Page 93 - Digital Electronics by harish

3.2.4.1 Four bit binary synchronous UP counter

The logic diagram of 4-bit binary synchronous UP counter is shown in figure. The UP
counter counts from 0000 to 1111. The synchronous counters are fast in operation but require
more hardware.

Figure : Four bit binary synchronous UP counter

Four JKMS flip-flops are used in this counter. J and K inputs of all the FFs are
connected to +5v (J =1, K = 1). This makes the FFs to operate as T (Toggle) flip-flop. The T
FF changes its state (i.e. from 0 to 1 or 1 to 0) for every input clock pulse. The clock input is
applied to all the flip-flops. The gate circuits are arranged in such a way that FF A toggles for
every clock pulse, FF B toggles for every two clock pulses, FF C toggles for every four clock
pulses and FF D toggles for every eight clock pulses. The Q output of all the flip-flops are
taken as the counter outputs ABCD.The output A is called the Least Significant Bit (LSB)
and the output D is called the Most Significant Bit (MSB). The truth table of the counter is
shown in figure.

Input Output
Clock D C B A
Reset 0 0 0 0
1 0 0 0 1
2 0 0 1 0
3 0 0 1 1
4 0 1 0 0
5 0 1 0 1
6 0 1 1 0
7 0 1 1 1
8 1 0 0 0
9 1 0 0 1
10 1 0 1 0
11 1 0 1 1
12 1 1 0 0
13 1 1 0 1
14 1 1 1 0
15 1 1 1 1
16 0 0 0 0

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