51.4 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, hence 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 causing a device reset. An additional resistor has been added in series with the switch to safely discharge the filtering capacitor, that is, preventing a current surge when shorting the filtering capacitor which again can cause a noise spike that can have a negative effect on the system.

Figure 51-6. External Reset Circuit Schematic

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

Table 51-3. Reset Circuit Connections
Signal Name	Recommended Pin Connection	Description
RESET	Reset low level threshold voltage V_DDIO = 1.6V - 2.0V: Below 0.33 * V_DDIO V_DDIO = 2.7V - 3.6V: Below 0.36 * V_DDIO Decoupling/filter capacitor 100nF⁽¹⁾ Pull-up resistor 10kΩ⁽¹⁾⁽²⁾ Resistor in series with the switch 330Ω⁽¹⁾	Reset pin

1. These values are only given as a typical example.

2. The SAM L21 features an internal pull-up resistor on the RESET pin, hence an external pull-up is optional.