Power on reset

Power on reset is an essential requirement for sequential circuit based modules. We consider a simple example, consider a remote controlled tube light or fan system, the circuit of interest is shown in below image.

The IR Rx is a infrared Receiver which trigger the DFF and switch ON/OFF the fan depending previous state of the DFF. The circuit is practically laid down on the PCB and is working fine. However, when the power is turned on some time the fan ON and some time OFF. It has happened that when supply  line is goes off, and supply arrive later the fan can turn on automatically. The problem is the DFF can attain logic high or low state and was unpredictable. 

In order to solve the problem, we need power on reset. Here R and C in the circuit do the job.

Whenever power is turned ON, while charging the capacitor, there is a current passing through the resistor and hence voltage across the resistor. This voltage uphold the reset pin of the DFF. The by selecting proper R and C values we can meet the reset condition of the DFF.After charging the capacitor C completely, reset pin is  grounded as there is no current through resistor. 

Very simple!