digital logic design

1. DIGITAL INTEGRATED
CIRCUITS
• Light introduction to the underlying
technologies
• Some of them historical, some of them
current

2. The Families
• RTL
• DTL
• TTL
• ECL
• N-MOS, P-MOS
• CMOS

3. {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.

4. Bipolar Transistor
• This uses the “classic” bipolar transistor
• In a few cases it was combined with CMOS
for chips marketed as Bi-CMOS

5. ECL
• Emitter Coupled Logic
• The fastest, if money and heat is not an issue
• Used in the supercomputers of the 80s and
90s

6. {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

7. CMOS
• THE family: Complementary MOS
• Currently the highest density, speed and
lowest power dissipation.
• Contains both positive and negative channel
transistors

10. 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

12. 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

13. Miniaturization Helps
• A big factor is stray capacitance
• Smaller circuits have (in general) smaller
capacitance

14. 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.

15. Propagation Delay
• The other big issue
• Used to be number one issue
• Directly affects clock rate

17. 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

20. 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

22. Resistor Transistor Logic
• The gates behave like analog amplifiers
• Low fan out
• Rather slow
• Power hungry
• Kind like a Hummer!

24. Diode Transistor Logic
• Some improvement in fan out
• Fewer resistors more diodes

27. Transistor Transistor Logic
• Small change over DTL
• Several other improvements were applied to
TTL
• Dominated the ICs in the early 80’s

28. 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)

29. Variety of Outputs
• Open collector where user supplies the
output resistor (wired-and)
• Totem pole
• Tristate

30. Nice trick
• Combines the diodes of the DTL into one
multi-emitter transistor

34. 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

35. How?
• When we need a large resistance we turn off
the transistor
• When we need a low resistance we turn it on

37. 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

42. 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

43. How it looks
• P-channel has p-doped source (emitter) and
drain (collector)
• Embedded in n-doped substrate
• The opposite for n-channel

45. 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

48. Solution
• Complementary MOS
• Have “islands” of p- or n-doped substrate
• Totem pole is really easy
• So is tristate

51. Transmission Gates
• Easy to build “gates”
• Either connect or disconnect a circuit
• Can be used to implement logic