8051 Serial Programming in C
Everything in a microcontroller is manipulated by writing and reading to registers. These registers are you API, so to speak. They are what your rather arbitrary, non-physical software manipulates to get things to physically happen in real things on the processor.
That means controlling the ADC is writing to registers. This includes settings (you write to a register to enable or disable settings and set values) and one-time actions (you write to a flag to get the ADC to do something a single time and the register might reset after that). Similarly, getting data from the ADC is also reading registers. You would have to look at the datasheet of the MCU you are using to figure out what those registers are, what they do, and how they work.
If you only need to TX, you just TX and ignore the fact that the RX exists entirely.