The max measurable voltage of P0_2 is 1.15 because I am using HAL_ADC_REF_125V as reference voltage here. If the input voltage is larger than 1.15V, the ADC reading adc_ain2 will be always 511. If I connect P0_2 to 0.5V, I will have ADC reading adc_ain2=223. Then, we can convert the reading 223 to measured voltage using the following equation.
measured voltage = 1.15x223/511=0.501...