On the AVR128DB48 Curiosity Nano board is a mounted 16.00 MHz crystal of type VXM7-9040-16M0000000, the crystal is accurate to within 20 ppm. Using the External High-Frequency Crystal Oscillator (XOSCHF) module in AVR128DB48 with the mounted 16.00 MHz crystal generates a clock signal that is significantly more accurate than the Internal High-Frequency Oscillator (OSCHF) module.
The 16.00 MHz crystal is connected to AVR128DB48 on PA0 and PA1, which are also routed to the edge connector through two solder points. PA0 and PA1 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 16.00 MHz crystal is a cut-strap (J217), 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 |
|---|---|---|
| PA0 | XTALHF1 (Crystal input) | Edge connector |
| PA1 | XTALHF2 (Crystal output) | Edge connector |
PA0 and PA1 can be used as GPIO. To do so, some hardware modifications are required.
- Disconnect the 16.00 MHz crystal by cutting the two straps on the top side of the board next to the crystal (J215, J216). 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 PA0 and PA1 on the bottom side of the board (J213, J214)
