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darkfader
Associate
May 14, 2026
Solved

RTC on STM32F769I-DISCO

  • May 14, 2026
  • 3 replies
  • 255 views

Hi,

 

I wanted to set up time keeping on my Discovery board for testing something.

I see the following in the documentation for the larger STM32F769-EVAL:

 

https://github.com/STMicroelectronics/STM32CubeF7/blob/master/Projects/STM32F769I_EVAL/Examples/RTC/RTC_InternalTimeStamp/readme.txt

 

- This example runs on STM32F779xx/STM32F777xx/STM32F767xx/STM32F769xx Devices.
- This example has been tested with STMicroelectronics STM32F769I-EVAL
board and can be easily tailored to any other supported device and
development board.
- STM32F769I-EVAL Set-up
- Make sure that JP15 is in position 2-3 to connect 3V battery to VBAT pin.

 

At first I didn't know -EVAL and -DISCO are two different things and kept searching through the manual for the -DISCO where JP15 seems to have a different role.

But the first line from the docs makes me think there has to be an RTC on the -DISCO board. I take it I need to put a 3.3V battery somewhere. In the diagram I see a number of 3.3V connected things, but most are related to ST-Link. It's totally unclear to me where I would need to connect a battery, and how to know that it'll only power the RTC (if there is one)

The diagram I'm looking at is at page 40 in the manual, here:

https://docs.rs-online.com/9ce5/0900766b81693555.pdf 

I'll take off the LCD later and search the board for anything that looks like an oscillator or clock chip, but I also searched the manual for "RTC" and found nothing. If there's none I also don't know how to best add it - there's a header for WiFi (ESP8266) or I2C. I got an ESP8266 around so I was gonna use it. I don't know if an RTC could be added to the same I2C interface?!!! 

Still, as the ethernet chip has hw timestamping, I think there must ought to be an RTC onboard so it all works together when people tested.

 

Do you know if there's an actual RTC on this board? Any other advice?

Best answer by mƎALLEm

Hello @darkfader and welcome to the ST community,

The MCU itself (STM32F769) has an RTC but unfortunately the VBAT pin is tied to VDD in the PCB on STM32F769I-DISCO as shown in the board schematic:

mALLEm_1-1778769997624.png

Contrarily to the EVAL board that has a battery socket.

I know this is a limitation I've already faced on one of my projects and asked to make VBAT accessible over jumpers or at least a solder bridge for future STM32 boards.

So in your case you need to use an external RTC chip over I2C, with a time saving with a battery.

3 replies

Andrew Neil
Andrew Neil
Super User
May 14, 2026
@darkfader wrote:

The diagram I'm looking at is at page 40 in the manual, here:

https://docs.rs-online.com/9ce5/0900766b81693555.pdf ?

That's on a 3rd-party site - note that it is Rev 3

Always best to go direct to the manufactures own site:

https://www.st.com/resource/en/user_manual/um2033-discovery-kit-with-stm32f769ni-mcu-stmicroelectronics.pdf

via: https://www.st.com/en/evaluation-tools/32f769idiscovery.html#documentation

Note that this one is at Rev 5 - so the one on the RS site is two revisions out of date!

 

Schematics for the DISCO board are here:

https://www.st.com/en/evaluation-tools/32f769idiscovery.html#cad-resources

 

@darkfader wrote:

Do you know if there's an actual RTC on this board? 

The RTC is part of the STM32F769I chip - so it is always present inside the chip, irrespective of what board you use.

RTC details are in the chip documentation - Reference Manual & Datasheet:

https://www.st.com/en/microcontrollers-microprocessors/stm32f769ni.html#documentation

A complex system that works is invariably found to have evolved from a simple system that worked.A complex system designed from scratch never works and cannot be patched up to make it work.
    mƎALLEm
    mƎALLEmAnswer
    Technical Moderator
    May 14, 2026

    Hello @darkfader and welcome to the ST community,

    The MCU itself (STM32F769) has an RTC but unfortunately the VBAT pin is tied to VDD in the PCB on STM32F769I-DISCO as shown in the board schematic:

    mALLEm_1-1778769997624.png

    Contrarily to the EVAL board that has a battery socket.

    I know this is a limitation I've already faced on one of my projects and asked to make VBAT accessible over jumpers or at least a solder bridge for future STM32 boards.

    So in your case you need to use an external RTC chip over I2C, with a time saving with a battery.

    To give better visibility on the answered topics, please click "Best answer" on the reply which solved your issue or answered your question.
      Andrew Neil
      Andrew Neil
      Super User
      May 14, 2026
      @mƎALLEm wrote:

      I know this is a limitation I've already faced on one of my projects and asked to make VBAT accessible over jumpers or at least a solder bridge for future STM32 boards..

      Indeed, this is often a limitation of the DISCO boards: they tend to be very tightly focussed on just one particular application/demo - which makes them very inflexible for use in other applications.

      @darkfader due you actually need battery backup?

      If you do, it might be better to look for a different board - the Nucleo boards are a lot more flexible:

      https://www.st.com/en/evaluation-tools/stm32-nucleo-boards.html

      A complex system that works is invariably found to have evolved from a simple system that worked.A complex system designed from scratch never works and cannot be patched up to make it work.
        D
        darkfaderAuthor
        Associate
        May 14, 2026

        Wow, so many so helpful and insightful replies. 

        I'm not gonna die from not having battery backup, I just wanted it to work with things like hwclock --systohc. But I think as long as I fetch the current time before enabling PTPv2 it should work as well. Still learning.

        I don't want to get another board at the moment - the original HW I want to make new firmware for was built off this one. (a Ubiquiti firewall that uses it as a very pretty display. It stops working if you replace the original OS with something more modern, so I want to re-implement that functionality). An RTC  battery is _not_ needed for that purpose. But since I bought the board to play with, I wanna see what I can do with it. Is it understandable like that.

        I'll assume I'm gonna postpone the RTC a bit. I can get a cheap external RTC circuit, and maybe I find someone more awake than me and we cut the VDD line there and use the builtin one.

        edit: I looked at the board a bit and the lines are so thin I'll not attempt anything anytime soon. I2C it is.

         

        Also thanks for the proper links. I'm very forgetful and so even if I might have found them on the ST website a week ago I might pick a different the next day. I'll save these and now I know I can look for the thread :)

         

          Andrew Neil
          Andrew Neil
          Super User
          May 14, 2026
          @darkfader wrote:

          Also thanks for the proper links. I'm very forgetful and so even if I might have found them on the ST website a week ago I might pick a different the next day. I'll save these and now I know I can look for the thread :)

          For each project, I keep a document with links I've found and notes about them.

           

          For ST, the Product Page is always the key link - everything starts from there!

          A complex system that works is invariably found to have evolved from a simple system that worked.A complex system designed from scratch never works and cannot be patched up to make it work.
            D
            darkfaderAuthor
            Associate
            May 15, 2026

            that's really good advice, thanks for taking the time to explain this.

            I do similar but my process is still from before I got a brain bug and it's not sufficient anymore. 

            I.e. right now the info on the display is in my notes on cavium/octeon CPUs, in the subdocument on the firewall model that had the STM32 based status display and there's a section on the STM32 HW. I'm sure the manual is linked there already, but since it was 3 levels deep I didn't access it, except for adding some notes. I break out stuff once a large body of knowledge is added (maybe) or when I turn it into web pages.

            If I had earlier moved the firewall doc and the stm32f769-disco into their own I'd have had my workflow OK.

            My old approach was for delaying any notetaking till I knew the grand picture of a topic and knew if I wanted to keep knowing about it. Back when 99% of that info was in my brain in parallel. I gotta adapt and what you do is a more reliable way. I'm insanely bad with structure, so I was already proud of how good docs I keep but it's time for another level. Thanks for stressing you just do that for every project. :)

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