Simple Prograqmmable Logic Device (SPLD)

A programmable logic device (PLD) is an IC that can be configured to implement a variety of logic functions, ranging from tens of gates to thousands of gates. PLDs became popular in the 1970s because PLDs could implement far more functionality in a single IC than was possible using SSI ICs.

A PLD device contains a prefabricated circuit with a set of external inputs feeding into a large AND-OR circuit structure, with the special feature of allowing the user to configure which external inputs connect to the AND gates.

Figure shows a basic PLD with three inputs feeding into three AND gates followed by an OR gate. The inputs feed into the AND gates in both true and complemented forms. Each wire feeding into each AND gate passes through a programmable node, which can either pass the node's input to the node's output or disconnect the node's input from the node's output. Thus, by programming the nodes, a PLD can implement any 3 term function of three inputs.

The programmable node design varies among types of PLDs. Figure shows two types. The type shown in figure (a) is fuse based PLD. A fuse conducts like a wire, unless the fuse is blown, meaning a higher than normal current is passed through the fuse, causing the fuse to literally burn up and break. A blown fuse obviously does not conduct electricity. The type shown in figure (b) is based on memory and a transistor programming a 1 into the memory causes the transistor to conduct, while programming a 0 causes the transistor to not conduct. We omit the details of how to program the fuses or program the memories themselves. Memory based PLDs can usually be reprogrammed, in contrast to fuse based PLDs that can only be programmed once, and that are known as one-time programmable (OTP) devices.

Fuse based PLDs are popular in electrically noisy applications, like space applications, since memories can have their contents changes from radiation in space. They are also popular in applications demanding high security, since malicious enemies cant reprogram the device. Memory based devices are more common, however, since they can be reprogrammed and thus reduce costs when we make design changes. The memories used are almost always nonvolatile, meaning the memories dont need power to retain their stored bits.

Complex Programmable Logic Device (CPLD)

As IC transistor densities grew in the 1980s, companies began to build PLDs to support thousands of gates. However, the PLD architecture described in the previous sections does not scale well to thousands of gates who needs one big huge circuit of two level logic. Instead, architectures evolved that consisted of numerous SPLDs on a single device, connected using switch matrices. These devices today are known as complex PLDs or CPLDs. CPLDs can typically implement designs with thousands of gates.