SM32F4 PCR_ON connected to GND

[Stefano Ugolini]

Hi,

we have an STM32F27ZI with VDD at 3.6V.

We connected by mistake PDR_ON to GND while we drive the NRST with an external voltage monitor as in the picture.

reset circuit to NRSTPower upPower down

Yellow -> 3.6, Purple -> Reset, Green-> PDR_ON

In addition, we also use VBAT to keep the RTC running and the BKPRAM on.

What consequence can it have?

So far we haven't experienced any "wrong", RTC keeps working as well as BKPRAM.

Only recently we had a case where in one our product, after a possible power off, the MCU could not boot anymore. We manage to boot the MCU again only flashing a new FW using the ST internal bootloader.

Thank you!

[Peter BENSCH]

What do you mean by PCR_ON?

Regards

/Peter

[Stefano Ugolini]

Sorry I meant PDR_ON, I correct it in the post

Thanks

Stefano

[Peter BENSCH]

The data sheet of the STM32F427 mentions two important details:

[3.21] When PDR_ON pin is not connected to VDD (Internal Reset OFF), the VBAT functionality is no more available and VBAT pin should be connected to VDD.

[3.17.2] PDR_ON controls the internal power reset circuitry, i.e. for PDR_ON = GND this section applies:

Internal reset OFF

This feature is available only on packages featuring the PDR_ON pin. The internal power-on reset (POR) / power-down reset (PDR) circuitry is disabled through the PDR_ON pin. An external power supply supervisor should monitor VDD and should maintain the device in reset mode [...]

As a consequence, you can work with PDR_ON=GND, but then you no longer have backup functionality via VBAT, so that if VDD fails, the backup domain is not buffered and the data in the backup RAM and in the RTC are lost.

You should also carefully check the function of your external reset module to see if it meets the requirements stated in the above data sheet, section 3.17.2.

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Good luck!

/Peter