4.2.3 32.768 kHz Crystal
On the PIC16F18076 Curiosity Nano board, a VMK3-9001-32K7680000 32.768 kHz crystal is mounted. The crystal is accurate to within 20 ppm.
The 32.768 kHz crystal is connected to PIC16F18076 pin RC0 and RC1, which are also routed to the edge connector through two solder points. RC0 and RC1 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.
The crystal has a cut-strap (J211) next to it that 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 Crystal Connection and Cut Straps.
| PIC16F18076 Pin | Function | Shared Functionality |
|---|---|---|
| RC0 | SOSCO (Crystal output) | Edge connector |
| RC1 | SOSCI (Crystal input) | Edge connector |
Some hardware modifications are required to use RC0 and RC1 as GPIO.
- Disconnect the crystal footprint by cutting the two straps on the top side of the board next to the crystal (J209, J210). The footprint must be disconnected if a crystal is mounted to prevent damage to the crystal.
- Connect the I/O lines to the edge connector by placing solder blobs on the circular solder points marked RC0 and RC1 on the bottom side of the board (J207, J208)