STM32C071 LQFP 32pin, need some help for first time design

[LMore.2]

I've already become familiar with the NUCLEO-C071 by creating some additional hardware (display, encoder, etc.). Now, I want to design a board using the smaller 32-pin version. However, in this smaller version, I find some differences that I don't know how to address (I have read the datasheet thoroughly multiple times but cannot find any answers):

1) In the 64-pin version, pin 6 is the VBAT to connect a battery or a supercapacitor to maintain the RTC operation in case of power loss. Where has it gone in the 32-pin version?

2) In the 64-pin version, both the quartz for the CPU and for the RTC can be mounted. Can you confirm that in the 32-pin version, the CPU only works without quartz? Even for managing USB?

Thank you to anyone who can help me.

Luca.

[Chris21]

I have not used this device, but it appears to allow one crystal connection on the smaller packages, used as either HSE or LSE.  From the Reference Manual:

"For the devices housed in 48-pin packages, the HSE and LSE oscillators have separate
input and output pins (see HSE_NOT_REMAPPED bit of the FLASH option bytes). On
packages with less than 48 pins, HSE and LSE oscillators have one common input pin
OSCX_IN and one common output pin OSCX_OUT, which restricts their use to one at a
time (the other must be disabled)."

There is no mention of a VBAT pin, apparently this microcontroller does not have that feature unlike most STM32s.

[Andrew Neil]

What's the full part number you're considering?

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@LMore.2 wrote:

1) In the 64-pin version, pin 6 is the VBAT to connect a battery or a supercapacitor to maintain the RTC operation in case of power loss. Where has it gone in the 32-pin version?.

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With the smaller packages, there just aren't enough pins for everything - this is one of the things you lose.

Application Note AN5673, Getting started with STM32C0 MCU hardware development:

https://www.st.com/resource/en/application_note/an5673-getting-started-with-stm32c0-mcu-hardware-development-stmicroelectronics.pdf

And check out all the other Application Notes - see the 'Documentation' tab on the Product Page for whichever part you'll be using; eg,

https://www.st.com/en/microcontrollers-microprocessors/stm32c071rb.html#documentation

[TDK]

> 1) In the 64-pin version, pin 6 is the VBAT to connect a battery or a supercapacitor to maintain the RTC operation in case of power loss. Where has it gone in the 32-pin version?

VBAT isn't on either chip. Pin 6 is PF3. Where are you seeing that it's VBAT?

[LMore.2]

You are right! My mistake, sorry.

The schematic of NUCLEO-C071 misled me as it lists pin PF3/VBAT (see attached image). I hadn't read the note that says 'fitted: G0 MCU with VBAT'. So using C071, VBAT is never present.

[TDK]

Great. Your other question is answered by @Chris21 above. 32-pin can't use a crystal as there is no OSC_OUT pin. The chip does support Crystal-less USB as a device.

[LMore.2]

Cool!, I didn't realize that the quartz was for both.

But without VBAT, how can the RTC stay functional when there's no power supply?

The only solution seems to power everything with a backup battery and try to consume as little power as possible by going into deep sleep mode.

Tnks

[Andrew Neil]

Yes - that!

PS:

As well as batteries, there's supercaps, etc ...

[Grant Bt]

AN4879 - now there's a document I wish I'd known about well before now. Thanks.

[TDK · Accepted solution]

> But without VBAT, how can the RTC stay functional when there's no power supply?

RTC is powered from VDD. The hardware should be designed to source power from a battery or the main power using a diode or other power selection. Have a pin indicate when the main source is gone and it's only being powered by a battery. The chip should enter a low power mode during that case and wait for power to return on a wakeup pin.