Single chip microcomputer scheme is classified according to purpose: general type: provide all exploitable resources (ROM, ram, I / O, EPROM) to users. Special type: its hardware and instructions are designed according to a specific purpose, such as recorder movement controller, printer controller, motor controller, etc. According to the classification of data bits processed by its basic operation: according to the width of bus or data register, single chip microcomputer is divided into 1-bit, 4-bit, 8-bit, 16 bit, 32-bit and even 64 bit single chip microcomputer. Most of the 4-bit MCU is used in calculators, vehicle instruments, vehicle anti-theft devices, pagers, wireless phones, CD players, LCD drive controllers, LCD game consoles, children's toys, scales, chargers, tire pressure gauges, thermohygrometers, remote controls and fool cameras; 8-bit MCU is mostly used in electric meter, motor controller, electric toy machine, frequency conversion air conditioner, pager, fax machine, caller ID, telephone recorder, CRT display, keyboard and USB, etc; 8-bit and 16 bit MCU are mainly used in the general control field, and generally do not use the operating system. Most of the 16 bit MCU are used in mobile phones, digital cameras and video recorders; 32-bit MCU is mostly used in modem, GPS, PDA, HPC, STB, hub, bridge, router, workstation, ISDN telephone, laser printer and color fax machine; 32-bit is used for complex processing occasions such as network operation and multimedia processing. Generally, embedded operating system should be used. 64 bit MCU is mostly used in high-level workstations, multimedia interactive systems, advanced video game devices (such as Sega's Dreamcast and Nintendo's Gameboy) and advanced terminals. The operating frequency of 8-bit MCU is between 16 ~ 50MHz, emphasizing simple efficiency and low-cost application. It still has a certain position in the current total market value of MCU, and many MCU manufacturers continue to develop energy-saving design of frequency regulation for 8-bit MCU to meet the product development needs of the green era. 16 bit MCU takes 16 bit operation, 16 / 24 bit addressing capability and frequency of 24 ~ 100MHz as the mainstream specification, and some 16bit MCU additionally provides 32-bit add / subtract / multiply / divide special instructions. Due to the emergence and continuous price reduction of 32bit MCU and the low price advantage of simple, durable and cheap 8bit MCU, the 16bit MCU market sandwiched in the middle is constantly squeezed and becomes a product with a low proportion of shipments. 32-bit MCU can be said to be the mainstream of MCU market. The quotation of a single MCU is between us $1.5 and US $4, and the operating frequency is mostly between 100 and 350MHz. It has better execution efficiency and a wide range of application types. However, due to the increase of operand and memory length, the length of program code with the same function will increase by 30 ~ 40% compared with 8 / 16bit MCU, which leads to the memory capacity of embedded OTP / flashrom can not be too small, while the number of external pins of the chip increases sharply, which further limits the cost reduction ability of 32bit MCU.