Question 7.PE.4: To investigate the 4-bit synchronous up counter For this exe...
To investigate the 4-bit synchronous up counter
For this exercise you will need the following components and equipment:
1 – 74LS161 ic (4-bit synchronous binary counter)
1 – 74LS00 ic (quad 2-input NAND)
4 – LED red (5 mm) and resistor (270 Ω), for the Q outputs
1 – LED green (5 mm) and resistor (270 Ω), for the carry output
1 – +5 V dc power supply
Procedure (a): to count to decimal 16
1 Make up a suitable circuit. The pin connection diagram for the 74161 ic is shown in Figure 7.12.
2 Make the following settings:
Load input – logic 1
Enable inputs (P&T) – logic 1
Data inputs – either logic 1 or logic 0 (to leave them floating can cause erratic behaviour of the counter)
Clear – logic 0 (to reset) then to logic 1
3 Apply a series of +5 V pulses to the clock input. The counter should count from ABCD = 0000 to ABCD = 1 1 1 1 + carry 1 and then reset to ABCD = 0000.
Note. Counting action takes place on the positive edge of the clock pulse.
Procedure (b): to stop the count before decimal 15
1 Modify the circuit to include the NAND gate whose pin connections are given in Figure 2.13 (b).
The action is now identical to that met in Practical Exercise 7.2, in that the counter will reset when the inputs to the NAND gate are both logic 1.
Suppose that we want the counter to count to decimal 10 which means resetting when outputs ABCD = 0101.
2 Make the following settings:
Load input – logic 1
Enable inputs (P&T) – logic 1
Data inputs ABCD – either logic 1 or logic 0 (as above)
3 Apply a series of +5 V pulses to the clock input. The counter should count
from ABCD = 0000 and reset at ABCD = 0101.
4 Repeat Procedure 2 with a reset value of your choice.
Procedure (c): to preset the counter
1 Return to the circuit of Figure 7.12. Suppose that we want the counter to start
counting at ABCD = 0101.
2 Make the following settings:
Data inputs ABCD – 0101 (= preset value)
Load input – logic 0 (this will disable the counter)
Enable inputs (P&T) – logic 1
3 Apply one clock pulse. The output should go to its preset value (on the positive edge of the pulse).
4 Now set the load input to logic 1 and apply further clock pulses.
The count should proceed from ABCD = 0101 to ABCD = 1111 + carry 1 and then reset to ABCD = 0000.
5 Reload the counter to a preset value of your choice.
