3 bit clocked up-down counter?

Question

If i have a question:
Design an 3 bit clocked up-down counter using
(i) D flip-flops
(ii) J-K flip-flops

what exactly do i have to do? I dont get it... do i need 3 inputs???

thnx so much

Answer 1

The counter will have a minimum of 2 inputs, preferrably 3:
1. clock
2. a bit to set the direction of the count (up or down)
3. a bit to reset the count to zero.

The output will be 3 bits (because it is a 3-bit counter).

Here is an example of one to help you get started. This is an up counter, only, using JK flops.. I'll leave it up to you to figure out the up/down parts. Look at the 2 most significant bits for the 7493 for the simplest example. Note this is a ripple counter, not a synchronous counter.
http://focus.ti.com/lit/ds/symlink/sn7490a.pdf

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Answer 2

Each JK f/f will toggle between 0 and 1, if you take the output of one of these f/f and feed it as the clock of the next f/f you will have a 2 bit counter, tack on another f/f and you got a 3 bit counter. Now, these f/f's have a Q and a NOT-Q output, if you hook up the clock pin of the next f/f to the Q output and the clock pin works on the up swing (or leading edge) then you will have a "down" counter. To get an up counter, you must connect the clk pin to the "Not-Q" output. If the clock pin is inverted, then it works in reverse.

Now, what do you mean by a "Clocked ... counter"? This could mean they want a synchronous output. The problem with cascading these f/f's is that each f/f adds a little time delay (after all, nothing is instanteous). To make it synchronous, use the D Flip flops on the "Q" outputs, and have them latch only on the down swing of the original clock input (lagging edge). Now just use the outputs of the D f/f's, they will be all simutaneosly updated on the down swing of the clock.

Answer 3

Your "I don't get it" comment (and the low complexity of the question) makes it sound like this is your first assignment.

The answer should look like a big box around a bunch of standard off-the-shelf parts. The inputs to your circuit are generally wires coming in through the left side of the big box and the outputs are wires going out through the right side. The inputs and outputs of your circuit should be labeled so that someone would know how to wire it into a bigger circuit.

Here's an example that uses two D flip-flops and an inverter to make a Master/Slave flip flop.
http://www.hobbyprojects.com/flip_flop/master_slave_D_type_flip-flop.html

The solution to your problem(s) will be of a similar form, but obviously contain a different circuit. Other responders have given you some good pointers on what exactly should go in your circuit.