I have the following setup:
Adafruit Feather 32u4 Bluefruit LE receiving USB power.
Arduino Pro Mini 3.3v powered from Feather 3v on the RAW pin.
Sparkfun SX1509 I/O Breakout powered from Feather 3v - Controlled by the Feather.
Sparkfun MicroSD Breakout powered from Feather 3v - controlled by the Pro Mini.
Everything of course shares a common ground.
I have created two of these setups which are identical. The first setup works without any problems.
The second setup will only write to the micro SD card if the Pro Mini is also connected to USB. It is being connected via a USB 2.0 to UART TTL 6PIN Connector Module Serial Converter, CP2102, WITHOUT the 3v pin connected.
The Pro Mini powers up when it is only receiving power from the Feather, but as I do not have a Serial connection when the Pro Mini is not connected to USB itself, it is really hard to trouble shoot.
Does anyone have an idea of what could be wrong? I guess the Pro Mini is not receiving enough power, but then I don't understand how it works in the first prototype I created.
I have omitted everything but the power setup, as everything connects and communicates properly when both microcontrollers are connected to USB.