I think that's the HID bootloader not atmel-dfu. i posted elsewhere and was pointed to ubaboot which is a similar size to halfkay.
a 39ohm resistor is neither rare or expensivethe 49.9Ω & 180Ω resistors added together in parallel give 39.08 ohms, close to the 39ohms of the datasheet. it's often necessary to use multiple components to replace a rare or expensive part.
The microcontroller should have a reference implementation in the datasheet or application notes, with the crystal and capacitor values specified.Thanks for the replies
Regarding the crystal load capacitors - the 2 load capacitors values are 10pf on the Teensy; 20pf on the HSGW Plaid; & 22pf on your mouse. Did you have to make calculations to figure out the correct load capacitance (or am I overthinking)?
Also, what's happening on the left side of the atmega on your schematic, with all of those 4.7k resistors that are also connected to VCC then to the switches? Is that hardware switch debouncing?