2-bit counter

Count 0, 1, 2, 3, 0, 1… every clock tick, with two T flip-flops. A ready-made counters circuit you can open in the TorchAnvil simulator.

Counting by toggling

A T flip-flop toggles its output on every rising clock edge — as long as its T input is held high. Press Run and FF0 flips every tick, producing 0, 1, 0, 1, …. That's counting in 1-bit binary.

The clever part: we feed FF0's output into FF1's clock. So FF1 only sees a rising edge when FF0 goes from low to high — which happens every two ticks. FF1 toggles half as often.

Read the lamps together as a two-bit binary number (Bit 1, Bit 0) and you get the sequence 00 → 01 → 10 → 11 → 00… — counting from 0 to 3 and wrapping.

How it works

• Clock → FF0.CLK — the base timing signal.
• HIGH → FF0.T — always toggle on rising edges.
• FF0.Q → FF1.CLK — the next bit's clock is the previous bit's output. This is called a ripple counter.
• HIGH → FF1.T — same deal.
• Each Q → Lamp shows that bit.

tick	FF1 (bit 1)	FF0 (bit 0)	decimal
0	0	0	0
1	0	1	1
2	1	0	2
3	1	1	3
4	0	0	0 (wrap)

Why it matters

Chain more T flip-flops and the counter grows. Four bits gets you 0–15. Eight bits gets you 0–255. The ripple pattern you see here is exactly how a frequency divider works, and it's the simplest kind of counter — every real CPU has a bigger, fancier cousin of this circuit somewhere inside.

Try this

• Press Run. Watch Bit 0 flicker, Bit 1 flicker at half the rate.
• Press Reset (in the toolbar). Both lamps go dark — flip-flops reset to 0.
• Try setting the clock period to 8 for a slower count you can read in real time.