Microcontroller Application Technology Selection (6)

Author: Chief Editor: He Limin
Publisher:
Publish Date: 1999-09-01
Features: Introduction to the Selected Topics of Microcontroller Application Technology (6) This book compiles 499 articles on microcontroller applications from more than 30 domestic science and technology journals in 1997, including 115 full-text articles and 384 abstracts. The book is divided into eight chapters: comprehensive application technology of microcontrollers, sensor interfaces, data acquisition and conversion processing, networking, communication, and data transmission; reliability design and anti-interference technology; control system and power interface technology; power technology; application examples; and article abstracts. This book has significant practical value. The new technologies, new devices, and software and hardware information of microcontroller application systems introduced in the book help reduce repetitive work in product development processes, improve the application technology level of microcontrollers, and serve as an important reference book for professionals engaged in microcontroller application development. Excerpt: 1.1 Development and Trends of Memory Technology Yangzhou University Normal College Department of Physics (225002) Dou Zhenzhong
1. Overview
Memory is one of the most important components in a computer. The von Neumann computer program storage principle utilizes the memory function of memory to store programs and data, allowing the computer to operate automatically without human intervention. Its access time and storage capacity directly affect the performance of the computer, and it was once the main part of the computer's size and cost. With the significant development of large-scale integrated circuits and storage technology, the integration of semiconductor memory has been increasing at a rate of doubling every three years, and the proportion of volume and cost occupied by the same capacity memory in computers has become increasingly smaller.
1. Classification of Memory
From the perspective of usage function, semiconductor memory can be divided into two major categories: volatile memory, which loses data when power is turned off, and non-volatile memory, which does not lose data when power is turned off. In the past, volatile memory that could randomly read and write information was called RAM (Random Access Memory). Based on different working principles and conditions, RAM is further divided into static RAM (SRAM) and dynamic RAM (DRAM). Non-volatile memory in the past was all Read-Only Memory (ROM), which could only be written to offline and could only read information during use without the ability to write or modify it. Non-volatile memory includes various memories with different principles, technologies, and structures. Traditional non-volatile memory can be further divided into Mask ROM (MROM), One-Time Programmable ROM (OTPROM), and Ultraviolet-Erasable Programmable ROM (UV-EPROM) based on writing methods and the number of writable times. In the past, OTPROM was all bipolar fuse-based, and such chips could only be programmed once, making it impossible to test product programming performance during the testing stage. Therefore, defects are often discovered only after programming, leading to failure, and some chips, although programmable, cannot operate normally due to their insufficient AC performance. As a result, the reliability of bipolar fuse-based PROM products is not high. Recently, many companies have introduced OTPROM using EPROM technology, where all chips are tested for programming and performance during production, and erased before packaging, ensuring that each chip is programmable and meets qualified performance standards.
2. Field-Programmable Non-Volatile Memory
In the memory market, the demand for non-ROM-type field-programmable non-volatile memory has been growing rapidly in recent years, and these chip technologies are rapidly changing the landscape of the memory world. This mainly includes Electrically Erasable Programmable Read-Only Memory (EEPROM), Non-Volatile Static RAM (NVSRAM) that uses lithium batteries as backup power for data protection, Flash Memory developed based on the technology of EPROM and EEPROM, and Ferroelectric RAM (FRAM) that uses the polarization direction of ferroelectric materials to store data. With the development of new semiconductor memory technologies, various field-programmable non-volatile memories have been introduced to the market. First is EEPROM (Electrically Erasable Programmable Read-Only Memory), which has a relatively slow write speed. To improve write speed, Xicor combined SRAM with EEPROM, introducing Non-Volatile RAM (NOVRAM), which combines the advantages of both. In 1983, Intel proposed the ETOX (Eprom Tunnel Oxide) principle based on the tunnel oxide layer of EPROM, which improved the erase/write performance of EPROM and introduced Flash Memory in 1988. Subsequently, Toshiba introduced Flash Memory based on the Fowler-Nordheim cold electron erase principle and the NAND architecture of EEPROM. From a principle perspective, Flash Memory belongs to ROM-type memory but can be rewritten at any time, and from a functional perspective, it is similar to RAM, making the traditional definitions and distinctions between ROM and RAM gradually obsolete. These non-volatile memories, whether EPROM or EEPROM, or Flash Memory, are based on charge storage principles to store information. The writing of information in these devices uses an electric field to force electrons through a semiconductor thin layer, and after multiple erases, the movement of these electrons eventually leads to the breakdown of the gate oxide layer, causing device damage. Another fatal flaw of these devices is the relatively slow time required to rewrite information. In 1984, Dallas Semiconductor introduced Non-Volatile Static RAM (NVSRAM) using long-life lithium batteries as backup power, adding data protection circuits to low-power SRAM chips, creating IBBSRAM (Integrated Battery-Backed SRAM). Other performance and usage are the same as static RAM, but the information can be preserved for 10 years in the event of a power outage. The drawback is the higher cost, and once the internal lithium battery fails, the chip cannot be regenerated. Additionally, a new type of memory has emerged on the market, which has improved the performance of non-volatile memory in many aspects and is expected to become a more ideal non-volatile memory. This is FRAM (Ferroelectric RAM) developed by Ramtron after 15 years of research and development, utilizing the polarization principle of ferroelectric materials under different electric fields. Recently, SIMTEK has introduced a new type of Non-Volatile Static RAM (nvSRAM), which may become a new star in the memory field.
3. Rapidly Developing Flash Memory
Flash memory does not require storage capacitors, allowing for higher integration and lower manufacturing costs than DRAM. It is user-friendly, offering the flexibility and fast access speed of SRAM reading and writing, as well as the non-loss of information feature of ROM when power is turned off. Therefore, flash memory technology has developed rapidly. Since the first commercial flash memory was introduced in 1988, more than 40 semiconductor companies have rushed to develop and manufacture to compete for a share of the market, with Intel, AMD, and Atmel holding the largest market share. Its capacity has evolved from the initial 64KB to current offerings of 16MB to 32MB from many manufacturers. In 1994, NEC succeeded in developing a 64MB flash memory and is now accelerating the development of 256MB flash memory. The number of rewrite cycles for flash memory has improved from the initial 100 to the current 1 million. As flash memory technology continues to develop, it is increasingly replacing EPROM, and when its price falls below that of EPROM, EPROM will eventually be phased out. Intel's flash memory based on ETOX technology requires two voltages: 5V for read operation and 12V for write operation. If a single power supply voltage is used, a DC-DC converter is needed to change the voltage. In April 1993, AMD introduced flash memory using Negative Gate technology, enabling it to operate under a single 5V power supply, making the chip more convenient to use and well-received by customers. To regain market share lost to AMD, Intel has recently adopted "Smart Voltage" technology in its new products: using two power supply pins—"read voltage pin" and "write voltage pin"—with read operations available in 3.3V or 5V and write operations available in 5V or high-speed 12V, offering four different operation voltage modes to provide flexibility based on needs (low voltage or high-performance requirements).