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Monitoring the internal voltage reference

Question asked by Lin Zi on May 30, 2018
Latest reply on May 30, 2018 by Lin Zi

Hello ST engineers,

I have some questions about ADC of MCU STM32F302RC, please help to answer.

In our application, operating voltage of MCU is 3.3V (VDD=VDDA=3.3V). In firmware, we want to use VREFINT of ADC to calculate the VDDA to detect it.

According to the reference manual RM0365, section 15.3.32 (Monitoring the internal voltage reference) at page 353/1080 described how to calculate VDDA with VREFINT. Here is the formula,

Under normal operation, there is no problem. The calculated value of VDDA is right, and the value is about 3.29V which is almost equal to the actual power supply voltage.

But as long as the input voltage of PA0 (TTa, this port is used as ADC function, channel is ADC1_IN1) is subjected to a voltage greater than 3.65V, such as 3.75V, the calculated value of VDDA will be abnormal and become to about 1.22V. My question are,

1), why this phenomena happen?

2), as long as any channel of ADC is subjected to a higher voltage (greater than 3.65V), the calculated value of VDDA based on this formula will be abnormal, such as become to 1.22V?

3), when a normal GPIO (no ADC function) of the MCU is input a voltage greater than 3.65V, will this phenomena happen?

 

Thank you in advance!

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