3 Control Law Accelerator (CLA)

Enabling extremely high performance computation and efficient processing is critical for solving today’s complex real-time control applications. Real-time control systems require minimal latency where the time delay between sampling, processing, and outputting must fit within a tight time window in order to meet performance objectives. For example, a typical digital power controller consists of an ADC to read the input signals (voltage and current), a math engine to compute the control law algorithms (PID, 2-pole/2-zero, and 3-pole/3-zero compensators), and a PWM channel to output the calculated waveform. Many advanced control systems would greatly benefit from an architecture that integrates these functions in such a way as to minimize latency, yielding the absolute minimum sample to output delay. Ideally, this architecture would execute time-critical control loops concurrently with the main CPU and free it up to perform other required tasks. In addition, the architecture must have a built-in protection mechanism to guard against over-current and over-voltage conditions. To address these important requirements, TI developed the CLA.

The CLA is a fully-programmable independent 32-bit floating-point hardware accelerator that is designed for math intensive computations. This accelerator can offer a significant boost to the performance of typical math functions that are commonly found in control algorithms. The CLA is designed to execute real-time control algorithms in parallel with the C28x CPU, effectively doubling the computational performance. This makes the CLA perfect for managing low-level control loops with higher cycle performance improvements over the C28x CPU. Another advantage of the CLA is that since it directly accesses memory, the overhead penalty for managing a data page pointer is removed. Additionally, the multiplier on the CLA does not require any delay slots, thus providing true single-cycle performance. A device using the CLA can achieve about a 1.3 times performance improvement over the C28x CPU for applications like motor control and solar, as shown in the table below. Furthermore, by using the CLA to service time-critical functions, the C28x CPU is freed up for other tasks, such as communications and diagnostics.

Another key benefit of the CLA, over hardware-based control law implementations, is flexibility. The CLA is a fully software programmable solution where developers can freely modify their control system without the time and high cost required to redesign a hardware-based solution. CLA in addition to these benefits can also perform compute intensive functions such as FFT (both complex and real). Table 3-2 provides the details of the cycles

The CLA is able to minimize latency because it has direct access to the various control peripherals such as the ADC and PWM modules. Utilizing this low-latency architecture and capability to directly access the various control peripherals provides a fast trigger response. The CLA is able to read the ADC result register on the same cycle that the ADC sample conversion is completed. This “just-in-time” reading of the ADC reduces the sample to output delay and enables faster system response for higher frequency control loops.

Programming the CLA consists of initialization code and tasks. A task is similar to an interrupt service routine, and once started it runs to completion. Each task is capable of being triggered by a variety of peripherals without CPU intervention. This makes the CLA very efficient since it does not use interrupts for hardware synchronization, nor must the CLA do any context switching. Compared with the traditional interrupt-based scheme, the CLA approach eliminates jitter, and furthermore the execution time becomes deterministic. It supports eight independent tasks, each of which is mapped back to an event trigger, such as a timer or the availability of an ADC result. Separate tasks can be used to support multiple control loops or phases at the same time.

Some C2000 devices feature an enhanced version of the CLA with the option of running the lowest priority task as a background task. Once triggered, it runs continuously until it is terminated or reset by the CLA or MCU. The remaining tasks in priority order can interrupt the background task when they are triggered. If needed, portions of the background task can be made uninterruptible. Typical uses of the background task include running continuous functions, such as communications and clean-up routines.

Another key benefit of the CLA, over hardware-based control law implementations, is flexibility. The CLA is a fully software programmable solution where developers can freely modify their control system without the time and high cost required to redesign a hardware-based solution.