MISC-TN-022: Advanced controller for DMX / RDM lighting applications

Info Box Applies to Machine Learning Applies to Machine Learning Technical Notes Applies to ORCA Technical Notes

History

Version	Date	Notes
1.0.0	May 2022	First public release

Introduction

This Technical Note (TN for short) illustrates the concept behind a custom product developed by DAVE Embedded Systems for a customer operating in the professional lighting market. This project is a useful use case for showing some interesting technologies that can address effectively common requirements in the world of embedded systems for industrial applications.

Overview

The following image depicts the product's block diagram.

Basically, the product is a dual-role device: it can operate either as a controller or as a recorder.

When working as a controller, it feeds slave devices (stage lighting dimmers, special effects machines, etc.) with DMX streams. DMX data can be generated programmatically on the fly or can be retrieved from previously recorded streams.

When working as a recorder, the product "sniffs" and stores DMX data traffic traveling on the connected buses. Each DMX frame is stored onto a permanent storage device — an e.MMC or a microSD card, for instance — with an associated timestamp. Thus, the data streams can be played at a later time with the same timings of the original ones.

As shown in the block diagram, the product features a rich set of I/O's ranging from the DMX/RDM channels to network interfaces. With regard to the Graphical User Interface, two displays are supported: an HDMI monitor and a local LVDS display. It is worth remembering that they can work simultaneously, for example in mirror mode.

The core of the system is the Orca SoM, which in turn is built around the i.MX8M Plus system-on-chip by NXP.

Heterogeneous asymmetric multiprocessing

From the computational standpoint, there are two domains running two different operating systems. In this regard, the resulting architecture is an example of heterogeneous asymmetric multiprocessing as it is based on different types of cores, namely the ARM Cortex-A53 and the ARM Cortex-M7.

The system architect chose this implementation because it is convenient to satisfy the diversified product's requirements. In particular, the DMX/RDM subsystem must meet real-time requirements in order to be compliant with the DMX/RDM standards. The DMX/RDM subsystem consists of an ARM Cortex-M7 core working in tandem with an FPGA. The M7 and the FPGA are connected with an I²S bus. Even though this bus is conceived for digital audio streams, it fits very well for conveying the data to/from the DMX/RDM channels.

The A53-centred domain runs a Yocto Linux distribution. The product's main application is executed in this domain. In particular, this application integrates the business logic and implements the GUI. Also, it deals with all the peripherals and interfaces not having real-time constraints such as the temperature sensor, the gyroscope, the Ethernet ports, etc.

The Linux domain is widely scalable in terms of computational power as multiple versions of the SoC are available featuring a different number of A53 cores (1, 2, or 4). These versions are ballout compatible, thus the same hardware platform fits them all.

Real-timeness

As stated previously, the ARM Cortex-M7 core and the FPGA are the building blocks of the DMX/RDM domain.

A real-time operating system (FreeRTOS) runs on the M7 core. The application executed on top of the RTOS communicates with the Linux domain through RPMsg-Lite, a lightweight implementation of the Remote Processor Messaging (RPMsg) protocol according to NXP documentation. Basically, the M7 acts as a bridge between the Linux domain and the FPGA, which is responsible for transmitting and receiving the DMX/RDM frames on the buses at physical level.

From a logical point of view, the whole transmitting/receiving chain looks like as shown in the following picture.

IoT and security

The product is an example of edge computing platform as well. It connects to the customer's cloud to upload data retrieved on the field and to manage OTA software updates. In order to use secure connections, the product is equipped with two secure elements:

• NXP EdgeLock SE050
• Microchip ATECC608A.

The first one is part of the Orca SoM, while the second is optionally populated at the carrier board level. These components allow to implement several schemes to address security-related issues. From the software perspective, they are supported by the Linux BSP so that the user space applications can access them through high-level API's. The Linux BSP is derived from the release L5.4.70 by NXP.