Download In building digital circuits, you will often use an LED as

Week 5: Digital Building Blocks
In building digital circuits, you will often use an LED as an indicator of the state, HI
or LO. ALWAYS USE A RESISTOR IN SERIES WITH THE LED. The series resistor
(220 Ω typical) is needed to avoid destroying the LED. The LED is a diode, and when
it is on, it has very little resistance. If you connected an LED directly between a
source and either ground or Vcc , it would try to pass an infinite amount of
current, which will burn up the LED.
With digital electronics, if you need your circuit to “forget”, power-off the
breadboard for at least 10 seconds.
Lab 13-1: Input and output characteristics of integrated gates:
TTL & CMOS (p311)
Read pages 281-308 on combinational logic in the lab manual.
Answer the following questions in your report:
1) Note that we are only using the CMOS part 74HC00 for this lab. For part (a), use
DIP switches with pull-up resistors (220 Ohm) to 5V to provide your logic
inputs. Use a red LED to get the logic output. Remember to ground the 6 unused
inputs on the 74HC00. Fill in the table in the lab manual with output logic levels
and voltages for each of the 4 input combinations.
2) Describe what happens to the CMOS NAND output when one input is pulled high
and the other is attached to a floating wire, as suggested in the lab manual.
3) What is the power consumption of the CMOS 74HC00 when all inputs are tied to
a variable input voltage, as described in the lab manual? Make a graph of power
consumption (supply voltage times current) versus the input voltage, using
enough points to show the interesting features. What do you learn from this
graph?
Lab 13-2: Applying NANDs to generate particular logic functions
(p314)
Answer the following questions in your report:
1) Hook up an LED with a 220-ohm series resistor as your "logic probe". Sketch a
combination of NAND gates that should calculate the AND logic function. Show
your truth table. Build and test your circuit.
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2) Sketch a combination of NAND gates that should calculate the OR logic function.
Show your truth table. Build and test your circuit.
3) Sketch a combination of NAND gates that should calculate the XOR logic function.
Show your truth table. You don't need to build this one. Note that you will need
at least 5 NAND gates to do this, but it is possible with only 4. Don't spend more
than 15 minutes in class on this. You can look up (and marvel at) the 4-gate
solution later.
Lab 14-1: A primitive flip-flop – NAND latch (p334)
This is a warm up for next week's sequential logic topics.
Answer the following questions in your report:
1) Build the circuit in figure L.14.1 but using the same DIP switches with 1k pull-up
resistors, instead of the 3-terminal slider switch shown. Hook up a LED logic
probe to the Q output, and fill in the 4 rows of the truth table. If it helps, you can
also add another LED logic probe (different color) to the other gate output.
2) Notice how this circuit uses feedback. Look at S and Q on your scope with a small
time scale. Make a sketch of your waveforms that shows this feedback in action.
Lab 14-5: Switch bounce (p339)
Answer the following questions in your report:
1) Use the same DIP switches with pull-up resistors to look for switch bounce on
your scope. Sketch an example of the waveforms you see.
2) Use your dual NAND gates to "de-bounce" your DIP switch with the circuit in
figure L.14.11. Describe how it works.
Note: You’ll need this circuit next week, so leave it setup on your breadboard.
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