System for coupling programmable logic device to external circuitry which selects a logic standard and uses buffers to modify output and input signals accordingly

Abstract

A programmable input / output device for use with a programmable logic device (PLD) is presented comprising an input buffer, an output buffer and programmable elements. The programmable elements may be programmed to select a logic standard for the input / output device to operate at. For instance, a given set of Select Bits applied to the programmable elements may select TTL logic, in which case the input and output buffers would operate according to the voltage levels appropriate for TTL logic (e.g., 0.4 volts to 2.4 volts). For a different set of Select Bits, the GTL logic standard would be applied (e.g., 0.8 volts to 1.2 volts). The invention enables a single PLD to be used in conjunction with various types of external circuitry.

Description

BACKGROUND OF THE INVENTION[0001]This invention relates to programmable logic devices (“PLDs”), and more particularly to a new architecture for the input / output (I / O) circuitry which couples the PLDs to external circuitry.[0002]Programmable logic devices are integrated circuits which are able to implement combinational and / or sequential digital functions which may be defined by a designer and programmed into the device. Thus, PLDs may be configured by a user to implement any Boolean expression or registered function with built-in logic structures. Once a PLD is configured, the user must connect the PLD to external circuitry which provides input signals to, and receives output signals from, the PLD.[0003]One deficiency of conventional PLDs and their I / O circuitry is that each PLD must be configured to operate with specific external circuitry. For example, if a user utilizes Transistor-to-Transistor Logic (TTL) or CMOS external circuitry, the PLD must be configured to provide the appr...

AI Technical Summary

Problems solved by technology

One deficiency of conventional PLDs and their I / O circuitry is that each PLD must be configured to operate with specific external circuitry.

This deficiency is even more apparent in view of the programmable nature of PLDs and the flexibility provided to the end users.

Further, the nature of PLDs, as semiconductor devices, is that they are susceptible to a wide range of potential hazards, such as electrostatic discharge (ESD).

One deficiency of Gunning and other known driver circuitry is the limited scope with which the circuitry may be used.

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