Custom sensors in cartridges: how does one read data?

Hi GBATemp community,

I'm trying to understand how custom sensors added to certain cartridges work, such as Boktai's sunlight sensor. the thread "boktai-solar-sensor-patch-kit.245127" (cant post a link...) lists a neat ROM hack where I can defer the address in the assembly where the sensor data is read. And I understand that any game can solder on any kind of sensor, it's game-specific, not platform-specific.

I'm a software guy, not a hardware one, so I wonder: to the devs just 'know' you...

Custom sensors in cartridges: how does one read data?