# Logic Families and Their Characteristics

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Logic Families and Their Characteristics
Chapter 9 Logic Families and Their Characteristics 1

Objectives You should be able to:
Analyze internal circuitry of a TTL NAND gate for both HIGH and LOW output states. Determine IC input and output voltage and current ratings from the manufacturer’s data manual. Explain gate loading, fan-out, noise margin, and time parameters. 2

Objectives (Continued)
Design wired-output circuits using open-collector TTL gates. Discuss the differences and proper use of the various subfamilies within both TTL and CMOS ICs. Describe the reasoning and various techniques for interfacing between the TTL, CMOS, and ECL families of ICs. 3

The TTL Family Bipolar transistors Physical model Symbol
Diode equivalent 4

The TTL Family Two-input NAND gate Multi-emitter transistor
Totem-pole output stage HIGH level output typically 3.4 V LOW level output typically 0.3 V 5

The TTL Family 7400 two-input NAND gate 6

TTL Voltage and Current Ratings
Input/output current and fan-out Source current – IOH Sink current – IOL Low-level input current – IIL High level input current – IIH 7

TTL Voltage and Current Ratings
Example of TTL gate sinking input currents from two gate inputs using logic symbols 8

TTL Voltage and Current Ratings
Example of TTL gate sinking input currents from two gate inputs using schematic symbols 9

TTL Voltage and Current Ratings
Example of TTL gate sourcing current to two gate inputs using logic symbols 10

TTL Voltage and Current Ratings
Example of TTL gate sourcing current to two gate inputs using schematic symbols 11

TTL Voltage and Current Ratings
Summary of I/O current and fan-out: Low-level input current IIL = 1.6 mA (-1600 μA) High level input current IIH = 40 μA (The minus sign indicates current leaving the gate) IOL – low-level output current = 16 mA (16,000 μA) IOH – high-level output current = -400 μA (-800 μA for some) (Max capability of a gate to sink or source current) Fan-out is max number of gate inputs that can be connected to a standard ttl gate output. Typically fan-out = 10. 12

TTL Voltage and Current Ratings
Input/Output Voltages and Noise Margin Noise margin: The difference between high level voltages and low level voltages 13

TTL Voltage and Current Ratings
Input/Output Voltages and Noise Margin (graphical representation) 14

Discussion Point Locate the voltage and current ratings covered so far on the typical data sheet given in Figure 9-8. 16

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Other TTL Considerations
Pulse-Time Parameters Rise Time – Measured from 10% level to 90% level 20

Other TTL Considerations
Pulse-Time Parameters Fall Time – Measured from 90% level to 10% level 21

Other TTL Considerations
Pulse-Time Parameters Propagation Delay (tPLH and tPHL) 22

Other TTL Considerations
Power dissipation Total power supplied to the IC power terminals Open-collector outputs Upper transistor removed from totem-pole Can sink current Can not source current Pull-up resistor used 23

Other TTL Considerations
Wired-output operation Outputs from two or more gates tied together Wired-AND logic 24

Figure 9–16 Wired-ANDing of open-collector gates for Example 9–4: (a) original circuit and (b) alternative gate representations used for clarity.

Figure 9–13 (continued) TTL NAND with an open-collector output: (a) circuitry; (b) truth table.

Figure 9–13 TTL NAND with an open-collector output: (a) circuitry; (b) truth table.

Figure 9–14 Using a pull-up resistor with an open-collector output
Figure 9–14 Using a pull-up resistor with an open-collector output. (a) Adding a pull-up resistor to a NAND gate. (b) When Q4 inside the NAND is on, Vout ≈ 0 V. (c) When Q4 is off, the pull-up resistor provides ≈ 5 V to Vout.

Other TTL Considerations
Disposition of unused inputs and unused gates Open inputs degrade noise immunity On AND and NAND – tied HIGH On OR and NOR – tied LOW Unused gates – force outputs HIGH 25

Other TTL Considerations
Power supply decoupling Connecing 0.01 to 0.1 F capacitor between VCC and ground pins Reduces EMI radiation Reduces effect of voltage spikes from power supply 26

Improved TTL Series 74HXX series Schottky TTL 74FXX
Half the propagation delay Double the power consumption Schottky TTL Low-power (LS) Advanced low-power (ALS) 74FXX Reduced propagation delay 27

Figure 9–17 Schottky-clamped transistor: (a) Schottky diode reduces stored charges and (b) symbol.

The CMOS Family MOSFETs
Metal oxide semiconductor field-effect transistors PMOS and NMOS type substrates 28

The CMOS Family MOSFETs Higher packing densities than TTL
Millions of memory cells per chip See Table 9-2 in your text 29

Table 9–2 Basic MOSFET switching characteristics.

Figure 9–19 CMOS inverter formed from complementary N-channel/P-channel transistors.

The CMOS Family Handling CMOS devices CMOS availability
Avoid electrostatic discharge CMOS availability 4000 series - original CMOS line 40H00 series - faster 74C00 series - pin compatible with TTL 74HC00 and 74HCT00 series Speedy, less power, pin compatible, greater noise immunity and temperature operating range 30

Figure 9–21 Wearing a commercially available wrist strap dissipates static charges from the technician’s body to a ground connection while handling CMOS ICs.

The CMOS Family CMOS availability
74- biCMOS series - low power and high speed 74-low voltage series See appendix B Nominal supply voltage of 3.3 V 74AHC and 74AHCT series Superior speed Low power consumption High output drive current 31

The CMOS Family 74AVC advanced very-low-voltage CMOS logic
Faster speed Very low operating voltages 3.3 V, 2.5 V, 1.8 V, 1.5 V and 1.2 V 32

Emitter-Coupled Logic
Extremely fast Increased power dissipation Uses differential amplifiers Figure 9-22 33

Emitter-Coupled Logic
Newer technologies Integrated injection logic (I2L) Silicon-on-sapphire (SOS) Gallium arsenide (GaAs) Josephson junction circuits 34

Comparing Logic Families
Performance specifications 35

Comparing Logic Families
Propagation delay versus power 36

Comparing Logic Families
Power supply current versus frequency 37

Interfacing Logic Families
TTL to CMOS 38

Interfacing Logic Families
TTL to CMOS Pull-up resistor 39

Interfacing Logic Families
CMOS to TTL 40

Interfacing Logic Families
CMOS to TTL 41

Interfacing Logic Families
Worse-case values See Table 9-4 in your text. 42

Interfacing Logic Families
Level Shifting Level-shifter ICs: B and 4050B 43

Interfacing Logic Families
Level Shifting Level-shifter ICs: B 44

Interfacing Logic Families
ECL Interfacing 45

Summary There are basically three stages of internal circuitry in a TTL (transistor-transistor-logic) IC: input, control, and output. The input current (IIL or IIH) to an IC gate is a constant value specified by the IC manufacturer. 46

Summary The output current of an IC gate depends on the size of the load connected to it. Its value cannot exceed the maximum rating of the chip, IOL or IOH. The HIGH- and LOW-level output voltages of the standard TTL family are not 5 V and 0 V but typically are 3.4 V and 0.2 V. 47

Summary The propagation delay is the length of time that it takes for the output of a gate to respond to a stimulus at its input. The rise and fall times of a pulse describe how long it takes for the voltage to travel between its 10% and 90% levels. 48

Summary Open-collector outputs are required whenever logic outputs are connected to a common point. Several improved TTL families are available and continue to be introduced each year providing decreased power consumption and decreased propagation delay. 49

Summary The CMOS family uses complementary metal oxide semiconductor transistors instead of the bipolar transistors used in TTL ICs. Traditionally, the CMOS family consumed less power but was slower than TTL. However, recent advances in both technologies have narrowed the differences. 50

Summary The BiCMOS family combines the best characteristics of bipolar technology and CMOS technology to provide logic functions that are optimized for the high-speed, low-power characteristics required in microprocessor systems. 51

Summary A figure of merit of IC families is the product of their propagation delay and power consumption, called the speed-power product (the lower, the better). Emitter-coupled logic (ECL) provides the highest-speed ICs. Its drawback is its very high power consumption. 52

Summary When interfacing logic families, several considerations must be made. The output voltage level of one family must be high and low enough to meet the input requirements of the receiving family. Also, the output current capability of the driving gate must be high enough for the input draw of the receiving gate or gates. 53

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