Design ideas and circuit design of external power-on delay reset circuit of PIC microcontroller

　　When designing a power-on delay reset circuit, the rise time of the power supply voltage VDD and the start-up time of the oscillator must be considered. The rise time of the power supply voltage VDD is related to the power supply and the circuits in the entire microcontroller application system that the power supply bears; the start-up time of the oscillator is related to the oscillator frequency. For example, when the oscillation frequency is 10MHz, the start-up time is about 1ms, and when the oscillation frequency is 1MHz, the start-up time is about 10ms.

　　In order to ensure reliable system reset, when the internal power-on reset function POR and two timers PWRT and OST of the microcontroller cannot meet the requirements, an external RC delay circuit can be designed to maintain the low level on the MCLR pin for a sufficiently long delay time. The hardware equivalent circuit related to the external power-on delay reset function is shown in Figure 1.

　　Figure 1 External power-on delay reset equivalent circuit

　　Figure 2 shows that when the RC value of the RC component is small, its RC time constant is smaller than the delay time of the timer PWRT. Figure 3 shows that when the RC value of the RC component is large, its RC time constant is greater than the sum of the delay times of the timer PWRT and the timer OST. At this time, the moment when the microcontroller leaves the reset state is determined by the low level delay time of the MCLR pin. This situation is very useful for testing or parallel operation of multiple PIC16F87X chips.

　　Figure 2 MCLR terminal connected to RC (smaller) components

　　Figure 3 MCLR terminal connected to RC (larger) component

　　Note that when the chip voltage VDD power-on rise time is greater than 100μs, the power-on reset function POR and power-on delay timer PWRT on the chip may not function normally, causing the chip reset (i.e. PG reset address) to be abnormal. In this case, it is generally recommended not to use PWRT.

　　If the vDD rise time is very long, the MCU usually needs to add a longer power-on delay circuit. A reliable power-on delay method is shown in Figure 4. An external reset circuit is connected to the MCLR terminal, and the delay time can be selected by the MCU developer.

　　2. Circuit design

　　When the PIC16F87X series microcontroller uses the low frequency oscillation mode, the microcontroller's external pin MCLR is connected as shown in (a) and (b) in Figure 4.

　　The maximum leakage current of the MCLR pin is 5μA, and the resistor R should be less than 40kΩ to ensure that its voltage drop is no more than 0.2V. R1 is used as a current limiting resistor with a value of 1.0Ω to protect the internal circuit of the MCLR pin.

　　Figure 4 MCLR connected to RC components

　　Diode D enables capacitor C to discharge quickly when the power is off. The charging process of capacitor C is useful to us, but the discharging process is not only useless, but also after a power outage, when C still has accumulated charge, if it is powered on again, the RC delay circuit will lose its delay function and cannot be reliably reset.