Question

The RESET circuit used on the four 3-Bit Counters analyzed in this activity reset the counts to zero (000). It makes sense for the up-counters to start at zero (000), but the down-counters should start at seven (111). What would you need to change so that the 3-Bit Binary Down Counter with J/K Flip-Flops you just created would reset to seven (111)?

Answer 1

Answer:

• "set" each flop, instead of resetting it
• invert each flop's output
• take the output from Q* instead of Q

Explanation:

The question asks how to establish an initial condition of 111 at the output of a 3-bit counter. (It does not ask how to make the counter count down, instead of up.)

The outputs can be made to be 1 by any of several means:

1. use the "set" input to establish the initial condition, instead of the "reset" input
2. invert the output of the flip-flop
3. take the output from Q* (the inverted output) instead of the Q output

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Additional comments

Note that either of the last two strategies automatically make it a down-counter instead of an up-counter. That is, the next state after 111 will be 110.

Using the first strategy alone does not alter the fact that the counter will count up. That is, the next state of the up-counter after 111 would be 000.