What is going on here? According to the circuit diagram there are two microcontrollers (U1 and U2, with slightly different partnumbers) on the Arduino Leonardo, but when I take a look at the PCB I can only find one. Is it a common way of drawing things and how does Eagle know it is one and the same device?
Unfortunately the project files only open in Eagle, so for what it is worth I included two screendumps. There is a pdf version of the circuit diagram too.
I looked at the Eagle CAD.
There are two different footprints, a QFN44 and a TQFP44. The pads are unique for either package, but the footprints are 'nested'. Typically the pin-out must be identical in sequence for both parts for this to work without vias.
Then Leonardo's tracks are routed in such a way that equivalent pins (i.e. pads) of each package are connected, and either part can be used. So either one can be fitted.
The pads are connected then connected to the remainder of the board, either part will work.
ST Micro use a similar technique across many of their MCU families. ST go even further, and have the pin order arranged in such a way that e.g. a 48pin part can nest within a 64pin part footprint. Tracks do not need to cross (which would require vias) to connect them both up.
Very sneaky!
