I’m currently working with a Boron 2G/3G and I can power it using:
USB - Only;
USB + Battery;
External 5V power supply (not from computer’s USB) - Only ;
External 5V + USB + Battery;
External 5V + USB;
Before I go any further I’m aware of the power requirements for the 2G/3G modem.
If I power my Boron using option 1) it is unable to complete the cloud connection as expected.
If I power my Boron using option 2) it is able to complete the could connection as expected.
If I power my Boron using option 3) it is able to complete the cloud connection as expected.
If I power my Boron using option 4) it is able to complete the cloud connection as expected.
If I power my Boron using option 5) it is unable to complete the cloud connection. NOT expected IMO.
My question is regarding the last scenario:
This behavior is determined by hardware (how the USB/charger controller is connected) or is it a firmware limitation that upon detection of the USB connection it limits the current draw in order to protect the USB host?
My guess – from similar experience – is the 5V from the USB is a bit low. Power switching in the Boron is odd-ish, and I found that connecting my Boron (admittedly an LTE version, not your 3G version) through a powered USB hub made all the difference. Yes, I’ve also powered through the Vin pin – not the same thing as powering through the USB connector.
It definitely is, that is why I wouldn't expect it to boot just from USB. However, having an external 5V while connected via the USB port (for flashing code, etc), I would expect that power would be provided by the external instead of the USB.
What is the USB power source? Computer, laptop, hub. tablet charger, etc.?
In case 1 and 5, the maximum input current is determined by DPDM, the USB negotiation for power limiting. For laptops and cheap chargers, this will likely default to 500 mA, which is not enough to power a Boron 2G/3G. (It is enough to power a Boron LTE.)
In the absence of USB, more specifically the lack of DPDM, case 3, the default is 900 mA which is still low for a SARA-U201 device, but usually works.
When a battery is connected, that makes up for any power deficit, so that works.
You can override the input current limit in the PMIC. If you plan on adding an external supply, that’s usually the best option. It’s limited in hardware to around 1.5A.
But you should change the PMIC power limit, even if 3) seems to work as expected. I learned this the hard way by observing random reboot after days or weeks, which are very hard to analyze.
I can confirm "usually" and would like to share my experience of "usually" here:
Without changing the default current limit, the Boron 2G/3G can work fine for days or even weeks, but there comes a day where it fails once or serveral times. Sometimes there seems to be a "bad day" where it always fails without connecting. Perhaps the location, the cellular signal quality an the mobile network operator availability has an impact on the "bad day" behaviour.
Fail means a reboot due to a voltage drop while blinking green.
Make sure that your 5V power supply is strong enough for the modem peak current and that the voltage does not drop. In case the PMIC current limit causes a reboot, the 5V input voltage is stable but VSYS (and +3.3V) drops. This can be measured with an oscilloscope. I soldered a wire to a capacitor to measure VSYS.
All this makes sense now! I should have thought about the PMIC limit since there’s no easy way to tell if you have an external power supply connected to the VUSB pin when you have that and the USB connection at the same time.
I’ll also make sure to change this current limit! Thank you for sharing your experience nils!
Just for reference @rickkas7:
DeviceOS 1.5.4-rc2
Micro USB connected to PC;
VUSB pin connected to a benchtop power supply;