Custom STM32H755ZI board not working

[incidentallyhandmade]

Hello everyone, 

I'm facing an issue with a custom PCB I made using the STM32H755ZI.

I also own the nucleo board for the MCU, so I'm sure the program works well; it's just a blink program.

The power schematics are inspired by the nucleo board schematics of the MCU.

I'm using an STLink-V2 programmer. 

There are mistakes in my design that have been rectified using soldering.

Issues are as follows:

1. The STM doesn't turn on. | Fixed : VDDLDO & VDD have to be shorted. Was initially floating.

2. The STM doesn't turn on. | Fixed : SMPS pins were left floating. VDDSMPS, VSSSMPS, VLXSMPS, and VFBSMPS have been shorted to ground.

3. Randomly getting detected. No consistency in detection.

 

The power is from a 3.3V LDO, I've verified that.I'm suspecting the issue is something related to power, as it shows up briefly for a while before it stops working.

Right now the VCAP capacitors are 0.1uF|100nF, which I'm suspecting is the primary issue, as in the SMPS Off/LDO On case we need a 2.2uF CAP with low ESR.

Apart from that, there might be other issues as well, which I'm hoping the experienced members of the forum could point out and guide in the right direction.

Any help on this is really appreciated; I've been racking my head for close to a week now.
This is my first forum post; advanced apologies if anything is wrong or missing. 

Thanks in advance.

[AScha.3]

Hi,

sorry, but when i read this, only critical or ironic comments come to mind.

Is this your first experiment with a cpu ?

I try it the other way : not talking about errors, but what is good; the list is smaller:

- power is from a 3.3V LDO : good.

All other ... not good. 

- STM32H755ZI and SMPS ?? isnt it a STM32H755ZIQ then ?

See simple circuit example, as on nucleo board and find the xx differences:

https://www.st.com/resource/en/schematic_pack/mb1363-h755ziq-d01_schematic.pdf

 

I just recommend: buy a 

NUCLEO-H755ZI-Q

and learn how to use this complex cpu/SOC - then try to make your own . (This one is for the trash.)

[incidentallyhandmade]

Hey,

Thanks for the reply.

 - Yes, this is my first time using this CPU in the PCB design.
- I already own the NUCLEO-H755ZI-Q and have used the schematics you attached as a reference for the PCB I made.

- The blink program I made for the NUCLEO-H755ZI-Q should work on my PCB provided it has the correct power going to it.

- In the reference manual RM0399 : Figure 22, there are multiple system supply configurations. I want to use configuration 1, which is LDO-ON and SMPS-OFF. To achieve this, I've VDDSMPS, VSSSMPS, VLXSMPS, and VFBSMPS shorted to ground, and VDDLDO and VDD have to be shorted.

- I've found a mistake in my capacitor selection for VCAP; it should be 2.2uF, but it's 100nF, so I will change them and update the results here.

- Apart from that, are there any other considerations for the power circuitry of this CPU?

- My issue is that the ST-Link is able to connect with the MCU, but very randomly, not consistently.

Thanks.

[AScha.3]

1. correct VCAP . 2 x 2.2 uF , or 4.7 uF + .1 , afaik. (If using LDO supply.) Otherwise no stable working cpu.

2. If using LDO supply, then use better STM32H755ZI , not -Q version with smps. more free pins then...

3. Check Vcore on Vcap , has to be about 1.3V , depending on your settings.

[incidentallyhandmade]

Hello,

An update on the situation.

I've changed the VCAP capacitor's to 2.2uF 16V capacitors. There are are 3 VCAP pins, all 3 have been changed.

The MCU is still not being detected by the ST-LINK. 

There is is now a weird behavior as follows.
 - The VCAP voltage is 0.99V when system is in reset, but when in normal mode, the VCAP voltage goes back to 0.
 - But now even if i reset it, the VCAP is always at 0