TESTING LOGIC LEVELS IN A CMOS TIMER CIRCUIT
3. To introduce a fault into the timer and detect the problem using logic probe readings
MATERIALS
Qty. Qty.
1 74HC08 two-input AND gate 1 keypad (N.O. contacts)
CMOS IC 8 LED indicator-light
1 74HC14 inverter CMOS IC assemblies
1 74HC393 binary counter 2 10-k, 1⁄4-W resistor
CMOS IC 1 5-V dc regulated power
1 logic switch supply
1 free-running clock 1 logic probe
SYSTEM DIAGRAM
Troubleshooting requires that the technician know the normal operation of the circuit. Normal operation from the technician’s or engineer’s view includes various inputs and expected output(s). It also includes expected nor-mal measurements of temperature, voltage/currents, and waveforms at vari-ous points in the circuit. Technical manuals and experience greatly aid in efficient troubleshooting. Besides technical knowledge of the circuit and testing, a technician’s or engineer’s powers of observation are important in troubleshooting.
A summary of the steps in troubleshooting include (1) the use of your senses (feel, look, hear, smell), (2) the use of a logic probe to check power to ICs, (3) the use of equipment manuals to determine the job of the circuit and to test unique output conditions, and (4) the use of test instruments to test input and output conditions to determine the problem.
The circuit in Fig. 3-24 is basically a timer with an 8-bit binary output. A block diagram of the system is shown in Fig. 3-24(a). Closing the clear input switch momentarily zeros the output to 00000000. Closing the count switch causes the control circuitry to permit the 10-Hz clock input to enter the up counter. When the count switch opens, the control circuitry blocks the clock pulses from getting through to the counter and the timer stops. The accumu-lated count on the output display indicates the number of tenths of a second the count switch was closed.
A wiring diagram for the timer circuit is detailed in Fig. 3-24(b). All the logic gates and counter are CMOS ICs. When the ON/OFF switch is in the ON position and when the 1 on the keypad is depressed, the counter will mea-sure the time (in tenths of a second) that the count key is closed. The binary output needs to be translated into decimal by you, the operator. The C on the keypad will clear the output display to 00000000. The free-running clock must be set at a frequency of 10 Hz for the timer to be accurate.
The ON/OFF switch can disable the timer, but it leaves the last display show-ing on the eight LEDs.
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Fig. 3-24 Timer circuit using CMOS ICs. (a) Block diagram. (b) Wiring diagram.
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It is suggested that the output LEDs be arranged with the LSB (1s LED) on the right and the MSB (128s LED) on the left. This will make the binary output easier to read. At this point, do not be concerned that you do not understand the exact operation of the counter IC. However, you should un-derstand how the control circuitry operates. This is a difficult circuit at this point in the course and should be attempted only by the more advanced students.
In this problem, you will concentrate on the control gating circuitry of the timer in Fig. 3-24. You will use a logic probe to test the logic levels around the 74HC14 inverter and 74HC08 AND gate CMOS ICs.
PROCEDURE
CAUTION
CMOS ICs can be damaged by static electricity. Store CMOS ICs with their pins in conductive foam or covered with aluminum foil.1. Power OFF. Insert the 74HC08, 74HC14, and 74HC393 CMOS ICs into the mounting board.
2. Power OFF. Connect power wires to the three ICs: use red wire for 5 V and black wire for GND. Connect all unused inputs to GND [see pin numbers in Fig. 3-24(b)] using black wire.
3. Refer to Fig. 3-24(b). Power OFF. Wire the entire circuit (switch, free-running clock, keypad, ICs, and LED indicator-light assemblies). Pin num-bers are shown on the wiring diagram. See your instructor if you have problems wiring this complex circuit.
4. Power ON. Check the operation of the timer using the following proce-dure:
a. Move ON/OFF switch to HIGH.
b. Press and release C on the keypad to clear display to 00000000.
c. Press 1 on the keypad for 10 seconds. The binary display should count upward.
d. After 10 s the display should read about decimal 100 (binary 01100100).
e. Adjust the clock frequency to get as close to 10 Hz as possible.
f. Move ON/OFF switch to LOW.
g. Try pressing 1 on the keypad to use the timer. The counter is disabled and should not count upward.
5. Power OFF. Connect a logic probe to the circuit. Read the instruction manual or ask your instructor if you need help in operating the logic probe.
Remember you are working with CMOS ICs.
6. Power ON. Operate the circuit for several minutes by running the timer and then clearing. Now, touch the top of all three ICs. Record your re-sults on Table 3-19.
7. Power ON. Follow the testing procedure in Table 3-19 and record your results. You are testing the normal operation of the timer circuit. It is im-portant you understand the normal operation of a circuit before you can troubleshoot a faulty circuit.
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TABLE3-19 Test Results—Normal Operation of Timer Circuit
Keypad Keypad
Keypad
Keypad
Keypad