Custom STM32h743ZI Board wont connect ("Failed to initialize DAP")

[QuirinS]

Hello Community,

I am struggling with a custom board design based on the STM32H743ZIT6 (LQFP144, non-SMPS version). I’ve replaced the MCU already, but the symptoms remain identical, suggesting a fundamental hardware/layout issue.

The Symptoms:

1. Current Draw: 10mA when BOOT0 = GND. Approx. 50mA when BOOT0 = 3.3V (System Bootloader).

2. VCAP Voltage: Measuring approx. 1.0V.

3. Debug Connection: Using Segger J-Link (V11). "Failed to initialize DAP" / "Can not attach to CPU". VTref is stable at 3.3V, NRST is at 3.3V.

4. USB DFU: When BOOT0 = 3.3V, Windows reports "Device Descriptor Request Failed" (Unknown Device) when connected via usb.

5. SWD Pins: PA13/PA14 show ~1.6V when floating (no debugger connected), which is highly unusual

6. Same Symptoms if 3.3V are supplied by ams1117 or laboratory power supply

My Schematic:

VBat, all Vdds, Vdda, VRef, PDR_ON are all connected to 3.3V

all GNDs and AGND are connected to GND

each Vcaps (2x) has a 2.2uF capacitor (alltough i am not sure about its esr)

each Vdd pin has its own 100nF or 1uF cap.

=> See stm32schematic.pdf

My Layout:

Vcaps:

 

 As this is a university projekt help would be much appreciated!

Thanks!

[mƎALLEm]

Hello,

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@QuirinS wrote:

each Vcaps (2x) has a 2.2uF capacitor (alltough i am not sure about its esr)

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For Vcap pins, you need just 2x 2.2uF and all VCAP pins need to be connected together.

 

Please refer to the AN4938 "Getting started with STM32H74xI/G and STM32H75xI/G " and check again your hardware based on the recommendations and the reference design provided in the section 8 Reference design.

[Andrew Neil]

You don't have NRST on the debug connector, and a I don't see the recommended capacitor on it.

As @mƎALLEm said, see the hardware development app note, AN4938.

Also the datasheet:

 

You could also look at ST board schematics, for reference.

[AScha.3]

Hi @mƎALLEm  , whats the pin U11 in your pic ?

[mƎALLEm]

U11 ball / TFBGA240 package:

 

[QuirinS]

Thanks for your response

i have the rst pin on a pad wich i connected to the jlink

[QuirinS]

Thanks for your response.

Ok, So vCap pins need to be connected. I will solder a new board and manually connect the vcap pins.

[QuirinS]

I will solder a new board just with the stm32h743 and:

100nF for each vdd and rst

1uf and 100nF for vbat, vdda, vref and vddusb

pulldown on boot0

I will supply 3.3V externaly

This minimal layout should allow me to comnect with the segger jlink, or am I missing something?

Re there any other pins that need more circuitry?

[mƎALLEm]

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@QuirinS wrote:

This minimal layout should allow me to comnect with the segger jlink, or am I missing something?

Re there any other pins that need more circuitry?

---

As suggested earlier, you need to follow the reference design provided in the AN4938. It provides a minimal hardware required.

[QuirinS · Accepted solution]

Adding a pullup Resistor to NRST solved the problem.

Thanks for your help!