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St-Link cannot connect to 429-based board

Question asked by maiorfi.lorenzo on Oct 10, 2014

We just got a custom board mounting a STM32F429.

Board has a 4 pin SWD connector with VCC, SDIO, SCK and GND pins.

COnnecting to it through ST-LINK Utility (version 3.4) and setting hot-plug mode (RESET isn't exposed by aforementioned connector) we get:

08:37:06 : ST-LINK SN : 52FF6D064980515755290187
08:37:06 : ST-LINK Firmware version : V2J21S4
08:37:06 : Connected via SWD.
08:37:06 : Connection mode : HotPlug.
08:37:06 : Debug in Low Power mode enabled.
08:37:06 : Device ID:0x419 
08:37:06 : Device flash Size : 2MBytes
08:37:06 : Device family :STM32F42xx/F43xx
08:37:06 : Can not connect to device!
08:37:08 : Disconnected from device.
08:37:08 : Connection to device is lost: check power supply and debug connection.
08:37:08 : If the target is in low power mode, please enable "Debug in Low Power mode" option from Target->settings menu.

Since it is a brand new un-programmed chip, what could be the most likely cause?