Study the Operation of Digital Counters 7490 & 7493

Jun 25 2023

Aim

Operate the counters 7490, 7493.

Apparatus Required:

Logic trainer kit, Counter ICs- 7490, IC - 7493 wires.

Theory

Asynchronous 74LS90 Decade Counter Digital counters count upwards from zero to some pre-determined count value on the application of a clock signal. Once the count value is reached, resetting them returns the counter back to zero to start again. A decade counter counts in a sequence of ten and then returns back to zero after the count of nine. Obviously to count up to a binary value of nine, the counter must have at least four flip-flops within its chain to represent each decimal digit as shown.

The 74LS90 BCD Counter The 74LS90 integrated circuit is basically a MOD-10 decade counter that produces a BCD output code. The 74LS90 consists of four master-slave JK flip-flops internally connected to provide a MOD-2 (count-to-2) counter and a MOD-5 (count-to-5) counter. The 74LS90 has one independent toggle JK flip-flop driven by the CLK A input and three toggle JK flip-flops that form an asynchronous counter driven by the CLK B input as shown.

The counters four outputs are designated by the letter symbol Q with a numeric subscript equal to the binary weight of the corresponding bit in the BCD counter circuits code. So for example, Q_A, Q_B, Q_C and Q_D. The 74LS90 counting sequence is triggered on the negative going edge of the clock signal, that is when the clock signal CLK goes from logic 1 (HIGH) to logic 0 (LOW). The additional input pins R₁ and R₂ are counter “reset” pins while inputs S₁ and S₂ are “set” pins. When connected to logic 1, the Reset inputs R₁ and R₂ reset the counter back to zero, 0 (0000), and when the Set inputs S₁ and S₂ are connected to logic 1, they Set the counter to maximum, or 9 (1001) regardless of the actual count number or position.