Use CTC mode of Timers to generate the following waveform on the OCnA (with n is 0,1,2 depending on the Timer you use). Figure 15-6. CTC Mode, Timing Diagram TCNTn OCnA (Toggle) 111 Im 3s 3s 3s 25 1s 4s (COMnA1:0 = 1) OCNA Interrupt Flag Set or ICFn Interrupt Flag Set (Interrupt on TOP) This figure is adapted from the definition of the CTC mode from datasheet (Page 100 in the attached datasheet on Canvas). You are freely to choose the type of Timer, the setting, and Interrupts if any. The generated delay time can be a bit off from the requirement (could be due to rounding the Timer Count from calculation).