It takes me something like 30 seconds from when my core boots until it is connectable via /dev/ttyACM0. Just after program start, the device pops up, but trying to get the output from there gives me:
$ cat /dev/ttyACM0
cat: /dev/ttyACM0: Device or resource busy
Is this usual, or is something derped up on my admittedly hackedy Linux Mint Laptop?
Hi @MattyG
The core will not appear as a USB device until your setup() function is run and Serial.begin(baud) executes, so some of that time is the core start-up and finding WiFi networks. Once you see the breathing cyan LED, it should be only a second or two at most.
What happens with your laptop when you plug in other USB devices? Do they take a long time too?
i have found after you start the serial you will need to do a
while (!Serial.available());
otherwise it will say port in use…
once your terminal opens you will have to push enter to get your program to continue
I also do something along the lines of
Serial.begin(9600);
sleep(5000);
to give a little boot-up delay that won’t hang up the rest of the code if I never use a console and press a key.
The best is 
while (!Serial.available()) SPARK_WLAN_Loop();
That way you don’t lose cloud connection and OTA.
Good call… i removed that when i turned the Wifi and Cloud off so it didnt create bugs (stupid slow internet here wont let it connect reliably)
I’ve also got a timeout on it now, and 2 modes… push a button and it changes between serial on and off. that way i don’t have to reset the core to get a terminal open the second time. Normal mode the core scans fingerprints continuously and the Programming mode lets me add users and fingerprints
Interesting notes all around. The boot up time of ~30 seconds was measured from the start of the aqua-“breathing” state. I am assuming setup() gets run moments after that, in which I do have the serial.begin() call.
Compared to the reports of others this seems long. Other usb devices (BBB/Arduino) come up quickly. I recall now adding a udev rule for the BBB which might be fuddling things, but I will check that in a bit. I only use the serial as debugging terminal really having to send something initially to get things primed seems a bit annoying. I will try it to see that that aids in the boot-time.
That while loop seems to be a good trick, thanks.