DIGITAL INTEGRATEDCIRCUITS

• Light introduction to the underlying technologies

• Some of them historical, some of them current

The Families

• RTL

• DTL

• TTL

• ECL

• N-MOS, P-MOS

• CMOS

{R, D, T}TL

• These are

– Resistor Transistor Logic

– Diode Transistor Logic

– Transistor Transistor Logic

• Only TTL survives, although not for long

• Until recently it was the fastest with reasonable price and power requirements.

Bipolar Transistor

• This uses the “classic” bipolar transistor

• In a few cases it was combined with CMOS for chips marketed as Bi-CMOS

ECL

• Emitter Coupled Logic

• The fastest, if money and heat is not an issue

• Used in the supercomputers of the 80s and 90s

{N, P}MOS

• Precursors to the CMOS

• Negative or Positive Metal Oxide Semiconductor

• FET: A fundamentally different kind of transistor than the one used in TTL

• Stands for Field Effect Transistor

CMOS

• THE family: Complementary MOS

• Currently the highest density, speed and lowest power dissipation.

• Contains both positive and negative channel transistors

Fan-Out

• How many circuits can be driven by the output of a particular circuit

• It is a property of the driving chip (we assume that the driven chips are from the same technology)

• Depends on the technology, clock rate etc

Power Dissipation

• The power needed to run the circuit

– each gate needs very little

– there are many gates

• Most of the energy is expended during switching

• Depends on the technology, clock rate, voltage etc

Miniaturization Helps

• A big factor is stray capacitance

• Smaller circuits have (in general) smaller capacitance

Definition

• The power is

– P = I * V

• I is the average current

• V is the supplied voltage

– A bit more complex than that actually

• We can reduce either.

Propagation Delay

• The other big issue

• Used to be number one issue

• Directly affects clock rate

Noise Margin

• All circuits have noise

• Some of it is inherent property of electricity

• Some of it is just too expensive to eliminate

• Some of it is just unmodeled issues in the circuit itself

Transistors are Funny

• The base-emitter voltage is about 0.7V when conducting

• The base-collector voltage is about 0.3V when saturated

• The collector current is about 50 times the base current

Resistor Transistor Logic

• The gates behave like analog amplifiers

• Low fan out

• Rather slow

• Power hungry

• Kind like a Hummer!

Diode Transistor Logic

• Some improvement in fan out

• Fewer resistors more diodes

Transistor Transistor Logic

• Small change over DTL

• Several other improvements were applied to TTL

• Dominated the ICs in the early 80’s

Improvements

• The flexibility of the family allows the manufacturers to offer a variety of grades

• One can trade off speed and power

• A great improvements was the introduction of the Schottky diode (prevents saturation)

Variety of Outputs

• Open collector where user supplies the output resistor (wired-and)

• Totem pole

• Tristate

Nice trick

• Combines the diodes of the DTL into one multi-emitter transistor

Totem Pole

• The output resistor gives us trouble:

– if it has many Ohms, the fan out is limited

– if it is has few, consumes too much power

• So, replace it with a transistor

– same trick in CMOS

How?

• When we need a large resistance we turn off the transistor

• When we need a low resistance we turn it on

Schottky

• A Schottky diode is a junction between aluminum and silicon

• Has only .4 Volt when conducting

• Can be used to prevent a transistor from going into saturation

FET

• Field Effect Transistor

– Junction FET

– Metal Oxide Semiconductor FET

• The gate (base) is insulated

• High fan out with little power consumption

• Can be miniaturized to death

How it looks

• P-channel has p-doped source (emitter) and drain (collector)

• Embedded in n-doped substrate

• The opposite for n-channel

Problem

• The substrate is part of the transistor

• Which means that one can have only one type transistors in the circuit

• So we cannot do totem pole easily (we need resistors)

• Resistors are half open transistors

Solution

• Complementary MOS

• Have “islands” of p- or n-doped substrate

• Totem pole is really easy

• So is tristate

Transmission Gates

• Easy to build “gates”

• Either connect or disconnect a circuit

• Can be used to implement logic