The Power-on-Reset (PoR) circuit composed with a combination of RC network that detects the power applied to the microcontroller chip. This RC network generates a reset impulse that goes to the entire circuit placing it into a known state. A schmitt trigger is used to detect the rising charged voltage of the RC network and to generates an impulse. This impulse is generated based on the two threshold voltages of the schmitt trigger. When the charged input voltage at the schmitt trigger input reaches the first threshold voltage the output of the schmitt trigger switches so that it generates the first edge of the input. The charging of the RC network should be long enough so that the PoR can reset all the internal circuits before the charging voltage reaches the other threshold voltage of the schmitt trigger and the output to switch back.
The reset pulse generated by RC circuit is quite sensitive of value of R and C and to the power-supply ramp characteristics. When the power supply ramp is rapid, the R and C values can be calculated so that the time to reach the switching threshold of the schmitt trigger is enough to apply a long enough reset pulse. When the power supply ramp itself is slow, the RC network tends to get charged up along with the power-supply ramp up. So when the input schmitt stage is all powered up and ready, the input voltage from the RC network would already have crossed the schmitt trigger point. This means that there might not be a reset pulse supplied to the core of the VLSI.