4.2.3 32.768 kHz Crystal

The AVR128DA48 Curiosity Nano board has a 32.768 kHz crystal mounted.

The AVR128DA48 is connected to the crystal by default, but the GPIOs are also routed to the edge connector through two solder points. The two I/O lines routed to the edge connector are disconnected by default to reduce the chance of an external signal causing contention with the crystal and to remove excessive capacitance on the lines. To use PF0 and PF1 as GPIO, some hardware modifications are required.

• Disconnect the crystal by cutting the two straps on the top side of the board next to the crystal (J210, J211). The crystal should be disconnected when using the pin as GPIO, as this might harm the crystal.
• Connect the I/O lines to the edge connector by placing solder blobs on the circular solder points marked PF0 and PF1 on the bottom side of the board (J207, J208)

The 32.768 kHz crystal has a cut-strap next to it (J209), which can be used to measure the oscillator’s 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.