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Computers
Published in John G. Webster, Halit Eren, Measurement, Instrumentation, and Sensors Handbook, 2017
A.M. MacLeod, P.F. Martin, W.A. Gillespie
An overview of a simple SBC is shown in Figure 89.2. The microprocessor, which contains the central processor unit, is responsible for executing the computer program and for controlling the flow of data around the SBC. The random access memory (RAM) acts as a temporary (i.e., volatile) storage area for both program code and data. On the motherboard of a PC, the read-only memory (ROM) is largely used for storing the low-level code used to access the input/output hardware, e.g., the BIOS (basic input–output system). The operating system and applications software are loaded into RAM from the disk unit. In small, dedicated systems such as an oscilloscope the ROM may be used to store the program code to avoid the need for a disk drive. For small production runs or development work, erasable programmable ROM (EPROM) is used as an alternative to ROM, allowing the code to be upgraded without the high cost of commissioning a new ROM chip from the manufacturers.
Basic concepts
Published in Zdravko Karakehayov, Knud Smed Christensen, Ole Winther, Embedded Systems Design with 8051 Microcontrollers, 2018
Zdravko Karakehayov, Knud Smed Christensen, Ole Winther
The ROM components are further classified into three essential types: Mask Programmable ROM (MPROM), which is frequently referred to as ROM, is a factory programmable component. The ROM may be a cost effective solution only if ordered in high volumes.The Programmable ROM (PROM), as you might expect, is programmed by the user. The PROM contains fuses that once blown can not be recovered. An equivalent term OTP (One Time Programmable) memory is very popular for the microcontroller’s internal PROM.Taking into account the requirements of the design process, we’ll see a desperate need for Erasable PROM (EPROM) components. At this level of the taxonomy we distinguish between UVEPROM (erasable by ultraviolet light) and EEPROM (electrically erasable PROM). The UVEPROM devices are similar to the OTP memories, but they have a transparent window in the package for exposure.The EEPROM is erased by a special electrical mode and there is no need for the device to be exposed with ultraviolet light, which takes a long time. The EEPROMs can be further divided into out-of-circuit programmable and in-circuit programmable (flash memories). Currently, the flash memory technology can offer higher density than other EEPROM processes. Flash memory is fast, reliable and suitable for a large spectrum of applications.
Widget Deconstruction #2: USB Flash Drive
Published in John D. Cressler, Silicon Earth, 2017
The very first ROMs were fairly primitive hardwired devices that contained a preprogrammed set of instructions. The contents of the ROM had to be specified before IC manufacturing began so that the actual data to be stored could be used to hardwire the connections to the transistors inside the ROM by use of the various metalization layers. One step up from this primitive ROM is the PROM (programmable ROM), which is manufactured and purchased in an unprogrammed state. If you examined the contents of an unprogrammed PROM, you would see that the data are made up entirely of “1s” (not very useful). The process of writing your data to the PROM involves a special piece of equipment called a “device programmer” or “PROM writer.” The PROM writer electrically writes data to the PROM one binary word at a time by applying a voltage to the input pins of the chip. Once a PROM has been programmed in this way, its contents can never be changed, and thus if the code or data stored in the PROM need to be changed, a trash can is the only option. An EPROM (erasable-and-programmable ROM), on the other hand, is programmed in exactly the same manner as a PROM; however, EPROMs can be erased and reprogrammed repeatedly. To erase the EPROM, you simply expose the device to a strong source of ultraviolet (UV) light. An easy indicator that you have an EPROM in your hands is the transparent window in the top of the package that allows the UV light to reach the transistors. After UV exposure, you essentially reset the entire chip to its initial unprogrammed state. This is getting handy now, although carrying around a strong UV lamp (can you say “tanning bed”) is admittedly a bit cumbersome! EEPROMs (electrically erasable-and-programmable ROMs—read: “double-E-prom”) represent the top of the ROM ladder, and are electrically erasable and programmable. Internally, they are similar to EPROMs, but the erase operation is accomplished electrically, rather than by exposure to UV light. In addition, data within an EEPROM can be selectively erased and rewritten, many times. Once the EEPROM is written, the new data are permanent. How on earth do we electrically erase and program a ROM memory? Very cleverly! In essence, we embed a second gate within the MOSFET and make it electrically floating (not connected to anything). As shown in Figures 6.18 and 6.19, we can then change the terminal voltages to induce electrons either into or out of the floating gate, changing the properties of the ROM cell transistor (i.e., shift the turn-on or threshold voltage of the device). For the not-so-faint-of-heart, this all happens via quantum-mechanical tunneling. The quantum world in action! If you are curious about the details of how this erase/program mechanism changes the behavior of the transistor, you’ll get some more exposure to how MOSFETs actually work in Chapter 8, so stay tuned. The primary trade-off for this improved functionality with EEPROMs is their increased complexity and hence higher cost, although write times are much longer than for DRAMs or SRAMs, preventing you from using EEPROMs for your main system memory, except in very unusual conditions. Drum roll please! Next stop, flash memory!
Radiation-Induced Damage–Based System and Method for Indirectly Monitoring High-Dose Ionizing Radiation
Published in Nuclear Technology, 2018
Karen Colins, Yu Liu, Liqian Li, Kiranpreet Birdee
Read-only memory (ROM) plays an important role in modern computing systems to store lookup tables for the evaluation of mathematical and logical functions, etc. Also, a typical IC is sensitive to radiation damage. In past decades, several types of nonvolatile ROM were invented and then widely used in different fields. Among them, ultraviolet-erasable programmable ROM (EPROM) needs only one transistor for one cell, but it requires high voltage for programming. In contrast, an EEPROM cell consists of two transistors, and the high voltage for program/erase is internally generated. An EEPROM is programmed by forcing an electrical charge (a range of 105 to 106 electrons) onto a polysilicon floating gate. When the charge is present, it represents a logical value of false, or 0, and when this charge is not present, it represents a logical value of true, or 1.