Problems with writing to SD card

Hi,

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.

SomethingClever:
I have created two of these setups which are identical. The first setup works without any problems.

So you're saying that you have two lots of the same kit..... ie. two sets of the same kit. And one kit works ...... and the other kit that uses the same brand/model components does not work... right?

SomethingClever:
Arduino Pro Mini 3.3v powered from Feather 3v on the RAW pin.

Not good.

You probably need arounf 5V on the RAW pin to power a Pro Mini properly.

If you think the PRo Mini is not getting enough power, what is the voltage you measured on the Pro Minis VCC pin ?

Southpark:
So you're saying that you have two lots of the same kit..... ie. two sets of the same kit. And one kit works ...... and the other kit that uses the same brand/model components does not work... right?

Yes. Same brand, same model. Bought from the same vendors. But the second setup works as well when the Pro Mini is connected to USB via the CP2102 - but WITHOUT the CP2102's 3v pin connected. Only RX/TX, DTR, and ground is connected.

srnet:
Not good.

You probably need arounf 5V on the RAW pin to power a Pro Mini properly.

If you think the PRo Mini is not getting enough power, what is the voltage you measured on the Pro Minis VCC pin ?

The Pro Mini's VCC pin is 3.01v and the Feather's VCC pin is 3.29v.

Update: I found out, that the setup actually only works if there is a serial monitor open. It does not matter if the USB is plugged in to the Pro Mini, if the monitor is not open.
There is not a while(!Serial) or anything like that in the code - in fact there is no Serial.print anywhere.

I don't think this was the case - it actually seems that there is a higher probability of everything working if the Pro Mini is restarted after everything is being powered.