The xBUS is the main connection between the xCHIP. Most xCHIPs will have a connection on all 4 sides, but some will have a connection sacrifices either for space or for other connectors, such as USB or antenna connectors.
The xPDI is the Programming and Debugging Interface. All low-level programmable core xChips will have such a connection on the south side, and all programmers able to do low-level programming will have a similar connector on its north side.
The xPDI is a programming and debugging interface supporting programming using SPI, JTAG, SWD and USB. Serial programming using a boot loader is supported on the main xBus.
The interface has to support debugging using OpenOCD[]. This can be achieved on Raspberry Pi using:
Power, Reset and Serial Connection is provided by the main xBUS and is not duplicated on the xPDI, therefore the xPDI will only work with xBus connected as well. Only 3.3v circuits are supported.