Книга: Real-Time Concepts for Embedded Systems

1.1.8 Software Storage and Upgradeability

1.1.8 Software Storage and Upgradeability

Code for embedded systems (such as the real-time embedded operating system, the system software, and the application software) is commonly stored in ROM and NVRAM memory devices. In Chapter 3, we discuss the embedded system booting process and the steps involved in extracting code from these storage devices. Upgrading an embedded system can mean building new PROM, deploying special equipment and/or a special method to reprogram the EPROM, or reprogramming the flash memory.

The choice of software storage device has an impact on development. The process to reprogram an EPROM when small changes are made in the software can be tedious and time-consuming, and this occurrence is common during development. Removing an EPROM device from its socket can damage the EPROM; worse yet, the system itself can be damaged if careful handling is not exercised.

The choice of the storage device can also have an impact on the overall cost of maintenance. Although PROM and EPROM devices are inexpensive, the cost can add up if a large volume of shipped systems is in the field. Upgrading an embedded system in these cases means shipping replacement PROM and EPROM chips. The embedded system can be upgraded without the need for chip replacement and can be upgraded dynamically over a network if flash memory or EEPROM is used as the code storage device (see the following sidebar).

Armed with the information presented in the previous sections, we can now attempt to answer the questions raised earlier. A personal computer is not an embedded system because it is built using a general-purpose processor and is built independently from the software that runs on it. The software applications developed for personal computers, which run operating systems such as FreeBSD or Windows, are developed natively (as opposed to cross-developed) on those operating systems. For the same reasons, an Apple iBook used only as a DVD player is used like an embedded system but is not an embedded system.

With non-volatile content and without the need for an external power source.

· Mask Programmed ROM - the memory content is programmed during the manufacturing process. Once programmed, the content cannot be changed. It cannot be reprogrammed.

· Field Programmable ROM (PROM) - the memory content can be custom-programmed one time. The memory content cannot change once programmed.

· Erasable Programmable ROM(EPROM) - an EPROM device can be custom-programmed, erased, and reprogrammed as often as required within its lifetime (hundreds or even thousands of times). The memory content is non-volatile once programmed. Traditional EPROM devices are erased by exposure to ultraviolet (UV) light. An EPROM device must be removed from its housing unit first. It is then reprogrammed using a special hardware device called an EPROM programmer.

· Electrically Erasable Programmable ROM (EEPROM or E2PROM) - modern EPROM devices are erased electrically and are thus called EEPROM. One important difference between an EPROM and an EEPROM device is that with the EEPROM device, memory content of a single byte can be selectively erased and reprogrammed. Therefore, with an EEPROM device, incremental changes can be made. Another difference is the EEPROM can be reprogrammed without a special programmer and can stay in the device while being reprogrammed. The versatility of byte-level programmability of the EEPROM comes at a price, however, as programming an EEPROM device is a slow process.

· Flash Memory - the flash memory is a variation of EEPROM, which allows for block-level (e.g., 512-byte) programmability that is much faster than EEPROM.

Also called Read/Write Memory, requires external power to maintain memory content. The term random access refers to the ability to access any memory cell directly. RAM is much faster than ROM. Two types of RAM that are of interest:

· Dynamic RAM(DRAM) - DRAM is a RAM device that requires periodic refreshing to retain its content.

· Static RAM(SRAM) - SRAM is a RAM device that retains its content as long as power is supplied by an external power source. SRAM does not require periodic refreshing and it is faster than DRAM.

· Non-Volatile RAM (NVRAM) - NVRAM is a special type of SRAM that has backup battery power so it can retain its content after the main system power is shut off. Another variation of NVARM combines SRAM and EEPROM so that its content is written into the EEPROM when power is shut off and is read back from the EEPROM when power is restored.

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