以MC68HC11K4和 MC68HC11N4微控制器为例,

PID (proportional, integral, derivative) compensation is one of the most common forms of closed-loop control. Control of closed-loop systems that require compensation is a growing area of application for embedded microprocessors. In these systems, analog signals must be converted into discrete digital samples before compensation or filtering can take place. Loop performance dictates the sampling rate, and calculations must be complete before the next sample time begins. These loop-related constraints and the Nyquist frequency requirement place an upper bound on digital control of closed systems with feedback error. If the controlled system has a resonance or other behavior with a time constant shorter than the sample and calculation time, chaos is the most likely outcome. Despite these limitations, increases in microprocessor clock rates and the addition of on-chip control-oriented hardware are expanding the number of medium performance control applications handled by 8-bit machines. While an expensive DSP-class processor is the correct choice for the most demanding applications, several members of the M68HC11 family have the speed and resources to control multiple PWM channels.