On the AVR128DB48 Curiosity Nano board is a mounted 32.768 kHz crystal of type VMK3-9001-32K7680000, the crystal is accurate to within 20 ppm. Using the External 32.768 kHz Crystal Oscillator (XOSC32K) module in AVR128DB48 with the mounted 32.768 kHz crystal generates a clock signal that is significantly more accurate than the Internal 32.768 kHz Ultra Low-Power Oscillator (OSC32K) module can generate alone. With the Auto-Tune feature, the 32.768 kHz can be used as a reference to improve the accuracy of the internal oscillator. This is an ideal reference clock for the Real-Time Counter module in AVR128DB48.
The 32.768 kHz crystal is connected to AVR128DB48 on PF0 and PF1, which are also routed to the edge connector through two solder points. PF0 and PF1 are disconnected from the edge connector by default to reduce the chance of an external signal causing contention with the crystal, and to remove excessive capacitance on the lines.
Next to the 32.768 kHz crystal is a cut-strap (J209), 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.
Cut straps and solder points can be seen in Figure 1.
| AVR128DB48 Pin | Function | Shared Functionality |
|---|---|---|
| PF0 | XTAL32K1 (Crystal input) | Edge connector |
| PF1 | XTAL32K2 (Crystal output) | Edge connector |
PA0 and PA1 can be used as GPIO. To do so, some hardware modifications are required.
- Disconnect the 32.768 kHz crystal by cutting the two straps on the top side of the board next to the crystal (J207, J208). 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 (J210, J211)
