Dip in analog input voltage during uart rx reception

[skuma.142]

Hi,

we are using STM32F030C8 mcu for our application. The port pin PA3 is configured for UART2 receive operation from a RS-485 transceiver and port pin PA5 is configured as an analog input ADC_IN5 and receives signal from an potentiometer. 

What we observe is, whenever there is a reception in port PA3 a dip of around 350 millivolt in analog voltage occurs in port PA5/ADC_IN5. We think the voltage dip is caused by the STM32F030C8 MCU internally. Following observations suggests that.

1) The 3.3 voltage input to the potentiometer and the mcu remains constant when the uart rx is working.

2) The dip in potentiometer voltage doesn't happen when the PA5 pin is disconnected. This we did by lifting up the mcu pin from the pad.

Please let us know if this is a known fault and if there is a way to workaround it. We have already fabricated the PCBs using this schematic and the product is failing in the field because of this.

Regards,

Pavan

[Tesla DeLorean]

Sounds more like an issue with the design, either a pin contending, or a supply/current issue. Would look at how power is routed to the RS485 transceiver. Is this a 2-layer board?

Disconnect the trace to PA3, and see if the issue persists