STM32L471RET6 Strange interaction about PB12 and PC12

[BamBam]

Hello all, I have encountered unexpected behavior on the voltage level of PC12 when I enable PB12.

My setup:

PC12: Level high, output mode PP, no PU no PD, Output speed High. 

PB12: Level low, output mode PP, no PU no PD, Output speed Low.

Without PB12 the High level of PC12 reaches 3.3V, enabling PB12 its high value becomes max 1.46V.

In my application PC12 was used as a chip select in an SPI comunication, It has a hardware pull up of 100K; PB12 was used for debugging purposes

[waclawek.jan]

Short on the board?

JW

[BamBam]

It was the first thing I checked, neither PB12 nor PC12 are shorted. Unfortunately I don't have a demoboard of this MCU for double checking.

I can add a detail: the same behavior described above works for PB12, its high level reaches 3v3 only if PC12 is disabled.

[waclawek.jan]

> It was the first thing I checked,

How?

Set one of them to output, the other to input (perhaps with a pullup or pulldown); wiggle the output (perhaps in debugger) and observe voltage on the other pin.

> chip select

It means, the short may be not only directly between these two pins; but VCC/2 is a quite good indicator of a short. You may also want to measure IDD.

JW