Decade Counter

A decade counter is a binary counter that is designed to count from 0000₂ to 1010₂. An ordinary four-stage counter can be easily modified to a decade counter by adding a NAND gate as shown in figure 3-25. Notice that FF2 and FF4 provide the inputs to the NAND gate. The NAND gate outputs are connected to the CLR input of each of the FFs.

The counter operates as a normal counter until it reaches a count of 1010₂, or 10₁₀. At that time, both inputs to the NAND gate are HIGH, and the output goes LOW. This LOW applied to the CLR input of the FFs causes them to reset to 0. Remember from the discussion of J-K FFs that CLR and PS or PR override any existing condition of the FF. Once the FFs are reset, the count may begin again. The following table shows the binary count and the inputs and outputs of the NAND gate for each count of the decade counter:

BINARY COUNT	NAND GATE INPUTS		NAND GATE OUTPUT
*******	A	B	*******
0000	0	0	1
0001	0	0	1
0010	1	0	1
0011	1	0	1
0100	0	0	1
0101	0	0	1
0110	1	0	1
0111	1	0	1
1000	0	1	1
1001	0	1	1

Changing the inputs to the NAND gate can cause the maximum count to be changed. For instance, if FF4 and FF3 were wired to the NAND gate, the counter would count to 1100₂ (12₁₀), and then reset.