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Generate reset signal to be input in IC at power-on

Latest Updated:09/30/2016

Question:

How to generate a reset signal to be input in IC when power is turned on?

Answer:

The easiest way is to use a reset IC.
Alternatively, the reset signal can be placed at low level by using an RC-circuit time constant.
The figure below shows an example of a circuit used to generate a reset signal when the power is turned on. In this example, the circuit also includes a reset switch to enable the reset signal to be generated at any time. The diode in the sample circuit is there to prevent the microcontroller being damaged by latch-up. In most ICs, the reset input is of the Schmitt trigger type, so the reset can occur even if the reset signal waveform rises slowly.
When using a capacitor with a high capacitance and low ESR, a current-limiting resistor (r in the figure below) is required between the diode and the capacitor for protection. Resistor r protects the capacitor from degradation from sudden discharge of electric charge from the capacitor when the power supply is cut off. Limit the current using a resistor with a low enough resistance value to obtain a voltage corresponding to a low level when the switch is switched on but still protect the buffer.


If the circuit contains only a pull-up resistor and no capacitor, the reset signal will rise together with the rise of the power supply voltage. In this case, a reset will not occur.
When considering a capacitor in terms of water and buckets, the job of the capacitor is to secure the time from when the bucket is empty until it overflows.
Note that recent microcontrollers feature on-chip power-on clear (POC) circuits. With these microcontrollers, the reset signal is generated internally, so there is no need to input a reset signal when the power is turned on.
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