Electrically Erasable PROMs (EEPROMs)

The term EEPROM (electrically erasable PROM) refers to a PROM which, unlike the EPROM, can also be erased electrically. With the more recent types, the voltage converter for generating the programming voltage and the timer for determining the programming pulse duration are incorporated in the memory chip.

However, despite these simple, fast erase and write procedures, one should not be tempted to use an EEPROM as a RAM. The number of possible write cycles is in fact limited: no byte must be written more than 10⁴... 10⁶ times (depending on the type). With a programming time of 1 ms, the end of the operational life of a byte or a page may be reached in as little as 10 s if continuous programming is performed.

Flash EEPROMs are intermediates between EPROMs and EEPROMs. Like EEP­ROMs, they can be erased electrically, but not byte by byte, as in the case of EPROMS - here, the entire block of 1... 32 kbyte is erased at once. This is the reason for their name. They are erased much more simply than EPROMs: a single erase pulse lasting for a few seconds is required. It is not necessary to take the package out of the circuit and put it into an eraser unit for 20 min. Flash EEPROM technology is simpler than that for stan­dard EEPROMs. Correspondingly, large integration densities can therefore be achieved, consistent with low prices.

10.4. Programmable Logic Devices (PLDs)

PLDs are used for storing logic functions. The categories in Fig. 10.1 show that there» are three variants: PLEs, PALs, and PLAs. The differences between them are in respect of programming flexibility. PALs (programmable array logic) are the easiest to program. They are therefore particularly popular and are available in a wide range of designs. PLAs (programmable logic arrays) are basically more flexible, but it is more complicated to program them. They have therefore ceased to be of major importance. Gate Arrays consist of a see of gates or primitive logical functions the connection of which is configured by programming. GAs are programmed by the manufacturer, therefore they are used with large quantities only. Very popular are the Field Programmable Gate Arrays FPGAs because they are user programmable like PLDs.

10.4.1. Programmable Logic Array (PAL)

PALs are the most important representatives of programmable logic devices (PLDs). They are available in a wide range of variants. The differences are in the implementation of the output circuit.

10.4.4. User Programable Gate Arrays

One group of logic circuits that can also be programmed by the user is that if the Field Pro­grammable Gate Arrays (FPGAs). They contain many Configurable Logic Blocks, CLBs that can be connected by column- and row-interconnect busses.

Дата добавления: 2014-01-11