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]

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@QuirinS wrote:

Adding a pullup Resistor to NRST solved the problem.

Thanks for your help!

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Hello,

I don't think adding NRST is a solution. Perharps it hides another problem.

There is an internal pull-up resistor inside the chip on NRST pin. See the screenshot from the datasheet posted by @Andrew Neil