Field Programmable Devices (FPDs)

1. Purpose

Allow designers to implement complex combinational and sequential logic without creating a custom or semicustom IC.
Provide very high levels of integration but are user-configurable.

2. Main Types of FPDs

Device Type	Description	Example / Components
PLDs (Programmable Logic Devices)	Purely combinational logic; configured by user.	PALs, PLAs, SPLDs, Complex PLDs
FPGAs (Field Programmable Gate Arrays)	Contain both combinational logic and flip-flops (i.e., sequential + combinational).	LUT-based arrays, include RAM blocks, sometimes CPUs

Definitions

PLD: User-configurable combinational logic IC.
SPLD (Simple PLD): Usually just one PAL or PLA.
PAL (Programmable Array Logic): Programmable AND-plane, fixed OR-plane.
PLA (Programmable Logic Array): Programmable AND-plane and OR-plane.
FPGA: Configurable array of logic blocks + flip-flops.

LOOK UP PAL vs PLA comparison

3. How FPDs are Configured

Configuration = deciding where to make/break connections.
Logic gates/flip-flops are static, but connections are programmable.

Two major connection types:

Type	Description	Pros	Cons
Antifuse	Permanent connection created at programming time.	Permanent, non-volatile.	Cannot be reprogrammed.
SRAM-based	SRAM contents downloaded at power-on control switches.	Reconfigurable, flexible.	Volatile; needs reload on power-on; slightly higher resistance.