DAC (PA4) giving constant 2.4V in STM32F446RE

[Paddy611]

Hi, 

Greetings!!

We have been using STM32F446RE MCU in the project. We have implemented the DAC in the application at pin PA4 and PA5. Both DACs are generating similar voltages for different application. 

The PA4 DAC is connected to ADC of another MCU through a cable with 15 cm length. In few boards we have observed that this DAC at pin PA4 stops working and gives constant 2.4V. Whereas the DAC at pin PA5 works as expected. The DAC at pin PA5 is not connected anywhere at this moment. 

What would be the reason for this situation. Kindly advise. 

We have not used any protection circuit from PA4 of STM32 to ADC pin of other MCU. It is simply connected through a cable. 

Thanks,

Purushottam 

[TDK]

If PA4 is stuck at a value, it is probably due to a design error or a hardware issue or error.

• Hardware issue, such as the pin is tied to something else via a solder bridge. Take a resistance measurement of the pin to ground when the power is out. It should be high. It should match the value you get from PA5.

Another thing you could do is measure the voltage on the chip while the chip is in reset (NRST low). If it's still 2.4 V, it's definitely not a software error but a hardware error instead.

[Paddy611]

Hi,

Thanks for the suggestion.

I do agree that the error is because of hardware. The things were working well for couple of months.. So I am curious why this might have happened and what will be the precautions that we should take to avoid this again

Thanks

[TDK]

Most likely event is an overvoltage condition somewhere. Perhaps it got shorted to 5V or similar. Could also be ESD damage, but that's much less likely. Perhaps something metallic was dropped on the board which shorted random pins together. Hard to guess without knowing the setup.

[SofLit]

Hello,

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@Paddy611 wrote:

The PA4 DAC is connected to ADC of another MCU through a cable with 15 cm length. In few boards we have observed that this DAC at pin PA4 stops working and gives constant 2.4V.

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What if you move this wire from PA4 to PA5? Do you have the same behavior? do you have the behavior when PA4 is disconnected?

[Paddy611]

We tried changing the wire from PA4 to PA5. 

PA5 is working as expected. Changing the DAC voltage at PA5. Whereas the PA4 status is same, i.e. @2.4V even if the wire is disconnected from PA4 and measured the signal at the pin.