Chapter 16 - Programmable ICs

Abstract

One of the first techniques that allowed users to program their own devices was—and still is—known as fusible-link technology. Devices based on fusible-link technologies are said to be One-Time Programmable (OTP), because once a fuse has been blown it cannot be replaced and there’s no going back. In its unprogrammed state, an antifuse has such a high resistance that it may be considered to be an open circuit. This is the way the device appears when it is first purchased. However, antifuses can be selectively “grown” by applying pulses of relatively high voltage and current to the device’s inputs. The first programmable integrated circuits were generically referred to as Programmable Logic Devices (PLD). The first of the simple PLDs were Programmable Read-Only Memories (PROMs), which jumped into the limelight in 1970. One way to visualize the manner in which these devices perform their magic is to consider them as consisting of a fixed array of AND functions driving a programmable array of OR functions. In order to address the limitations imposed by the PROM architecture, the next step up the PLD evolutionary ladder was that of Programmable Logic Arrays (PLAs), which first became available circa 1975. These were the most user-configurable of the simple PLDs, because both the AND and OR arrays were programmable. In order to address the speed problems posed by PLAs, a new class of device called Programmable Array Logic (PAL) was introduced in the late 1970s.