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Varadharajan V
Varadharajan V
Associate III
April 26, 2017
Question

STM32F746 Discovery VBAT

  • April 26, 2017
  • 8 replies
  • 1500 views
Posted on April 26, 2017 at 19:03

Why the big featured STM32F746 Discovery Lacks in External VBAT pins 

This makes the board completely unusable for backup test and RTC time keep up

The designer had concentrated only on big features like LCD, SDRAM, FLASH and others but simply forgot about external VBAT for testing its Backup facility

Is there any other way to make the VBAT external so that we can connect and run RTC even there is not main power

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    8 replies

    Varadharajan V
    Varadharajan VAuthor
    Associate III
    April 26, 2017
    Posted on April 26, 2017 at 19:05

    The same applied on STM32F769i Discovery also - No External VBAT Terminal to Connect External Battery

    What a great Design ?

    Tesla DeLorean
    Tesla DeLorean
    Guru
    April 26, 2017
    Posted on April 26, 2017 at 22:12

    Yes, they have a $500+ EVAL series board for people like you to evaluate the chip more completely.

    Tips, Buy me a coffee, or three.. PayPal Venmo (See Profile) Up vote any posts that you find helpful, it shows what's working..
    Varadharajan V
    Varadharajan VAuthor
    Associate III
    April 27, 2017
    Posted on April 27, 2017 at 06:57

    So for connecting a CR2032 which is less than 1$ we need to pay $500 extra 

    Wow Simply super 

    RomainR.
    RomainR.
    ST Employee
    April 27, 2017
    Posted on April 27, 2017 at 09:22

    You can also use a NUCLEOF746ZG that has all the signals including the VBAT.

    The Discovery Kit is generally designed to cover the important features of the devices (DSI, SRAM, QSPI, USB ...).

    For a more specific application that requires backup domain like RTC and VBAT, the Nucleo are more flexible.

    To give better visibility on the answered topics, please click on Accept as Solution on the reply which solved your issue or answered your question.
    Varadharajan V
    Varadharajan VAuthor
    Associate III
    April 27, 2017
    Posted on April 27, 2017 at 10:43

    Thank you Mr.Romain Reicher

    I'm already using those boards (NUCLEO - 144 (429ZI,746ZG) and even bought 767ZI for my applications

    for a simple home automation based on time and date I tried this discovery since every things are in one board like (LCD with TOUCH, simple IO for Relay Switching and LAN for Connected Control)

    So while trying i've noticed that it does not support external RTC battery backup which makes RTC reset on every power fail. Now I attached 5V external battery with manager for continuous on

    Is there any touch LCD for Nucleo like plugging directly on Board (if so it would be very easy for my projects since as you said it holds all the required things on board)

    RomainR.
    RomainR.
    ST Employee
    April 27, 2017
    Posted on April 27, 2017 at 11:45

    You can use this compatible shield 

    https://www.adafruit.com/product/802

    The Cube has a HAL librairy suitable for this product and is supported by the MBed environment as well.

    But it is not touch screen 

    >:(

    You can also buy this 1.8inch LCD screen alone at all Asian suppliers for under $ 5USD. This LCD is based on the ST7735 controller and works with the SPI.

    You can then build your own Nucleo shield and integrate a CR2032 3V battery for the VBAT and the LCD screen.

    regards

    To give better visibility on the answered topics, please click on Accept as Solution on the reply which solved your issue or answered your question.
    Varadharajan V
    Varadharajan VAuthor
    Associate III
    May 17, 2018
    Posted on May 17, 2018 at 13:11

    It would be fine to re design the board with external terminal for VBAT which makes it much more comfortable

    While designing Discovery Kit, designers have to consider minor point like external VBAT with jumper, since once designed they simply make it mass production which is useless

    Adding External RTC to discovery is like Having Food in Plate but going to buy groceries for cooking food and eat

    I don not whether the design is analysed before development

    Since Time to Market is the only aim, there is no analysis took place

    Missing VBAT external seems that ST don't want to use RTC with Battery (may be causing issue to entire board)

    so the board having features will be demolished by simple Battery

    Every one advised to connect External 5V to board which makes the board with stands for only few days and if they provided the VBAT we can expect RTC backup for more than years (with smile and no problem)

    Hope future design meet all this otherwise products may not be usefull

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