ECEN 1400
HW 7
Seven segment display
ECEN 1400, Introduction to Analog and Digital Electronics HW 7: Seven segment display (50 pts) This homework will walk through the use of seven segment display chips and the
specialized logic chips that decode BCD into the seven LED signals for the display.
1: CREATE A TWO-DIGIT COUNTER WITH ARBITRARY ROLL OVER (10 PTS)
From homework 4, problem 5, you have a multisim circuit with two cascaded counters.
The first counter cycles through 0 to 15 and the second counter counts the number of
these cycles, also counting 0 to 15. Here we will customize the two counters so each has
an arbitrary maximum. You will need counters that progress from 0 to 5 and 0 to 9 for
your clock.
The ~LOAD pin (9) on the 74161 timers causes the values on pins 3-6 to be loaded into
the counter on the next clock event. Now imagine you had a simple digital logic circuit
to detect when your counter has reached a particular count. If this is inverted (since
~LOAD is active low) and connected to pin nine, the counter will load zeros at the next
clock event, rolling over. Using ANDs and inverters from digital logic lab, design the
required logic to cause the fast, first counter to count 0-6 and the slow, second counter to
count 0-7. These are arbitrary numbers, just to exercise your design skills. Modify your
existing two counter circuit in multisim and use a simulated scope to verify the operation.
Make sure your counter rolls over at the desired value. Turn in 1) a screen capture of the
circuit and 2) a scope trace showing the input clock and the two ~LOAD signals.
Confirm that the scope trace verifies your design. Hint: See the extra credit for lab 5.
2: UNDERSTAND THE DISPLAY DECODER (20 PTS)
a) Fill in the Karnaugh map for the “a” truth table of a BCD to seven segment decoder
(use the truth table in the notes). Assume that states beyond BCD digit 9 are “don’t
care”. On the map, neatly show the groupings that will result in a minimal Boolean logic
expression. Write this logic expression for a as a function of A,B,C and D. Note that
there are multiple ways to group the terms which result in the same complexity.
b) Copy the 1, 0 and X states to the second map and now write a function for NOT(a).
That is, group the zeros on the map and write a function that returns 1 when a is either
zero or “don’t care”. Counting ANDs, ORs and NOTs, which expression would take the
fewer gates to implement?
Version 1.2, 8/25/13
R. McLeod
1
ECEN 1400
HW 7
Seven segment display
BA
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0
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DC
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BA
3: PUT IT TOGETHER WITH SEVEN SEGMENT DISPLAYS (20 PTS)
1. Insert two BCD to seven-segment display decoders under
TTL…74LS…74LS47N. Put each just to the right of your counter. Connect
QA…QD of each counter to A…D of the decoder. Wire pins 3,5, and 4 of the
decoder to 5V.
2. Calculate an appropriate current limiting resistor for the RED LEDs in the seven
segment display. Connect one of these resistors to each of the outputs OA…OG
of both decoders (14 resistors in all).
3. Insert two common anode seven-segment displays under
Indicators…HEX_DISPLAY…SEVEN_SEG_COM_a. Connect the common
anode (labeled CA) to 5 V and the inputs A…G to the outputs of the decoder.
Hit the Simulate arrow and, if all is done right, you should see the two displays counting
from 0 to 6 and 0 to 7. Turn in a screenshot of your schematic.
Version 1.2, 8/25/13
R. McLeod
2