I don't know if this more appropriate in "Setup" forum, or "Interfacing" forum, and it isn't really specific to Arduino, but here goes:
Many Arduino boards, particularly older revisions, have an FTDI FT232RL chip to provide a USB connection. The FTDI Windows driver makes this visible on your computer as a virtual com port. I have discovered that when I have my Arduino 1280 plugged in, and I connect to the virtual COM port with any software (Terminal, PuTTY, BraysTerminal), the clock (time of day) reported by Windows starts drifting. The same thing occurs with my FTDI cable. Basically the Windows clock loses 3 seconds per minute, until it's about 6 minutes behind, and then suddenly corrects itself (or something external is correcting it). Clock drift only occurs when BOTH these conditions are met:
- Arduino (or separate FTDI cable) is connected to computer's USB port.
- Software on the computer connects to the associated COM port.
I've Googled every combination of terms I can think of. My computer was recently wiped & rebuilt (Win7 pro, FTDI VCP driver 126.96.36.199), and has all updates applied. Port params: 115200, 8N1. I do not know if this is new behavior, because until recently I didn't leave the serial port connected long enough to notice.
I've sent a more detailed report to FTDI support, but I thought I would ask here, since most of you probably have this same combination of chip, driver, and OS. I would appreciate any pointers or suggestions you can provide.