单片8位微处理器可满足很多嵌入式控制应用需要,因为它能以很低的代价把很多系统控制功能连同处理器集成在一块芯片上.但是,随着系统的需要越来越复杂,微控制器单元(MCU)在芯片上所占的面积也日益增大,从而使芯片的价格变得非常昂贵.此外,很多应用子任务(计数、定时、串行和并行通信、脉宽调制,等等)并不需要很高的CPU性能,通常只需一种几兆赫的CPU即可满足需要,因为片上逻辑支持这些功能. A single 8-bit microprocessor caters to many embedded control applications because it integrates many system control functions with the processor on a single chip at a fraction of the cost, but as the system needs become more complex, Microcontroller units (MCUs) are also becoming more expensive on chips, making them expensive. In addition, many of the application subtasks (counting, timing, serial and parallel communications, pulse width modulation, Etc.) does not require high CPU performance and usually only needs a few megahertz CPU to meet the need, because on-chip logic supports these functions.