Digital Electronics: Principles, Devices and Applications

Computer Fundamentals 629erasability feature of EEPROM without compromising the high-speed access of both. Structurally, thememory cell of a flash memory is like that of an EPROM. The basic memory cell of a flash memoryis shown in Fig. 15.19. It is a stacked-gate MOSFET with a control gate and floating gate in additionto drain and source. The floating gate stores charge when sufficient voltage is applied to the controlgate. A ‘0’ is stored when there is more charge, and a ‘1’ when there is less charge. The amount ofcharge stored on the floating gate determines whether or not the MOSFET is turned ON.It is called a flash memory because of its rapid erase and write times. Most flash memory devicesuse a ‘bulk erase’ operation in which all the memory cells on the chip are erased simultaneously. Someflash memory devices offer a ‘sector erase’ mode in which specific sectors of the memory devicecan be erased at a time. This mode comes in handy when only a portion of the memory needs to beupdated.Figure 15.20 shows the basic array of a 4 × 4 flash memory. As in the case of earlier memories,there is an address decoder that selects the row. During the read operation, for a cell containing a‘1’ there is current through the bit line which produces a voltage drop across the active load. This iscompared with the reference voltage, and the output bit is ‘1’. If the memory cell has a ‘0’, there isvery little current in the bit line. Memory sticks are flash memories. They are available in 4, 8, 16, 32,64 and 128 MB sizes.To sum up, while PROMs are least complex and low cost, they cannot be erased and reprogrammed.UV EPROMs are a little more complex and costly, but then they can be erased and reprogrammed bybeing taken out of the circuit. Flash memories are in-circuit electrically erasable either sectorwise or inbulk mode. The most complex and most expensive are the EEPROMs, but then they offer byte-by-byteelectrical erasability in circuit.15.6.3 Applications of ROMsThe majority of ROM applications originate from the need for nonvolatile storage of data or programcodes. Some of the common application areas include firmware, bootstrap memory, look-up tables,function generators and auxiliary memory.The most common application of ROM chips is in the storage of data and program codes thatmust be made available to microprocessor-based systems such as microcomputers on power-up. Thiscomponent of the software is referred to as firmware as it comes embedded in the hardware with themachine. Even consumer products such as CD players, microwave ovens, washing machines, etc., haveembedded microcontrollers that have a microprocessor to control and monitor the operation accordingto the information stored on the ROM.FloatinggateControlgateDrainSourceFigure 15.19Basic cell of flash memory.