It is enough to use just UART and ground (TXD, RXD and ground,). These are the essential pins that need be controlled during programming. You may also need Pin number 4, which is explained in the next question: https://radiocrafts.com/kb/riim-how-can-i-use-pin-4-the-bsl-pin/
Last Update: September 29, 2021
September 29, 2021 777 RIIM Q&A