USB mostly works, but disconnects and all hell breaks loose

sarodrig · Joined: 14 Aug 2009 Posts: 6 Location: Seattle, WA, USA

I'm using USB as a serial port on a PIC18F2550. It works like a charm 99% of the time. However, sometimes after several hours or days, I lose communication and can't reconnect.

If I have the serial port open (with Hyperterminal, or whatever), and comm is lost, then it hangs. I have to manually close the port, disconnect the USB cable, and then reconnect it in order to be able to reopen the port. Simple closing and opening the port without disconnecting the cable does not work. I think the problem is that the Windows driver (Win XP, with cdc_NTXP.inf) doesn't let go of the COM port on a disconnect.

This is easily repeatable by momentarily disconnecting the USB cable and then reconnecting it while the COM port is open by another program, such as Hyperterminal.

I'm fairly certain this is a PC-side driver problem. Anybody else seen this or come up with a workaround?

Thanks!

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

I see two points. Some Windows applications (e.g. Hyperterminal, at least previous versions) don't handle a driver fault like a disconnect correctly. The other point is, that interferences, e.g. electrostatic discharges or plug/unplug of a power supply can cause the USB connection to be suspended. Apparently only disconnection of the USB cable can revive it, although I guess, there's an option to force a reconnect by io_requests to the driver.

jbmiller · Joined: 07 Oct 2006 Posts: 73 Location: Greensville,Ontario

Welcome to the woes of the Useless Serial Bus....sigh. Ever since 'they' got rid of real RS-232 ports, PC programming has been a 'challenge'. The sheer amount of overhead for USB is astounding, mind boggling that it ever works.
Two workarounds. I've used both, #1 is my favorite, #2 was to learn yet another language.
1...use a PCI serial card if possible, get rid of USB unless you want to tear out your hair tracking down the problems in the driver.
2...Cut your own code using Borland's Delphi. It'll create tight, fast, self contained code bypassing all those hooks to flakey .dlls,etc.
I'm sure others will praise USB as the cat's meow but I require fast realtime interrupts something USB cannot ever give you.
Jay