@ maxint - OK. I was able to reproduce the pattern you’re seeing, but only by using the following code (note that I commented out the USBSerial declaration):
//USBSerial pc; // tx, rx
Serial device(P0_9, P0_10); // tx, rx // any other pins than P0_19, P_018 cause blinking
/* not needed to reproduce blinking
Now that I’m seeing it, it does look like some kind of error indication.
I also commented out the line of code that imports the USBSerial library, and the blinking symptom remains, so we can eliminate that as the source of the issue.
There’s some interesting stuff up on the mBed site for Serial PC comms, but I saw some info that suggests the required driver may not work on Windows 8 (I’m on 8.1, and could not get the driver to install…kept telling me that no mbed microcontrollers were found). Then again, other posts indicate that folks have successfully installed the driver, so who knows. Here’s some info, anyway, in case it’s helpful.