Lab 2 Logic Gates Characteristics

CSCE 2302 - Digital Design 1 Lab 
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Lab 2
Logic Gates Characteristics
Objectives
➢ Measure the propagation delay of CMOS logic gates
➢ Measure VIL and VIH of CMOS logic gates
➢ Introduce the Analog Discovery Kit and its usage in testing logic circuits made out of CMOS chips
Material
➢ 74HC00 (Quad 2-input NAND)
➢ 74HC08 (Quad 2-input AND)
➢ Analog Discovery Kit
➢ Digital Logic Trainer
➢ Wires
Introduction
a. The physical characteristics of logic gates
The physical characteristics of logic gates often play an important role in the design of digital circuits.
These include:
➢ Voltage and current levels
➢ Noise margin
➢ Fan-out and loading
➢ Propagation delay
➢ Threshold Voltage
Some of these physical characteristics are discussed below.
i. Current Direction:
According to IEEE standards, currents are directed into devices. Therefore, if a current in a specification is
positive, it is entering the device. If a current in a specification if negative, it is leaving the device.
ii. Propagation Delay
Propagation delay is the time that it takes a gate to switch logic levels. Logic gates often have a different
propagation delay switching from LOW to HIGH than from HIGH to LOW, so two types of delay are defined:
tPLH = propagation delay when the OUTPUT switches from LOW to HIGH calculated at 50% of input-output
transition, i.e. maximum time from the input change crossing 50% to the output LOW to HIGH change
crossing 50%.
CSCE 2302 - Digital Design 1 Lab Fall 2023
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tPHL = propagation delay when the OUTPUT switches from HIGH to LOW calculated at 50% of input-output
transition, i.e. maximum time from the input change crossing 50% to the output HIGH to LOW change
crossing 50%.
Figure 1 demonstrates propagation delay. The average propagation delay tp = ½ (tPLH + tPHL).
Figure 1. Propagation Delay calculation
iii. Voltage and Current Levels:
Several voltage and current levels are of interest when working with logic gates, including:
➢ VOL = Low-level output voltage (output voltage when the output of the gate is logic 0)
➢ VOH = High-level output voltage (output voltage when the output of the gate is logic 1)
➢ VIL = Low-Level input voltage
➢ VIH = High-level input voltage
➢ IOL = Low-level output current
➢ IOH = High-level output current
➢ IIL = Low-level input current
➢ IIH = High-level input current
iv. Noise Margins
Noise margin is a quantitative measure of noise immunity offered by a logic family. Noise margin allows
you to determine the allowable noise voltage on the input of a gate so that the output will not be
corrupted. The specification most commonly used to describe noise margin (or noise immunity) uses two
parameters: the logic low noise margin, NML, and the logic high noise margin, NMH.
With reference to Figure 2, NML is the difference in maximum LOW input voltage recognized by the
receiving gate and the maximum LOW output voltage produced by the driving gate.
CSCE 2302 - Digital Design 1 Lab Fall 2023
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Thus, NML = LOW level noise margin = VIL(max) – VOL(max)
Where VIL(max) is the maximum LOW input voltage and VOL(max) is the maximum LOW output voltage.
This implies that, when the LOW voltage output of one device feeds the input of another, there is an
available margin of NML. That is, any positive voltage spikes of amplitude less than or equal to NML on the
signal line do not cause any spurious transitions.
Also the value of NMH is the difference between the minimum HIGH output voltage of the driving gate
and the minimum HIGH input voltage recognized by the receiving gate.
Thus, NMH = HIGH level noise margin = VOH(min) – VIH(min)
Where VIH(min) is the minimum HIGH input voltage and VOH(min) is the minimum HIGH output voltage.
This implies that, when the HIGH voltage output of one device feeds the input of another, there is an
available margin of NMH. That is, any negative voltage spikes of amplitude less than or equal to NMH on
the signal line do not cause any spurious transitions.

Figure 2. CMOS High and Low Noise Margins definitions
As seen in Figure 2, VOL (max) is lower than VIL (max) to allow for noise and signal deterioration. Similarly,
VOH (min) is higher than VIH (min). These differences in voltages are those referred to as noise margins. If
either NML or NMH for a gate are too small, the gate may be disturbed by noise that occurs on the inputs.
Typically, in a CMOS inverter VOH will equal VDD and VOL will equal the ground potential. In practice, noise
margins are the amount of noise that a logic circuit can withstand. Noise margins are generally defined so
that positive values ensure proper operation, and negative margins result in compromised operation, or
outright failure.
Inputs between VIL and VIH are said to be in the indeterminate region or forbidden zone and do not
represent legal digital logic levels. Therefore, it is generally desirable to have VIH as close as possible to VIL.
Logic ‘0’
Logic ‘0’
Logic ‘1’
Logic ‘1’
Intermediate
Input voltage
Range
Disallowed
Output
voltage Range
CSCE 2302 - Digital Design 1 Lab Fall 2023
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b.Analog Discovery Kit
The Digilent Analog Discovery is a multi-function instrument that can measure, record and generate
analog and digital signals. The small, portable and low-cost Analog Discovery was created so that
engineering students could work with analog and digital circuits anytime, anywhere - right from their PC.
The Analog Discovery Kit (shown in Figure 3) provides all capabilities necessary for test and measurement
of circuits. It provides the ability to apply voltages (signals) to a circuit or device and measure the response.
The Analog Discovery combines a dual-channel oscilloscope, a two-channel waveform generator, a 16-
channel logic analyzer and many other instruments into a USB-powered, low-cost device. The Analog
Discovery works with the Waveforms software that offers intuitive interfaces to the oscilloscope,
waveform generator, etc.
The Analog discovery kit will be used in different lab experiments to generate digital signals and observe
the waveforms generated from logic circuits. The connector has the following pins (as shown in Figure 3):
➢ V+ & V-: Fixed power supplies (+5 V (+50 mA) and -5 V (+50 mA))
➢ W1 & W2: variable power supplies (Wave function generator)
➢ 1+,1- ,2+ & 2-: Two channel voltage measurement (oscilloscope)
➢ 0 to 15: Digital input output (I/O) (D0, D1, …., D15)
➢ ↓: Ground
Figure 3. The Analog Discovery Kit and its pin-out
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Pre-lab Questions
Use Logisim software to connect the circuit shown in Figure 4 using pin-to-pin diagram. Use the proper
chips from the 7400 lib. DO NOT USE individual gates. Provide a screenshot of the circuit on Logisim
along with the .circ file.
Experiments
1. Measuring the Propagation Delay at 5V input supply:
a. Connect the circuit as shown in Figure 4. Connect pin 14 to +5V from the power supply area
on the logic trainer and connect the pin 7 to ‘GND’.
Figure 4. A simple circuit
b. Connect the Analog discovery pins 'D0' & 'D1' to ‘A’ and ‘X’ respectively. Also, connect the
Analog discovery pin ↓ to your circuit ground to create a common ground.
c. Open the WaveForms software (C:\Program Files (x86)\Digilent\WaveForms3)
d. Apply a square wave to ‘A’ with “Pattern Generator” instrument:
i. Add → Signal → select DIO 0, as shown in Figure 5
ii. Change the signal parameters as follows
- Type: Clock
- Output: PP
- Parameter1: 5MHz
iii. Press to start generating the signal
e. View the 'A' and 'X' signals using the 'Logic Analyzer' instrument :
i. Change the parameters as follows
- Base: 500 ns/div
- Press 'Run’
ii. After a few seconds press Double click to enable the hot track to be able to
measure data from signals. Click on the point where you want to start recording and click
again to finish recording.
iii. Use the track to record the propagation delays as shown in Figure 6
f. Record the values in Table 1 in the Lab Report.
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Figure 5. Adding a signal in WaveForms Digital Input mode
Figure 6. Zooming hot tracks in WaveForms
2. Measuring VIL_max and VIH_min:
a. Connect the circuit as shown in Figure 7, and select a good Vcc value as in the datasheet.
b. Set the potentiometer knob initially to 0V.
c. Use the "Voltmeter" instrument of the Analog Discovery to measure the signals at B & X;
connect '1+' to B, '2+' to X, '1-' and '2-' to your circuit ground.
d. Turn the knob gradually to increase the input voltage while monitoring the voltage on X. Keep
doing that till the voltage at X is greater than VOH_min (check the datasheet).
e. Record VIH_min = ___________
f. Set the potentiometer knob to 5V.
g. Turn the knob gradually to decrease the input voltage while monitoring the voltage on X. Keep
doing this till the voltage on X is less than VOL_max (check the datasheet).
h. Record VIL_max = ___________
Figure 7. A simple circuit
CSCE 2302 - Digital Design 1 Lab Fall 2023
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Lab Report [10 Pts]
(Deadline: Tuesday of next week 2:00pm) (Individual submission)
1. [1.5 Pts] Record the values obtained in Experiment question 1 in Table 1 below.
Table 1
Propagation delay (tP_LH)
Propagation delay (tP_HL)
Average propagation delay tp
Propagation delay (tP_LH) per gate
Propagation delay (tP_HL) per gate
Average tp per gate (measured)
Average tp per gate (from datasheet)
Is the recorded tp value per gate same as in the datasheet? If not then how can you explain this
difference?
2. [1.5 Pts] Record the values obtained in Experiment question 2 in Table 2 below.
Table 2
VOH (from datasheet)
VOL (from datasheet)
VIH_min
VIL_max
Are the recorded values same as in the datasheet? If not then how can you explain this difference?
3. [2 Pts] Why should VIL_max be greater than VOL_max? And similarly why should VOH_min be greater than
VIH_min?
4. [2.5 Pts] Research question: Describe briefly in 1-2 sentences the following physical characteristics
of a logic gate. Also mention the problems that may arise if their values surpass their thresholds.
a. Fan-out
b. Current levels IOL , IOH , IIL , IIH
5. [2.5 Pts] Extract for the AND IC datasheet attached on BB the typical values for the following
physical characteristics. Mention any conditions required to replicate these values in lab.
a. Fan-out
b. Current levels IOL , IOH , IIL , IIH