The AVR16EB32 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 AVR16EB32 by default, as the GPIOs are routed to the
edge connector.Figure 4-1. 32.768 kHz Crystal Block
Diagram
Some hardware modifications are required to use a crystal with the AVR16EB32.
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, J107 and J108. Marked PA0 and PA1 in
the silkscreen
Connect the crystal by soldering on a
solder blob on each circular solder point next to the crystal on the top side of the
board, J109 and J110
Add suitable capacitors to C104 and
C105
Add a suitable series resistor to
R100
Figure 4-2. Crystal Cut Straps
Info: The series resistor R100 may
be required to limit the drive strength of the crystal. If no resistor is required, the pads
of R100 can be connected with a solder blob.
Important: PA0 and PA1 are used as the I2C pins on the edge connector. Disconnecting PA0 and PA1 from the edge connector may
disable some functionality with connected base boards.
The crystal has a cut strap (J111), which can be used to measure the oscillator safety factor
and is done by cutting the strap and adding a 0402 SMD resistor across the strap. The AN2648 application note from Microchip contains more
information about oscillator allowance and safety factors.
Figure 4-2 shows the cut
straps and solder points.