4.2.3 Crystal

ATmega4809 Curiosity Nano has a 32.768 kHz crystal footprint made for standard 3.2 mm by 1.5 mm surface-mount crystals with two terminals.

The crystal footprint is connected to the ATmega4809 by default, but the GPIOs are routed out to the edge connector through not mounted resistor footprints (J207 and J208).

The two I/O lines routed to the edge connector are disconnected by default to both reduce the chance of contention to the crystal as well as removing excessive capacitance on the lines when using the crystal. To use the PF0 and PF1 pins as GPIO on the edge connector, some hardware modification is needed.

Any crystal should be disconnected when the pins are used as GPIO, as this might harm the crystal
To connect the routing solder a 0 Ω resistor or add a solder blob to the footprints J207 and J208

The crystal has a cut-strap next (J209) to it, which can be used to measure the oscillator safety factor. This is done by cutting the strap and adding a 0402 SMD resistor across the strap. More information about oscillator allowance and safety factor can be found in the AN2648 application note from Microchip.

The cut straps and solder points can be seen in Figure 4-4.

Table 4-3. Crystal Connections
ATmega4809 Pin	Function	Shared Functionality
PF0	TOSC1 (crystal input)	Edge connector
PF1	TOSC2 (crystal output)	Edge connector

Figure 4-4. Crystal Connection and Solder Points