I tested this and got a similar response with my Arduino expansion board. I used an adjustable power supply and several USB cables of different brands and got different booting times for each scenario.
12V/2.5A Power supply: ~30s
9V/2.5A Power Supply: ~32s
Both USBs (same brand): 45s
Both USBs (different brand): No boot. Using this configuration the board didn’t had enough power to boot so it went through all the booting process and started again after it displayed the login/password prompt. The cable was not damaged however because I used it to transfer sketches to the IDE while powering with an external power supply.
What is your goal? Do you need to boot as fast using the USB powering method? I noticed my board took more time to boot than yours. I’m using 70% of my /dev/root folder. How much space left did you had left when you did these tests?
first thanks a lot for your tests!
I do not have a lot extra stuff on the system as I'm only running a small Arduino sketch. Maybe that's the reason why the boot time I encountered is shorter than yours.
I also totally understand that the Edison cannot boot from USB power if not enough power is supplied either because of bad USB cables or the computer USB port does not provide enough power, but I absolutely do not understand why - if there is enough power to boot - the boot time is not always the same!?
Personally I would prefer powering it from USB as I do have a 5 V power source (also used from other project components outside the Edison) and therefore using the USB connection would be the best solution for me, but not if this causes a doubling of the boot time. I could try to find some DC-DC converter to get my 5 V power supply converted to 9 V or 12 V for powering the Edison, but as I understood the hardware scheme correctly the Edison directly converts that back to 5 V internally.
Therefore I really would like to provide this 5 V directly and don't understand why the boot time is longer this way...
Is there any way to provide the Edison with 5 V directly without losing boot time?
Have you already seen the Arduino Hardware guide Intel® Edison Boards — Intel® Edison Kit for Arduino* Hardware Guide ? If you take a look at Section 4 Figure 6 you’ll notice that the power converter only converts Vin and J1. I also looked in the Edison hardware guide Intel® Edison Boards — Compute Module Hardware Guide . Section 4.12 has some information on the booting process. My guess is that based on the tests, the booting time varies depending on the powering method, regardless if you are meeting the power requirements.
Have you already tried powering the Arduino expansion board with a lithium-ion battery connected to J2 to see the results?