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FEZ Cerb40 power problem


Hi, I have some question in powering the FEZ Cerb40.
In Wiki, "Users have the option of providing external 3.3V to FEZ Cerb40 or soldering a 3.3V regulator and 47uF capacitors like T520B476M008ATE035 Digikey Mouser."
Does the board have the place to soldering a 3.3V regulator and 47uF capacitors? Anyone can provide a figure to point me? Thanks.
Other question is, If I soldering a 3.3V regulator and 47uF capacitors, could I power the FEZ Cerb40 via external USB( 5V) ?


To supply the Cerb40 you have two options :
1 : By soldering a 3.3V regulator and 47µF. The location of these components are reserved on the board. See Wiki :
I have not tested this solution.
The USB is used to power the Cerb40 and for debugging, not to communicate.

2 : By a direct 3.3V supply connected on 3.3V pin.
If you choose this option, the Cerb40 can be powered from 1.8V to 3.6V with success, I tested it.
The USB is used for debugging, not to communicate.


So IC2 is the regulator and the C10 is the 47uF capacitors, right?

What do you mean “the USB is used to power the Cerb40 and for debugging, not to communicate”.
Does debugging mean deploying the app to the Cerb40 ?
and the communicate is to upgrade the firmware?


So IC2 is the regulator and the C10 is the 47uF capacitors, right? => Yes

The USB allows to debug and deploy an application in the Cerb40. The USB is used to communicate with the Cerb40 during debugging.
It is not possible to develop an application, deploy it to the Cerb40 and that standalone application then communicates to the outside via USB.


I want to connect the Cerb40 with Xbee Pro.
Could I use two AA Battery(1.5+1.5) to supply the Cerb40 and Xbee Pro?
Or I should use a regulator(3V to 3.3V)?



To use two 1.5v AA batteries you will need a boost converter. You need to be careful in this scenario to make sure you test the output over a range of battery voltages (state of discharge) to make sure its stable for the required output you also need to be sure your circuit can handle the current requirement of the mcu and xbee.


According to the STM32F4 datasheet, you should be good for full-speed operation anywhere between 2.7V and 3.6V. Assuming your other components can handle the lower voltages, you should be able to run for a little while, at least, on 2 AA batteries.

The “absolute maximum values” table says you’re (theoretically) safe up to 4V, which means that 3x NiMH kinda works, if they’re not topped up completely. Don’t try that at home, though.

The MCU itself will run down to as little as 1.8V if it’s clocked slower. You could do that with the Register class, given enough time and effort.

If you want to do it the easy way, however (Brett’s way), then something like this would suit you well: That’s only good for 200mA (and that only under some circumstances), though, so be sure you understand the datasheet.


Thank you.

I think I will design a board with battery circuit for the FEZ Cerb40 and Xbee.


Does the VBAT is for internal RTC?
Does the 3.3v pin is Power in or out? only can choose either? I don’t understand the explain in the web(“In if no 3.3V is soldered and out if soldered”)
If I soldering a 3.3V regulator and 47uF capacitors to FEZ Cerb40, do I need to use other 3.3V regulator in my external power?


VBAT is indeed for the internal RTC (and battery RAM).

The 3.3V pin is connected to the 3.3V net on the board. One way to supply power to this net is to supply 3.3V on that pin. Another way to supply power to that net is to solder the regulator and capacitors and supply 5V to the 5V pin. If you use a regulator with sufficient capacity, you can use the 3.3V pin to supply power to other components. Remember to consider heat when using linear regulators.


Is it possible to use the 2 ways.
When the cerb40 works normal. He use external power. (with cap & ic2)
And when I need to reprogram the cerb40 i use the usb cable en let the external power connected?
Is that a problem or do I need te remove the external power?


@ d-ray - You can supply additional 3.3V to the 3.3V pin, or additional 5V to the USB pin directly opposite that one.
The only issue is that if you need to hard-boot your Cerb, you will have to unplug the USB and also turn off the power supply.


Recently, I bought some FEZ Cerb40 II for my project.
I have a little confused.

Now the FEZ Cerb40 II has a 3.3V Regulator on board, so how much the voltage can supply to the FEZ Cerb40 II? 5V? 9V? which pin is used for the power input? 3.3v pin or USB pin?

I have a component need a 5V input, does FEZ Cerb40 II can power it, or I need external power supply?

Other question is LODR pin, if I want to update the firmware, which pin should be connected to LODR pin? (3.3V?)



@ Tzu Hsuan - The schematic is available here: