45.4 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 cause 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 45-4. External Reset Circuit Schematic
Figure 45-5. External Reset Circuit Schematic (EFT Immunity Enhancement)
Note: This 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.
Table 45-3. Reset Circuit Connections
Signal NameRecommended Pin ConnectionDescription
RESET

Reset low-level threshold voltage

VDDIO = 1.6V - 2.0V: Below 0.33 * VDDIO

VDDIO = 2.7V - 3.6V: Below 0.36 * VDDIO

Decoupling/filter capacitor 100 pF(1)Pull-up resistor 2.2 kΩ(1)(2)Resistor in series with the switch 330Ω(1)		Reset pin
  1. These values are given as a typical example.
  2. The SAM D21 features an internal pull-up resistor on the RESET pin, hence an external pull-up is optional.