Hi
Lets understand the role of capacitor in reset circuit of power-on reset. In fig1,initially,when +5V is applied to capacitor, one plate of capacitor gets charge accumulated whereas other plate is deprived of charge. This charge imbalance generates potential across capacitor plate which makes current flow in capacitor and capacitor acts as short circuit. This state of capacitor makes micro-controller reset (since RESET in 8051 is active high) at power-on of system. After some t+ time,other plate of capacitor also gets equal amount of charge,thus potential drop between plates vanish which stops any current flow through capacitor and capacitor is called fully charged.At this stage, capacitor works as open circuit to the current coming from +5V supply.
But the real question is what is the role of resistor in reset circuit ?
The resistor available in this circuit is also called as pull down resistor. When the capacitor is fully charged, since it acts as open circuit, the charge accumulated on RESET pin need a discharging path, which is given by this pull down resistor. The sole purpose of pull down resistor is to pull down the RESET pin to GND when it is not connected to +5V such that state of RESET pin could be defined at any point of time.
In fig2, simply by connecting switch across capacitor, we create a bypass path for the current coming from +5V, such then when needed RESET pin can be activated as per requirement (even though capacitor acts as open circuit due to its fully charged state) and make the micro-controller reset.
Happy learning :-)
Lets understand the role of capacitor in reset circuit of power-on reset. In fig1,initially,when +5V is applied to capacitor, one plate of capacitor gets charge accumulated whereas other plate is deprived of charge. This charge imbalance generates potential across capacitor plate which makes current flow in capacitor and capacitor acts as short circuit. This state of capacitor makes micro-controller reset (since RESET in 8051 is active high) at power-on of system. After some t+ time,other plate of capacitor also gets equal amount of charge,thus potential drop between plates vanish which stops any current flow through capacitor and capacitor is called fully charged.At this stage, capacitor works as open circuit to the current coming from +5V supply.
But the real question is what is the role of resistor in reset circuit ?
The resistor available in this circuit is also called as pull down resistor. When the capacitor is fully charged, since it acts as open circuit, the charge accumulated on RESET pin need a discharging path, which is given by this pull down resistor. The sole purpose of pull down resistor is to pull down the RESET pin to GND when it is not connected to +5V such that state of RESET pin could be defined at any point of time.
In fig2, simply by connecting switch across capacitor, we create a bypass path for the current coming from +5V, such then when needed RESET pin can be activated as per requirement (even though capacitor acts as open circuit due to its fully charged state) and make the micro-controller reset.
Happy learning :-)
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