49.4 External Reset Circuit

The external Reset circuit is connected to the RESET pin when the external Reset function is used. The circuit is not necessary when the RESET pin is not driven low externally by the application circuitry.

The reset switch can also be removed, if a manual reset is not desired. The RESET pin itself has an internal pull-up resistor, therefore it is optional to add any external pull-up resistor.

A pull-up resistor makes sure that the reset does not go low and unintentionally cause a device reset. An additional resistor has been added in series with the switch to safely discharge the filtering capacitor, which prevents a current surge when shorting the filtering capacitor which again can cause a noise spike that can have a negative effect on the system.

Place the components illustrated within one-half inch (12 mm) from the RESET pin.

Figure 49-4. External Reset Circuit Schematic

Note: These values are only given as a typical example. Refer to the 46.2 Power Supply to determine proper values given system parameters to meet reset timing requirements (T_RST).

The following reset circuit is intended to improve EFT immunity, but does not filter low-frequency glitches, which makes it not suitable as an example for applications requiring debouncing on a reset button.

Figure 49-5. External Reset Circuit Schematic (EFT Immunity Enhancement)

Note: Reset circuit recommendations to improve EFT/ESD/EMI immunity can be found in the Basic 32-Bit MCU Design and Troubleshooting Checklist guide on the Microchip web site.