BCD Counter Using D Flip Flops

This BCD counter uses d-type flip-flops, and this particular design is a 4-bit BCD counter with an AND gate. BCD counters usually count up to ten, also otherwise known as MOD 10. Since a 4-bit counter counts from binary 0 0 0 0 to binary 1 1 1 1, which is up to 16, we need a way to stop the count after ten, and we achieve this using an AND gate to initiate a reset. The 4-bit binary pattern for decimal ten is 1 0 1 0, and this is the only time when we see this pattern, therefore we can use the 1’s and feed them into an AND gate, which should produce a HIGH output, which can then reset all the flip-flops simultaneously, thereby stopping the count.

Understanding MSB and LSB

The first thing to identify is the most significant bit (MSB), and least significant bit (LSB) concerning the outputs Q₀, Q₁, Q₂, Q₃. If you do not get this part right, then you will trip-up when it comes to connecting the AND gate.

The way I remember it is that this counter always starts with the smallest number, which appears first at Q₀, therefore it has to be the LSB end.

As you can see from the table above, the outputs Q₃ and Q₁ contain the 1’s and therefore we connect these outputs to the AND gate.

4-bit BCD Counter Truth Table