TABLE OF CONTENTS Ian Grout
Early electronic circuits were analog, and before the advent of digital logic, signal processing was undertaken using analog electronic circuits. The invention of the semiconductor transistor in 1947 at Bell Laboratories [Ref. 7.1] and the improvements in transistor characteristics and fabrication during the 1950s led to the introduction of linear (analog) ICs and the first transistor-transistor logic (TTL) digital logic IC in the early 1960s, closely followed by complementary metal-oxide semiconductor (CMOS) ICs. The early devices incorporated a small number of logic gates. However, rapid growth in the ability to fabricate an increasing number of logic gates in a single IC led to the microprocessor in the early 1970s. This, with the ability to create memory ICs with ever increasing capacities, laid the foundation for the rapid expansion in the computer industry and the types of complex digital systems based on the computer architecture that we have available today. The last fifty years have seen a revolution in the electronics industry.
Fundamentally, a digital circuit will be categorized into one of three general types, each of which is created and fabricated within an integrated circuit:
Combinational logic, in which the response of the circuit is based on a Boolean logic expression of the input only and the circuit responds immediately to a change in the input.
Sequential logic, in which the response of the circuit is based on the current state of the circuit and the sometimes the current input. This may be asynchronous
