5.2.5 32.768 kHz Crystal
On the AVR64EA48 Curiosity Nano board, a VMK3-9002-32K7680000TR 32.768 kHz crystal is mounted. The crystal is accurate to within 20 ppm. Using the external 32.768 kHz Crystal Oscillator (XOSC32K) module in AVR64EA48 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, which is an ideal reference clock for the Real-Time Counter module in AVR64EA48.
The 32.768 kHz crystal is connected to AVR64EA48 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), used to measure the oscillator safety factor by cutting the strap and adding a 0402 SMD resistor across. More information about the oscillator allowance and the safety factor is available in the AN2648 application note from Microchip.
Figure 5-5 shows the cut straps and solder points.
| AVR64EA48 Pin | Function | Shared Functionality |
|---|---|---|
| PF0 | XTAL32K1 (Crystal input) | Edge connector |
| PF1 | XTAL32K2 (Crystal output) | Edge connector |
Some hardware modifications are required to use PF0 and PF1 as GPIO.
- Disconnect the 32.768 kHz crystal by cutting the two straps on the top side of the board next to the crystal (J208, J207). The crystal must be disconnected when using the pin as GPIO, or it 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)
