I'm just trying to read analog data (from potentiometer) through the ADC of an STM32F303 (on the discovery board).

My minimum programming involves:
let rcc = unsafe { &*RCC::ptr() };
let adc = unsafe { &*ADC1::ptr() };

rcc.ahbenr.write(|w| w.adc12en().enabled());
adc.cr.write(|w| {
    w.advregen().intermediate();
    w.advregen().enabled();
    w.aden().enabled()
});
// some other stuff
hprint!("{}\n", adc.isr.read().adrdy().is_ready()); // to check if adc is ready

When I run the print statement in a loop to check if the adc is ever ready for conversion, it always reads "false." Am I missing any step for setting up the ADC? I'm referring to the data sheet for the STM32F303 to see the startup sequence.

A related question: Given that there is a startup time for the ADC after writing to the advregen, how do I know that I allow sufficient time between successive writes to the advregen bits?