[Technology Report]
Divide And Conquer: On-Chip Hardware Adjuncts Accelerate MCUs
Enhanced on-chip peripherals or asymmetric multiprocessing helps optimize performance and power consumption, avoiding the need to resort to higher-power processors.
Matching microcontrollers to most applications is a relatively simple feat. The task becomes more onerous, though, when high performance meets low-power requirements. Portables, remote monitoring systems, and even low-cost motor controls fall into this category. Choosing MCUs augmented with specialized hardware or additional processors is one way to garner the performance without moving up to higher-performance processors.
Software delivers the goods at one end of the spectrum, while ASICs lie at the other extreme. Sitting somewhere in between are most MCUs. One 8-bit example, Zilog's Z8 Encore! XP, has a 20-MHz eZ8 processor surrounded by the usual complement of serial and parallel interfaces. It also adds a few useful twists like a UART port with IrDA encoder/decoder, a temperature sensor, and a transimpedance amplifier. These peripherals can simplify the job that the processor must perform, but the processor remains the center of attention.
To reduce power, many MCUs can power down the processor while letting peripherals continue to operate. The peripheral restarts the processor when a particular action occurs, such as receiving a character or the occurrence of a timeout. MCUs typically weave in a standard set of peripherals, including serial ports, general-purpose I/O (GPIO) ports, and timers. Analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) are also standard for MCUs servicing analog interfaces.
Servicing this array of peripherals keeps an MCU humming. However, power becomes an issue when the processor must run at full speed and all peripherals are active. The ability to power down unused or idle peripherals can reduce consumption, but the processor is typically the most power-hungry component. Thus, minimizing the processor's power requirements will take a serious bite out of overall system consumption.
SPECIALIZED PERIPHERALS In applications such as motor control, specialized peripherals can help low-end 8- and 16-bit MCUs boost performance. They tend to deliver more performance so these MCUs can handle a chore. In addition, they can slow or shut down 32- and 64-bit MCUs while continuing to run. Specialized peripherals often handle bit manipulations that are less efficient with 32- or 64-bit registers.
A host of specialized peripherals is available on standard MCUs. Many target a specific application or class of application. Texas Instruments' (TI) MSP430FE42x ESP430 power-meter MCU incorporates a set of ADCs and a modified version of the main MSP430, the DSP430 (Fig. 1). The subsystem is specifically designed to obtain power-meter readings where overall system power consumption must be minimized. A programmer can't modify the subsystem, which was programmed by TI. So, it's a specialized but fixed-function peripheral.
Another TI chip, the MSP430FW42x Single-Chip W-Meter, is designed as a water-meter recording device. It blends together a 10-bit DAC and comparator along with two state machines that are accessible to the programmer. This allows for more customization, enabling the peripheral to operate on its own so the main processor can shut down when idle.
DMA (direct memory access) support is one reason why TI's MSP430 line can take advantage of its peripherals. Thanks to the DMA flexible implementation, an application can configure and forget both DMA and specialized-peripheral support.
MOTOR CONTROL AND TIMING In some cases, DMA support and application-specific peripherals simply don't provide enough flexibility. This comes up often in motor control, where timing details can become very complex. In this instance, a programmable solution that's timing-specific but less than a digital-signal-processor (DSP) coprocessor may be the right choice.
To that end, Motorola/Freescale Semiconductor has a range of Time Processor Units (TPUs) with various MCUs. The eTPU (enhanced TPU) is the latest incarnation, winding up in products like the Coldfire MFC5232 (Fig. 2). The eTPU consists of one or two programmable micro engines. Unlike specialized peripherals, the eTPU requires developers to create a substantial amount of code to get useful work out of the peripheral. Here, it's a matter of handling timing-based I/O such as pulse-width modulation (PWM) and quadrature encoder support often used with motor-control applications.
For simple PWM control, developers can employ standard support code for the eTPU. Or to make the eTPU sing, they can turn to third-party programming products. Ash Ware's eTPU Development Kit provides a C compiler for the eTPU, enabling MCU programmers to configure the eTPU.
These intelligent on-chip peripherals essentially bring asymmetric multiprocessing to MCUs. Ubicom's hardware-based multiprocessing support also brings symmetric multiprocessing to its MCU.
Still, don't overlook MCUs specifically designed for such applications as motor control. NEC's 8-bit, 20-MHz D78F0714 and the 32-bit, 32-MHz V850ES/IK1 target three-phase motor control for a single motor. While it may appear from the spec sheet to have the usual collection of timer and analog interfaces, a closer look reveals a set of interconnected peripherals that readily handle a specific task within a range of parameters.
MULTIMEDIA Mobile multimedia devices resemble motor-control applications in that a preselected set of peripherals can often address an application. The processors often reside at the high end of the spectrum, but they're typically augmented with peripheral hardware that provides even more performance with lower power requirements.
Take the Renesas SH-MobileV2 RISC-based MCU, which targets portable multimedia devices (e.g., cell phones). It has a 32-bit, 133-MHz SH3-DSP CPU core with Java support and built-in DSP functions, plus a 2D/3D graphics engine cache. Intelligent peripherals include the on-chip MPEG-4 acceleration, a two-channel sound I/O unit, and a color-management unit that generates color conversions tailored to the characteristics of various displays. Of course, the SH-MobileV2 has typical MCU peripherals like an IrDA interface and a USB 2.0 interface.
CAN COMMUNICATION Targeting a specific aspect of an application works even better when a standard exists, such as CAN (controller-area network). CAN support can be implemented with a simple serial port, but an intelligent implementation can significantly reduce system overhead (Fig. 3).
A sophisticated CAN interface like that found in the Infineon C505 line or in MCUs from companies like Microchip and Mitsubishi Electric have an application interface based on a mailbox architecture. Here, the main processor simply drops data in a specific location for a remote device. The CAN interface moves the data automatically, including handling of all the necessary protocol overhead. As with many intelligent peripherals, these CAN interfaces often operate independently of the main processor. They wake the processor when an event occurs, such as receiving data from another device.
Reprints
