74LS90 is basically a MOD-10 ten counter that produces a BCD output code. It consists of four internally connected master-slave JK flip-flops to provide the MOD-2 (count up to 2) counter and the MOD-5 counter. The 74LS90 also has an independent pass-through JK flip-flop driven by CLKA, and the other three are driven by CLKB.
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