Fez Cerb 40 II - Power Supply and Pins Questions

I’ve got my Fez Cerb 40 II up and running, but I have some questions as the site information isn’t perfectly clear to me and the documentation mainly represents the the Non II model. First version (without the 3.3 volt regulator and RTC crystal)

Anyway, It seems to power itself via the USB and enable debugging, etc without any addition power sources, but the documentation does not mention anything about this. Also, I’m interested in powering other devices that will work in with the Cerb.

Anyway here are my questions/ clarifications on some of the Pins and the capabilities/should/should nots

Pin: USB - 5V from the USB port. (Also Input VCC from a power supply if USB is not present?)
Pin: 3.3v - Regulated down from USB pin from 5v to 3.3v. This also powers the device. OUTPUT.
Vbat - only to power the RTC crystal when there is no power.

That all being said, When USB in not present, USB pin is the power in? Off the cuff, would a simple LM7805 regulator circuit be sufficient to power the device and some other small ICs and LEDs?

OR do I really need an external regulated power supply while USB is present?

Any clarification/corrections on the above is appreciated


You really need to look at the “resources” tab on the Cerb40 page and look at the schematic.

The pin labelled “USB” on the Cerb40 image is VBUS. VBUS is the “power” pin on the USB header - which is a “regulated” 5v output from a PC or USB charger. VBUS is used as the input to the onboard 5v to 3v3 regulator, and the regulator’s output is labelled VCC. VCC is used to power the processor. VCC is also exposed on the pin labelled “3.3v” on the device header.

So if you are wanting to power this device on its own, you have two options:

  1. provide REGULATED 3v3 power on the VCC net via the “3.3v” pin.
  2. provide REGULATED 5v power on the VBUS net via the “USB” pin or via an actual USB cable.

Note: there is no “input VCC” labelled on the headers - the VCC net is 3v3.

A 7805 should be fine, depending on total power consumption of your Cerb40 and your other peripherals/chips; it is a linear regulator so not high efficiency but if for example you’re using a 12v battery it is a simple way to bring power down to the 5v required and let the 3v3 regulator onboard give you a stable source to power the processor. But if you also need 3v3 for the other processors, I’d suggest you use appropriate smaller regulators for each device; the LM1117-3v3 that is on the Cerb40 is capable of powering a small device like Cerb40 but it’s not something I’d load up by trying to power other devices with it, I’d use multiple LM1117’s off a common, higher current, 5v rail.

The LM1117 is good for 800ma, the Cerb40 will use only a little of that, so assuming your cooling and input voltage are sufficient, you should be fine to power your leds, small screen, sensors, etc from it.