Sidekick64 is a versatile cartridge/expansion for the C64 and C128 (and new: C16/+4) whose functionality is entirely defined by software running on a Raspberry Pi 3 (RPi). The connecting circuitry is quite simple and does not include any programmable ICs. Example use cases are connecting the RPi to the expansion port where it can emulate
a GeoRAM/NeoRAM-compatible memory expansion,
a (freezer or Easyflash) cartridge, or
run a (Dual-)SID plus FM emulation (the SID emulation would also work when connecting the RPi to the SID-socket on the board).
But many more things are imaginable, e.g. 80 column cards with HDMI video output, custom accelerators/coprocessors etc.
Sidekick64 is a result, or cumulation, of the RasPIC64 project, which is the framework enabling a RPi to bidirectionally communicate on the bus of a Commodore 64/128. Currently, Sidekick64 is set up to work with a Raspberry Pi 3A+ or 3B+ and has only be tested with PAL-machines (C64 various revisions, C128).