A talk of RubyKaigi 2019 in Fukuoka. Writing mruby/c firmware applications is like writing mrbgems. You need to make some C functions and mruby wrapper of them in order to handle peripherals like sensor, flash memory or BLE. Easy to imagine it's hard to develop for a team in a situation of TIGHT COUPLING, right? I will talk about some tools, mrubyc-test and mrubyc-debugger, which I made with CRuby for testing and debugging to keep our team slack coupling.