, it does so with a much more user-friendly interface for sending data back and forth.

You can also use the remoteproc framework to control other cores (power on/off, load firmware), alongside RPMSG to communicate with the remote cores.

3. SPI Communication 

SPI (Serial Peripheral Interface) is a synchronous serial communication protocol where a master device communicates with one or more peripheral devices in full-duplex mode through four main lines—MOSI, MISO, SCLK, and SS/CS—enabling bidirectional data exchange in embedded systems. 

The versatility and simplicity of SPI make it a popular choice, especially in scenarios with multiple peripherals, while the full-duplex capabilities enable simultaneous input/output.

As you can imagine, the integrity of this type of signal degrades quickly with distance, even a few meters, and is really optimized for use on a PCB. Wiring can also be complex, especially as each SPI slave device requires a dedicated SS/CS line. 


4. GPIOs for Basic Signaling 

GPIO, or General Purpose Input/Output, is a standard feature in many microcontrollers that allows for basic signaling.

GPIO pins can be configured as either inputs or outputs, and they are commonly used for simple signaling tasks, such as controlling LEDs and buttons. The simplicity of GPIOs is a double-edged sword, as they are less suitable for more intricate or high-speed communication. 


5. Real-Time Operating Systems (RTOS)

Dedicated protocols are not the only way to communicate between cores, you can also use an RTOS.

While something like FreeRTOS is not designed specifically for inter-core communication, skillful use of task management and synchronization mechanisms can facilitate efficient communication.

FreeRTOS is open-source and enjoys fantastic community support, which helps with customization and maintenance. On top of that, FreeRTOS is built using a modular architecture and is easily portable, allowing for greater scaling. 

Using an OS for inter-core communication comes with some drawbacks, the most obvious of which is the additional draw on system resources. Using an RTOS forces developers to factor in the overhead of the OS itself, which can be a deal-breaker in embedded systems with tight memory constraints.

There is also the consideration that using RTOS this way demands greater development expertise, especially if you require more nuanced communication patterns. 

Final Thoughts 

Selecting the best inter-core communication method requires care and consideration. Your choice will depend on your project goals and constraints.

You may prefer shared memory and rpmsg when the A and M cores are on the same chip, as they are much more likely to share the same memory. However, if the cores are in different chips on a PCB then SPI and GPIO are likely needed. 

If you need help with your embedded software or Linux BSP development, Witekio is your trusted partner in development. As an official partner of ARM with over 20 years in embedded and IoT development, we can help you get the most from your device.

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Patrick HADDAD - Copywriter
15 January 2024