I am having an issue with VCAP pin and dont know how to fix it.

[CEagl.1]

Hi,

This is my first time using an STM32, but not my first time using a MCU, my previous experience all comes from PIC.

To avoid any generic questions being put to me such as "have you checked your wiring" etc, It is worth pointing out that i am a PCB design engineer in my day job. So i am familiar with debugging and finding a cause for failure.

That being said, On this particular board that i have designed, after 2 days of debugging, I am absolutely stumped!!

So, I have a custom board that has on it an STM32F411CCU6TR.

Initially i populated the entire PCB including several peripheral components. A sensor, an RTC and cryptographic security IC.

I powered up the board and checked voltages etc, All good, nice stable VDD at 3.3v

Tried to connect ST LINK....Nothing.

Started debugging and digesting the datasheet. First checks were RESET pins and BOOT0 pin. Both checked out and working. Reset pulled high, boot pulled low. push buttons make them do the opposite.

So im thinking well what else can it be? >> Fast forward hours of googling >>

The VCAP pin!!! This thing is becoming the worst PITA i have ever had in a device.

So i read its supposed to have 1.25v on the pin. Checked it on a meter and the values were all over the place. Checked it on a scope and found it to be pulsing at around 1.5Hz. The scope shows it having a sharp rise up to about 0.6v then a 1.5second drop down to 0.2v ish this just repeats itself. Doesnt look good, right?

This particular IC has no external controls to turn off the internal Vreg so cant have messed that up and it "should" just work. at least that is my understanding of it.

So with the internal regulator seen to be acting strange, i set about looking into what could cause it.....

>>>>>fast forward hours of googling and probing>>>>>

Drawing a blank, nothing i have found can tell me why the internal regulator acts like this. So i think to myself, it must be broken.

So i assemble a second board with no peripherals on it. Just a STM32, decoupling caps, Oscillator, reset and boot circuits (pullups/downs and cap) and the VCAP capacitor.

Literally the bare minimum to get this chip running.

I test my VDD, Perfect.

I test my VCAP, Nothing, Nothing at all.

>>>> Fast forward some words i cannot type in an open forum along with my opinion on the STM32, LOL>>>>

At this point i have had enough, its 2am so time for bed.

I woke up this morning with a bright idea. Why not just apply 1.2v to the VCAP pin and see if it does anything different. Whats the worst that could happen. So i applied a stable 1.2v to the VCAP Pi on the fully populated PCB. The pulse was still there up to 0.6v, but instead now it only drops to 0.4v, great, wheres my other 0.8v gone to??

I tried the same again on my bare build PCB, Exactly the same. However no pulse, just fixed at 0.4v. With the scope attached i disconnected the 1.2v supply from VCAP pin and watched as the voltage went back up to 1.2v.

That where i am up to.

I am sitting here now thinking that i have zero control over this regulators output and i cannot see what i can do in terms of circuitry to make it not put out this voltage. I have tested the peripherals on the PCB by connecting into the I2C bus and i can communicate with all of them, so clearly my VDD voltages are all good enough to support comms via I2C and none of the devices are causing an issue. It seems isolated to the STM32 but internally.

So my experience of STM32 devices so far is that they are sensitive little buggers and i have a 100% (2 devices) failure rate. I have trawled over my schematics and i am thinking that even redesigning it wouldn't help me because based on my understanding of how it works, i wouldn't want to change a thing.

I have attached the STM32 schematic, The quality isnt great as the software i am using doesnt yet export to PDF. S i have added another one to clear it up so you can see the chips pin out better.

Also in the schematic it shows a 4.7uF 0402 cap for Vcap. Throughout the above debugging it has been swapped for a 10uF 0603 X5R.

All of the pinouts go to headers so on my bare populated PCB there all those pins are floating.

I am thinking that due to not properly understanding the STM32 that i have possibly used a pin for a task and its an important pin. But despite looking over and over i cant see where it would be.

Can anyone offer anything at all that might help to understand this issue???

[Tesla DeLorean]

VREF+ (VDDA) should be powered

[CEagl.1]

Clive, You mate, are a legend. I left VREF+ disconnected since i wasnt using the ADC. Also from my experience of using PIC i have never needed to use it.

Connected VREF+ and it starts up and connects.

[Tesla DeLorean]

I looked over the Data Sheet, it is definitely not sufficiently clear/decisive about this. I think a minimal test circuit would be a great help.

It is definitely too easy to assume VDDA/VREF pins only impact designs using ADC/DAC. I rarely use them myself, which seems a bit on an anomaly, but I'm clearly not alone.

[Piranha]

AN4488 section 2.2 point 3 says "VDDA pin must be connected" and figure 2 shows "Analog: RCs, PLL, ...". By RCs they mean RC oscillators - HSI and LSI. As after reset MCU starts from HSI, VDDA is absolutely necessary. Also, because of starting from HSI, crystal/oscillator is not required for a bare minimum and not required at all, if HSI tolerance is enough for your application. SPI, I2C and USART run as a charm from HSI based clock... =)

[T J]

Yes, well done @Community member​, as the resolving expert you should be recompensed for every post. @Camilo LEMOS​ 

The VDDA was my Pain once too (recently) , similarly pulled it to 3v and I was up.

but still I am concerned about the 4.7uF 0402, the startup surge could eventually kill the pcb.

that is dependent on how fast the 3V comes up and that affects your reset.

I would suggest the you use plain old ceramic caps,

I use a 1uF and a 0.1uF, I use 0805 or 1206 1uf and 0603 0.1uF on every power pin,

Never have any issues with stability, usually the power rails are squeeky clean.

This method uses many low cost caps but no expensive reels.

Power pins include , each VDD + VDDA, VCap1 and VCap2.

that is if you are using A/D pins, otherwise I just link VDDA to VDD, (just use the one set of caps for those two pins.)

I also recommend that you use a reset chip.

I use the MIC 803 series 140mS seems to work perfectly on every PCB I have made.