What should you use, a digital signal controller (DSC) or a conventional microcontroller? Designers frequently face that question, especially when an application sits on the boundary of the performance envelope. Because DSCs span a wide range of performance and functionality, choosing the right DSC for a particular application can be daunting. Issues such as battery operation and cost are often as important as performance and hardware support for a particular application like motor control.
Computing Power And Migration
Selecting a DSC with the appropriate resources is, of course, crucial. Program memory requirements and MIPS performance aren't directly comparable from one vendor to another. For example, standard benchmarks show that C-generated code from a base benchmark can vary in size over 50% from vendor to vendor!
The basic requirements for computing power and memory need to be examined in the light of the processor's architecture. Multiple multiply-accumulates (MACs), dual address manipulation, and zero overhead loops can affect how efficiently an algorithm is implemented or how much hardware support is idly wasted.
Likewise, DSCs frequently are available with peripherals that target a specific type of application. These peripherals can significantly offload the processor, allowing a considerable reduction in price and performance to meet the needs of the application.
One other consideration is the range of options available when working on a design. The ability to switch to a DSC or MCU with higher or lower performance— without changing the application source code—can provide a developer with a considerable number of options.
For instance, Microchip's 16-bit PIC24F is a low-cost general-purpose microcontroller that's compatible with the higher-performance dsPIC DSC. Seamless migration through to the dsPIC33F is possible because of code compatibility. All of these 16-bit families possess the same base instruction set, and common peripherals have been mapped to the same pins. The dsPIC DSC families add signal-processing instructions and peripherals that target DSP-centric applications, such as audio, high-performance motor control, and power-supply management.
Motor-control applications benefit from on-chip pulse-width modulators (PWMs) specialized for motor control. Look for PWM provisions for programmable deadtime settings, fault overrides, analog-to-digital converter (ADC) triggering, and multiple duty-cycle sgenerators. These PWMs also can be used for power-conversion applications, such as uninterruptible power supplies (UPSs) and LED lighting. Libraries and tools can help reduce development time for these applications, including tools for sensor or sensorless brushless direct-current (BLDC) motors, alternating-current induction motor (ACIM) control, and sinusoidal control of permanent magnet synchronous motors.
Switch-Mode Power Supplies (SMPSs)
Pulse-width-modulation control is critical in power-supply control. Because of the high switching speeds and fine control needed for SMPS designs, DSCs typically require PWMs with 1-ns resolution, a high-speed ADC, and analog comparators that make for cost-effective digital control of the power-conversion feedback loop. In contrast to a motor-control PWM, a PWM designed for SMPS support can accommodate a wide variety of power-supply topologies, such as push-pull, half-bridge, and full-bridge modes. Designers will take full advantage of full digital loop control for both voltage-and current-mode operation, as well as defining new digital topologies and control strategies beyond today's textbooks.
Speech And Communications
Speech and communications applications often require the computing power of a DSP and the appropriate peripherals. Many customers want to add DSP-enabled features, but they're discouraged by the cost and time required to add a DSP chip to the design and license the appropriate library. DSCs can provide invaluable assistance in these cases, because the microcontroller and DSP resources are contained in one device.
Moreover, Microchip adopted a strategy in which engineers can use its application libraries for development at low or no cost. Current speech libraries from Microchip include echo cancellation (acoustic and line), speech compression and decompression (several standards), noise-suppression, and U.S. English speech recognition. Example communication libraries include soft modems, a TCP/IP stack, and encryption libraries.
DSCs are appearing more frequently in battery-powered, portable applications. Features such as very low sleep current and a wide range of operating voltages and clock frequencies are critical to proper operation. High-endurance EEPROM can be very useful in this type of environment.
Selecting the right MCU or DSC involves more than selecting a device with the right onchip resources. Software, tools, and their impact on time-to-market are also major factors. Choices made today will affect time-to-market and market competitiveness for generations of products to come.