Digital Electronics: Principles, Devices and Applications

334 Digital Electronicsthat have evolved over the years for programming PLDs include fuses, EPROM or EEPROM floatinggatetransistors, static RAM and antifuses.Each one of these is briefly described in the following paragraphs.9.9.1 FuseA fuse is an electrical device that has a low initial resistance and is designed permanently to break anelectrically conducting path when current through it exceeds a specified limit. It uses bipolar technologyand is nonvolatile and one-time programmable. It was the first user-programmable switch developedfor use in PLAs. They were earlier used in smaller PLDs and are now being rapidly replaced by newertechnologies.9.9.2 Floating-Gate Transistor SwitchThis interconnect technology is based on the principle of placing a floating-gate transistor betweentwo wires in such a way as to facilitate a WIRE-AND function. This concept is used in EPROM andEEPROM devices, and that is why the floating-gate transistor is sometimes referred to as an EPROMor EEPROM transistor. Figure 9.30 shows the use of floating-gate transistor interconnects in the ANDplane of a CPLD or SPLD. All those inputs that are required to be part of a particular product termare activated to drive the product wire to a logic ‘0’ level through the EPROM transistor. For inputsthat are not part of the product term, relevant transistors are switched off.This technology is commonly used in SPLDs and CPLDs. A floating-gate transistor based switchmatrix, however, requires a large number of interconnects and therefore transistors. For example, aCPLD with 128 macrocells with four inputs and one output each would require as many as 65 536interconnects for 100 % routability. A large number of interconnects also adds to the propagation delay.VccInputWireProductWireFigure 9.30Floating-gate transistor interconnect.