STM32H745: need to check schematics: check power on a custom board

[Hank_Schrader]

Post spread from STM32H745: No device found on target on a custom board

Hi, and thank you for your previous answer and for pointing me to RM0399 / figure 22.

I am quite new to the STM32H7 power configuration, so I may have misunderstood the datasheet and reference manual. I am using an STM32H745BI with only an external 3.3 V LDO (no SMPS components on my board).

In CubeMX I have changed the power configuration from PWR_DIRECT_SMPS_SUPPLY to PWR_EXTERNAL_SOURCE_SUPPLY

Right now my board is wired like this:

• VDDx: 3.3 V, each with 100 nF decoupling

• VSSx, VSSA: GND

• VBAT: not connected

• VSSSMPS: GND

• VLXSMPS: not connected

• VDDSMPS: not connected

• VFBSMPS: not connected

• VCAP1 / VCAP2 / VCAP3: 2.2 µF to GND

• VDDLDO_1 / VDDLDO_2 / VDDLDO_3: not connected

• PDR_ON: 0 Ω to GND

SWD is not working reliably and I suspect my power-pin connections are wrong.

For a pure LDO-only supply (no SMPS hardware) could you please tell me how these pins should be connected?

1. VBAT

2. VDDSMPS

3. VDDLDO_1 / VDDLDO_2 / VDDLDO_3

4. VLXSMPS, VFBSMPS, VSSSMPS (when SMPS is not populated)

5. PDR_ON – is tying it to GND with 0 Ω correct in this configuration?

If possible, a short checklist or a small sketch (similar to figure 22) for this exact case would help me a lot.

Thank you again for your support!

 

[Hank_Schrader]

Hi, thank you very much for your reply.

My VCAP pins are connected with 2.2 µF capacitors to GND as recommended in the datasheet.

At this point I think the issue may be related to the LDO/SMPS power configuration.
My board does not have any SMPS components populated, and I originally selected an SMPS mode in CubeMX by mistake. I have now switched to PWR_EXTERNAL_SOURCE_SUPPLY, but some pins like VDDLDO_x and VDDSMPS were left unconnected on my PCB.

I suspect this mismatch between the CubeMX power mode and the actual hardware (LDO-only) might be the reason why SWD fails.

Thanks again for your help!

[mƎALLEm]

I think you are mixing between the function of your 3.3V LDO regulator and the External SMPS function.

So you have an external 3.3V LDO to be connected to VDD of the MCU, right?

So use LDO mode instead (the LDO internal to the chip), and refer to one of the design reference:

Look at the NUCLEO-H745 schematic and inspire from. It proposes many power config:

Hard to explain all the stuff here. You need to refer to the reference design.

[AScha.3]

Hi,

so from what  @mƎALLEm  found, i think, you have no power for the core now.

Try : connect vdd to vddldo 1,2,3 ; check voltage at vcap: should be 1.2V , about.

this is the way:

 

Then connect with CubeProgrammer...should work.

And set LDOsupply in Cube ...to have it working on.

[Hank_Schrader]

Hi, Thank you very much for your help. You were right — the issue was indeed caused by the VDDLDO / core power not being supplied. After connecting VDD to VDDLDO1/2/3 and checking VCAP, the problem is now solved. Thanks again for your guidance.

[mƎALLEm]

@Hank_Schrader 

You need to decide which config you plan to use. To simplify I suggest LDO mode:

SMPS has an extra coil and a capacitor. So I don't think it's convenient for you if you are newbie.

Moreover you can reach 480MHz of the system clock with LDO contrarily to SMPS that you cannot exceed 400MHz.

You can refer to the AN4938

[Hank_Schrader]

Hi, Thank you very much for your help. You were right — the issue was indeed caused by the VDDLDO / core power not being supplied. After connecting VDD to VDDLDO1/2/3 and checking VCAP, the problem is now solved. Thanks again for your guidance.