Industrial-grade features are critical in embedded processing

Rick Gentile

Analog Devices

Many general-purpose signal processors have good computational performance and basic connectivity features that make them acceptable for industrial applications. On the other hand, there are some important peripheral enhancements that can significantly improve the capabilities of the processors, making them suitable for more demanding industrial systems. This article will discuss two examples of enhancements for networking and motor control applications.

Ethernet interface

For traditional industrial applications, an Ethernet controller provides basic network connectivity. The controller (MAC) is usually located on the same chip as the processor. It is usually used in conjunction with an external PHY chip to form a complete interface.

An external MAC/ PHY chip can also be used , often connected directly to the processor’s asynchronous memory interface. While Ethernet MAC/PHY combo chips continue to drop in price to the point where they are almost on par with standalone PHY chips, their transfer rates cannot match those of an integrated MAC plus external PHY solution. This is because the internal MAC is typically connected to the system DMA channel and can be set up to send or receive data with minimal interaction with the core processor. The internal MAC controller can generally achieve near line-speed performance, depending on the protocol.

Another important aspect of performance is the processor load required to achieve a given throughput rate. This is part of the overall performance and is the biggest difference between internal MAC solutions and external MAC solutions.

In industrial networks, Ethernet can provide basic system time using the Network Time Protocol (NTP). For NTP-based systems, synchronization of the entire controlled network is measured by the " human-machine interface " time scale. While this protocol is suitable for general system timing information, it is not precise enough and is not suitable for many industrial control systems that require more precise synchronization.

To improve accuracy, the industry developed the IEEE 1588 Precision Time Protocol (PTP) standard to work with Ethernet controllers and network stacks to synchronize "local" clocks on the network with a master clock. That is, each processing or control node is synchronized to the master reference time of the drive system.

By maintaining a precise timing relationship across an industrial network, time events such as when analog/digital converters sample, when digital/analog converters are driven, and when I/O lines are activated to perform system control can be synchronized to sub-millisecond levels.

IEEE 1588 PTP requires the exchange of specific packets to provide time information from two nodes. These packets are used to calculate the time and frequency differences between the clocks of each node. In addition, the protocol provides a way to continuously adjust the clocks so that the clocks remain synchronized.

The IEEE 1588 PTP protocol can be implemented either entirely in software or as a combination of hardware and software. A hardware-based solution provides the best accuracy, resulting in the best synchronization between nodes. With a hardware solution, the packet can be timestamped as close as possible to the point where it interacts with the PHY. This results in lower jitter between nodes.

PWM Unit

A standard peripheral for microprocessors and DSPs is a general-purpose timer that provides standard timer functions based on one or more clock references internal or external to the chip. On the pin interface, it can also provide width capture or pulse counting functions, as well as single-ended pulse width modulation (PWM) output waveforms. These PWM outputs usually have programmable pulse width and period and can be used in many tasks and industrial control applications, including DC level generation and noise-resistant analog signal transmission (with appropriate low-pass filtering).

However, to make it truly usable for AC motor control, the basic PWM functionality needs to be upgraded in several ways. Figure 1 shows a motor control block diagram where the PWM output from the processor drives the high-side and low-side power devices differentially to regulate the torque and speed of the motor. The ADC is used to provide current measurement feedback to the processor so that the PWM duty cycle can be managed in a closed-loop system with tight timing to control the motor.

Figure 1: Schematic diagram of the motor control signal chain.

The PWM units used for motor control have several enhancements compared to the PWM modules of general-purpose processors. As mentioned above, the motor control PWMs are used in pairs to alternately drive the high-side and low-side power switches in a given motor phase. For a three-phase AC motor, three pairs of PWM units are required.

As shown in Figure 1, isolation must generally be provided between the processor's PWM control unit and the gate drive device of the power transistor. This isolation is usually achieved using an optocoupler or a pulse transformer. Therefore, some PWM units provide a gate drive unit that allows the output to be mixed with a high-frequency chopped signal for connection to a pulse transformer, and also provide pin drivers that can drive most optocouplers with sufficient source and sink current.

It is important that the motor control PWM provide a certain guaranteed "dead zone" between the end of assertion of one power device and the beginning of assertion of the other complementary power device. Otherwise, the power switches may experience a DC short circuit.

Additionally, there must always be a way to asynchronously disable the PWM outputs immediately to avoid an erroneous condition where multiple output phases are enabled simultaneously. This “PWM jump” feature allows all PWM outputs to be disabled using an external asynchronous signal, regardless of the state of the processor clock.

Finally, while it is common practice to synchronize the start of general-purpose timers, synchronization of PWM timers has even greater significance for motor control. An internally or externally applied “PWM Sync” signal can be used to generate an interrupt (sometimes more than once per cycle) so that the processor can adjust the duty cycle according to the control algorithm and the ADC can acquire and transmit the next current measurement.

At this point, it is obvious that while many industrial applications may choose a processor with a common peripheral set, it is wise to first consider which "industrial upgrades" will benefit the current application. In this article, we have only chosen to discuss two examples, network connectivity and PWM functions, but the same principles apply to many other subsystems, including memory structures and data conversion interfaces. The added value brought by the expansion of peripherals and system modules can improve the stability and system control capabilities of industrial products.