STM32- Nucleo-L476RG schematic: Why is PB12 (Pin 33) connected to V_ref/VDDA (Pin 13) ?

[FHopp.1]

Hello,

I'm designing a new schematic for am IOT project with STM32L476RGT and looked for reference

in the schematic of the Nucle-L476RG. In this schematic the  V_ref/VDDA (Pin 13) is connected to PB12 (Pin 33). Why are these two pins connected?

[Uwe Bonnes]

I did not check, but I guess for L4, SB37 is bridged, while SB34 is removed. Only for F37 with ths SDADC, SB34 is bridged and SB37 is removed.

[FHopp.1]

Thanks for your fast answer. But why is it bridged?

[FHopp.1]

I made a picture of the bridges:

[Uwe Bonnes]

For the F37, you need SD_VREF on PB12. Otherwise you want PB12 to be accessible on the external connectors.

[FHopp.1]

My aim is to use the V_ref pin as input for external Voltage reference. I couldn't find any hints in data sheet why this connection is made. What happens fpr example when I don't connect it?

[Amel NASRI]

Hello @FHopp.1​ ,

You need to refer to AN4555 (Getting started with STM32L4 Series and STM32L4+ Series hardware development) where you will find following details:

The internal voltage reference buffer supports two output voltages, which are configured with VRS bit in the VREF_CSR register:

– VREF+ around 2.048 V. This requires VDDA equal to or higher than 2.4 V.

– VREF+ around 2.5 V. This requires VDDA equal to or higher than 2.8 V.

VREF- and VREF+ pins are not available on all packages. When not available, they are bonded to VSSA and VDDA, respectively.

When the VREF+ is double-bonded with VDDA in a package, the internal voltage reference buffer is not available and must be kept disable (refer to datasheet for packages pinout description).

VREF- must always be equal to VSSA.

So all depends on the package you selected and the availability of VREF+. You need to check the datasheet for such purpose.

-Amel

[FHopp.1]

Exectly. But this don't answer the question why V_ref/VDDA (Pin 13) is connected to PB12 (Pin 33) on the Nucleo Board. Why did ST do this?

[Amel NASRI]

There is no SD_Vref+ for STM32L476. So when SB37 is bridged, this does not mean PB12 is connected to Vref+.

You need to look to SB57: it is ON which means "VDDA/VREF+ on STM32 is connected to VDD" (referring to UM1724).

-Amel

[FHopp.1]

OK, thanks!