4.2.3 Crystal

The ATtiny1627 Curiosity Nano Board has a 32.768 kHz crystal footprint made for standard 3.2 mm by 1.5 mm surface mount crystals with two terminals.

The crystal footprint is not connected to the ATtiny1627 by default, as the GPIOs are routed out to the edge connector. To use the crystal, some hardware modifications are required.
  • The two I/O lines routed to the edge connector must be disconnected to reduce the chance of contention to the crystal, and to remove excessive capacitance on the lines.
  • Disconnect the edge connector by cutting the two cut-straps on the bottom side of the board, marked PB2 and PB3.
  • Connect the crystal by soldering on a solder blob on each of the circular solder points next to the crystal on the top side of the board, J210 and J211.

The crystal has a cut-strap next (J209) to it, 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.

The cut straps and solder points can be seen in Figure 4-4.

Table 4-3. Crystal Connections
ATtiny1627 PinFunctionShared Functionality
PB3TOSC1 (Crystal input)Edge connector
PB2TOSC2 (Crystal output)Edge connector
Figure 4-4. Crystal Connection and Cut Straps

A crystal with 7 pF equivalent load capacitance and 70 kΩ ESR (XC200) and two 4.7 pF external matching capacitors for C203 and C204 have been tested to yield good results on this board. See the Assembly Drawing for footprint locations.