Datasheet

42.4 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.
Figure 42-4. External Reset Circuit Schematic
GND
RESET
Ω
V
DD
330Ω
2.2k 100pF
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, i.e.,
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.
Table 42-3. 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 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.
42.5 Clocks and Crystal Oscillators
The SAM D21 can be run from internal or external clock sources, or a mix of internal and external
sources. An example of usage will be to use the internal 8MHz oscillator as source for the system clock,
and an external 32.768kHz watch crystal as clock source for the Real-Time counter (RTC).
SAM D21 Family
Schematic Checklist
© 2018 Microchip Technology Inc.
Datasheet Complete
DS40001882D-page 1166