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I'm facing the firmware downloading on a custom board for the first time after its testing version was produced. This custom board design was an heritance from an externarl designer but I have found some hardware issues that must be fixed. The firmware was allready tested on the Nucleo Board and it's working fine.

Now I'm wondering about the regulator VCAP pins. I'm just comparing the Nucleo schematics with this custom board and they have differences. So I read about this two pins on datasheets datasheet and reference manuals from here technical docin order to understand about the hardware needs and its configuration.

So after doing this, I have understood some things but there is other questions that I have not answered looking at the technical documents that I read.

What I have understood:

  • These two pins will ever need some kind of external circuitry connection.
  • Voltage regulator is enabled by default
  • The only way for being disabled is by hardware (using IRROFF and REGOFF pins, and they are only present on WL64 package) so you can add external regulation
  • Voltage regulator has 3 operation modes (configured by software says)
  • Voltage regulator needs two 2.2uF capacitors connected to VCAP0/1 and GND for stabilizing 1.2V that supplies internal digital circuits

My unclear conception, hence my questions:

  1. this internal voltage regulator also handles POR/PDR and voltage supervision. Is it? how is it done: When 1,2V output drops or grows too much it resets the system, is it? I guess it's not refered to the 3.3V supply.
  2. after connecting 2.2uF capacitor what kind of software configuration does the voltage regulator need? I was generating configuration code using cubeMX tool from stm32CubeIDE and I have not seen options about this (internal voltage regulator). Does it mean that using cubeMx it is configured with default values?
  3. the proposal of my custom board is to use an external voltage supervisor circuit for reset signal (micro.jpg). Does the VCAP pins still needing these capacitors? I guess that it does.. but I would like to be 100% sure. micro

Another data:

I have measured voltage at VCAP pins (without any capcitor connected):

  • VCAP1: 3.2V
  • VCAP2: 1.2V

Does it mean that microcontroller will not be able to setup the system?

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1 Answer 1

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VCAP pins are the MCU core supply pins.

Either you use the internal regulator and put external bypass caps, or you can disable internal regulator and externally provide the core supply yourself, which still needs bypass caps.

Regulators neee bypass caps to work properly, so if there are no bypass caps, the board may work poorly, or may not work at all, or in worst case, even damage if the regulator is unstable and starts to oscillate.

  1. You don't need to worry how it works when you use internal regulator. The point is that when internal regulator is used, the MCU can handle power-up reset itself. And in case you supply 3V3 and 1V2 separately, it can't, and needs external POR.

  2. The software usually does not need to know how the hardware is powered. However, it may be that running at full speed might consume so much current that you can't use internal regulator. This does not seem to be the case with F207.

  3. Caps are required for the regulator, regardless of if you use external reset circuitry or not.

However, VCAP should have 1.2V on it. If you measure 3.3V on one of them, it might mean the MCU has been damaged.

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  • \$\begingroup\$ Thanks @justme. After reading that VCAP needs capacitors for stability I was afraid that damages could be happened. About voltage regulator configuration it's a good thing for me not to be worried about software config. But I was reading some "configured operation mode by software" explanation on datasheet and I get confused. Now it is clear. Another question: Those 1.2V, does this voltage should be between VCAP1 and VCAP2? should both pins be 1.2V? \$\endgroup\$
    – Suvi_Eu
    Jan 25, 2023 at 12:32
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    \$\begingroup\$ Both pins should be 1.2V as per the datasheet. And yes, if you intend to use sleep modes on the CPU you can control different regulator modes if you want, and can even go into sleep mode where CPU core is unpowered. \$\endgroup\$
    – Justme
    Jan 25, 2023 at 12:37

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