Counter

Toggling Q1 on a negative edge of Q0 is the same as toggling Q1 on a positive edge of Q0',

so connect the negated output Q' of each flip flop to the input of the next flip-flop:

Timing diagram for this 3-bit counter:

toggle x1

toggle x2

Read values in each column from left to right:

000, 001, 010, . . .

Variation: how would we use D flip-flops instead of T flip-flops?