Name _____________________________________
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
6.091 Hands-On Introduction to EE Lab Skills
Laboratory No. 4 Digital Circuits
IAP 2008
1 Objective
In this laboratory, you will become familiar with basic digital logic, design and
implementation through the use of LSTTL (Low power Schottky Transistor Transistor
Logic) and HCT (High-speed CMOS with TTL compatible inputs). You will also use a
68 channel logic analyzer. Please get a bag of parts from the LA. Return the bag with
the parts when your exercise is completed.
1.1 Ring Oscillator
Digital circuits have finite propagation delays. Typically the delays are on the order of
10 ns per gate. In this exercise you will construct a ring oscillator – much like a dog
chasing its tail.
1.1.1
Exercise – Ring Oscillator
Wire up the above circuit using a 74LS04. Note that power (Vcc = 5V) and
ground are not shown but need to be attached. Use the BK Precision bench
power supply which has a fixed 5 volt supply. The IC pin out for the 74LS04 is
shown below:
VCC
14
13
12
11
10
9
8
Using the oscilloscope, take a look
at the output and describe what is
happening. What is the period of
the waveform? Would an even
number of inverters work?
(The oscilloscope in the 6.111 lab
is slightly different from the one
previously used.)
1
2
3
4
5
6
7
GND
Figure by MIT OpenCourseWare, based on Motorola datasheet.
1.2 4 Bit Synchronous Counter
Counters are very commonly used in digital circuits. When each bit of the counter is
clocked by the same signal, the counter is called synchronous. Wire up the crystal
oscillator and counter. Power and ground is shown for the crystal because it is not a
standard integrated circuit. Power and ground for the 74LS163 counter is not shown.
+5
7
10
+5
QA
T
QB
14
13
12
9
1
1.8432
Mhz
crystal
osc.
P
LD
QC
CLR
QD
11
74LS163
counter
triangle is symbol
for clock input
Look at the output of the crystal oscillator. Now look at QA, QB, QC, and QD. What
is the relationship between the Q outputs? Which is the MSB (most significant bit)?
Attached the logic analyzer “flying leads’ to QA, QB, QC, and QD. QA should go to
logic analyzer A2(0), QB to A2(1), etc.
You should be able to view the counter in graphical expanded mode, decimal mode,
and hexadecimal mode. The counter should increment to 15 and then start at zero
again.
Using the vertical bars on the logical analyzer, measure the period of the clock.
Similar to an oscilloscope, you can also select a trigger point. Try to set the trigger
so that the display triggers when the count is 7 (binary: 0111).
If you have a USB drive, you can save a copy of the image using the PRINT
SCREEN function and the PAINT program.
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6.091 Lab 4
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1.3 Digital to Analog (DA) Conversion
In lab 1, you built a R-2R ladder to perform a digital to analog conversion.
R
R
R
R
Vo
R
2R
2R
+5
+5
Bo
2R
2R
+5
B1
B2
+5
B3
In place of the switches, you will now drive the R-2R ladder with the output of the
counter. Note that B0 is the least significant bit and the output is
Vo =
1⎡
1
1
1
⎤
B3 + B2 + 2 B1 + 3 B0 ⎥
⎢
2⎣
2
2
2
⎦
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6.091 Lab 4
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In order to drive R-2R ladder to +5 and ground, you will need to buffer the output of
the counter with a 74HC08. This IC is configured as a AND gate but we will use it as
a buffer.
R
R
R
R
Vo
R
2R
2R
2R
2R
B0
QA
B1
QB
B2
QC
B3
QD
1A
1
14 V
CC
1B
2
13 4B
1Y
3
12 4A
2A
4
11 4Y
2B
5
10 3B
2Y
6
9
3A
GND
7
8
3Y
Top View
1A 1
14 VCC
1B 2
13 4B
1Y 3
12 4A
2A 4
11 4Y
2B 5
10 3B
2Y 6
9 3A
GND 7
8 3Y
Figure by MIT OpenCourseWare.
If you successfully wired up this circuit you should see a 0-5V ramp. Notice the
discrete quantum jumps. Place a 100pf capacitor on the output. How does the
waveform change? Most of our personal electronics and computers are digital in
nature. Audio is generated by DAC, digital to analog converters. Music CD players
have 16 bit DAC. We will discuss this in more detail in lecture 5.
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