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Digital Electronics
that have evolved over the years for programming PLDs include fuses, EPROM or EEPROM floatinggate transistors, static RAM and antifuses.
Each one of these is briefly described in the following paragraphs.
9.9.1 Fuse
A fuse is an electrical device that has a low initial resistance and is designed permanently to break an
electrically conducting path when current through it exceeds a specified limit. It uses bipolar technology
and is nonvolatile and one-time programmable. It was the first user-programmable switch developed
for use in PLAs. They were earlier used in smaller PLDs and are now being rapidly replaced by newer
technologies.
9.9.2 Floating-Gate Transistor Switch
This interconnect technology is based on the principle of placing a floating-gate transistor between
two wires in such a way as to facilitate a WIRE-AND function. This concept is used in EPROM and
EEPROM devices, and that is why the floating-gate transistor is sometimes referred to as an EPROM
or EEPROM transistor. Figure 9.30 shows the use of floating-gate transistor interconnects in the AND
plane of a CPLD or SPLD. All those inputs that are required to be part of a particular product term
are activated to drive the product wire to a logic ‘0’ level through the EPROM transistor. For inputs
that 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 switch
matrix, however, requires a large number of interconnects and therefore transistors. For example, a
CPLD with 128 macrocells with four inputs and one output each would require as many as 65 536
interconnects for 100 % routability. A large number of interconnects also adds to the propagation delay.
Input
Wire
Product
Wire
Vcc
Figure 9.30 Floating-gate transistor interconnect.
Digital Electronics
that have evolved over the years for programming PLDs include fuses, EPROM or EEPROM floatinggate transistors, static RAM and antifuses.
Each one of these is briefly described in the following paragraphs.
9.9.1 Fuse
A fuse is an electrical device that has a low initial resistance and is designed permanently to break an
electrically conducting path when current through it exceeds a specified limit. It uses bipolar technology
and is nonvolatile and one-time programmable. It was the first user-programmable switch developed
for use in PLAs. They were earlier used in smaller PLDs and are now being rapidly replaced by newer
technologies.
9.9.2 Floating-Gate Transistor Switch
This interconnect technology is based on the principle of placing a floating-gate transistor between
two wires in such a way as to facilitate a WIRE-AND function. This concept is used in EPROM and
EEPROM devices, and that is why the floating-gate transistor is sometimes referred to as an EPROM
or EEPROM transistor. Figure 9.30 shows the use of floating-gate transistor interconnects in the AND
plane of a CPLD or SPLD. All those inputs that are required to be part of a particular product term
are activated to drive the product wire to a logic ‘0’ level through the EPROM transistor. For inputs
that 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 switch
matrix, however, requires a large number of interconnects and therefore transistors. For example, a
CPLD with 128 macrocells with four inputs and one output each would require as many as 65 536
interconnects for 100 % routability. A large number of interconnects also adds to the propagation delay.
Input
Wire
Product
Wire
Vcc
Figure 9.30 Floating-gate transistor interconnect.