Understanding Smart Sensors - Nomads.usp

Using MCUs/DSPs to Increase Sensor IQ 105MIPS rating. The features of a specific 16-bit DSP, the DSP56L811, includethe following:• 40-MHz, 20-MIPS operation from 2.7V to 3.6V;• One-cycle multiply-accumulate shifter;• 16-bit instruction and 16-bit data word;• Two 36-bit accumulators;• Three serial ports;• Sixteen I/Os, two external interrupts;• Power dissipation of 120 mW at 40 MHz.The need for real-time processing in several systems is causing control systemengineers to evaluate and use DSP technology for the control function.This growing class of functions cannot work effectively with traditional tablelook-up and interpolate functions to make the control decision. Instead, amultiply-accumulate unit allows state estimator functions to be implementedwith an algorithm defining the state. However, as shown in Table 5.2, MCUshave an advantage over DSP units in many areas except real-time operating efficiency.The question that system designers must answer is, “What sample rateand performance level are required for the application?” Also, the possibility ofcombining the new system with a previously developed or concurrently developedsystem should be evaluated. Depending on the nature of the system(s)and the availability of control alternatives, a change from MCU to DSP maynot be required.DSP technology is evolving in two ways. First, DSP-like performance canbe approached when a time processor unit (TPU) is included in an MCUdesign, which is possible in higher performance MCUs. The TPU is a programmablemicrostate machine that addresses requirements for computationand greatly reduces the overhead on the main processor. That allows the mainprocessor to calculate strategy-related items and not have to decide when a specificactivity is initiated, such as firing the next sparkplug in an automotiveengine control system. In some applications, the main signal processor is aDSP. In other cases, multiple devices can be integrated on the same chip, especiallywith the capability of integrating an increasing number of transistors perchip. For example, multiple CPUs are used for fault tolerance in safety systems,such as automotive antilock braking systems.Second, in high-end applications, such as an automotive near-obstacledetectionsystem or noise-cancellation system, a dedicated 24-bit DSP is used.