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Interfacing multiple analog-to-digital converters on a Sitara™ processor

In this training series, we demonstrate how to use the PRU-ICSS subsystem on a Sitara processor to interface between multiple SAR ADCs using SPI.

Applications like protection relay and grid automation use multiple analog-to-digital converters (ADC) with Serial Peripheral Interfaces (SPI) to acquire data from various voltage and current transducers. This along with the need for adding redundant channels for reliability is driving the need for expanding the input channels that should all be synchronized on the data acquisition (DAQ) system. With limited SPI peripherals and data-capture rate in micro-controllers, interfacing multiple ADCs becomes difficult. With a high performance ADC — like those found on Sitara processors — engineers can achieve better accuracy. The challenge is in synchronizing, simultaneous sampling, and achieving coherency (sampling same number of samples for varying input frequency). We'll show you how to address these topics in this training series.

Our Flexible Interface (PRU-ICSS) Reference Design for Simultaneous, Coherent DAQ Using Multiple ADCs is an elegant solution for interfacing multiple SAR ADCs to achieve synchronization and simultaneous sampling. We use six ADCs with mux inputs to keep the cost low. Each ADC runs at 256 ksps (32ksps/channel and 1536ksps across 6 ADCs). The sampling rate can be easily adjusted to meet IEC61850-9-2 standard for protection and measurement. The PRU-ICSS subsystem provides the ability to dynamically adjust the ADC interface timing and thereby extract highest AC performance from ADCs resulting in more accurate data acquisition. Also using the zero-cross detect scheme, coherency is achieved by adjusting the sampling rate of the ADC which simplifies hardware design. 

1. Introduction to grid protection equipment and data acquisition

This video introduces the electrical grid and the different types of faults. Tune in as we cover a variety of grid protection equipment and their requirements, including signal processing for data acquisition.
# Title Duration
1.1 Introduction to grid protection equipment and data acquisition
This video covers a variety of grid protection equipment and their requirements, including signal processing for data acquisition.
08:28
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2. Decoding PRU-ICSS (hardware and software) for data acquisition

This video dives deeper into the details of the Programmable Real-Time Unit and Industrial Communications Subsystem (PRU-ICSS). The programmable nature of the PRU-ICSS, along with its access to pins, events and all system-on-chip (SoC) resources, provides flexibility in implementing fast, real-time responses, custom peripheral interfaces and the ability to offload tasks from the other processor cores of the SoC. The video also includes a comparison of PRU between different processors. Pseudo code is covered, demonstrating easy and flexible interface between the ADC and PRU for different types of interfaces. Lastly, we will discuss how to achieve simultaneous sampling across multiple ADCs and coherent sampling.

# Title Duration
2.1 Decoding PRU-ICSS (hardware and software) for data acquisition
This video dives deeper into the details of the Programmable Real-Time Unit and Industrial Communications Subsystem (PRU-ICSS).
17:39
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3. Flexible interface (PRU-ICSS) for data acquisition using multiple ADCs

This video showcases the PRU-ICSS interfaced to six 8-channel SAR ADCs (ADS8688), capturing data from all of the 48 channels at 32ksps/channel. Architecture of PRU implementation is demonstrated, achieving simultaneous sampling across multiple ADCs and coherent sampling to obtain high AC performance and minimal spectral leakage necessary for the data acquistion.
# Title Duration
3.1 Flexible interface (PRU-ICSS) for data acquisition using multiple ADCs
This video covers flexible interface between the PRU-ICSS and multiple ADCs to achieve simultaneous and coherent sampling (TIDA-01555).
07:11
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