Page 127 - Handout Digital Electronics

state, since it is triggered by the clock connected to the Q output of FF0. The same applies to FF2 and
FF3. Initially all the flip flops are set to 1, that is FF0 = 1, FF1 = 0, FF2 = 0.
The first flip flop (FF0) operates on a 1 o 0 clock transition (just like in the up counter) but FF1, FF2 and
FF3 operate on a 0 to 1 clock transition since their clocks are connected to the Q outputs. Counting
down always starts from the first flip flop (FF0). So, on a 1 to 0 transition FF0‟s present state (Q = 1) is
toggled to a 0 and at the Q output the 1 displaces the zero which is passed on to the clock and at FF1 the
situation is J = K = 1, C = 0. This means FF1 does not change its state. The same applies to FF2 and
FF3. (See example below)

FF3 FF2 FF1 FF0

1 1 1 1 Pre-set value (15)

1 1 1 0

. . .

0 0 0 1

A time diagram explains even better the operation of a down counter. Figure 57 shows the operation of a
a4-bit asynchronous binary down counter.

Figure 57: 4-bit asynchronous binary down counter time diagram

Advantages and disadvantages of asynchronous counters

Two major advantages of asynchronous counters are:

• They are simple to design and implement
• They were less expensive

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