Software on PIC will come in loop after disconnect USB connection

[Ronald123]

My application is connected via USB to a PC. Within my application I'm using a 18F2550 PIC. A status change of a I/O will be send via USB to PC.

The code which I'm using for outgoing:

USBwriteStatus:
USBService ' Must service USB regularly
USBOUT 3, USBBUFFER ,CNT_UIT, USBWRITESTATUS
RETURN

The problem is when I disconnect the USB cable it seems that the the PIC program commes in an unendless loop of "USBwriteStatus. I have mode some modifications, see below, however this will still not solve the problem.

USBwriteStatus:
USBService ' Must service USB regularly
USBOUT 3, USBBUFFER ,CNT_UIT, exitusbwritestatus
exitusbwritestatus:
RETURN

When I'm taking this SUB out of my application everything works also without USB connection to PC.

Is there somebody who knows what I'm doing wrong?
Is it possible to see the status of the USB connection (connect/disconnect) before calling the SUB USBwriteStatus.

[Bruce]

Hi Ronald,

There's a very simple solution to this. Assign a single I/O-pin as the USB bus-attach input.

Here's how;

ground --100k resistor----PIC input pin ----4.7k resistor----+V from USB connector.

When the USB cable is plugged in, the input will see +V from the USB port. When the USB
cable is removed, it will see ground.

Don't attempt to send/receive any data until you see +V on the USB bus-attach input pin.
Very simple. Very effective.

[Squibcakes]

Bruce,

Thats a great tip, (although I havent seen this problem come up before).

I'm wondering if this would be concidered a fail safe "Design Rule" and should be implemented into all USB PIC based circuits?

Cheers
Squib

[Bruce]

Hi Squib,

I can't take credit for the idea. It's an option on the Microchip USB development board, and
in the USB C firmware.

There's no reason to enable the USB module if it's not attached, so it's not a bad idea to
add in.

[Ronald123]

Hi Bruce, thank you for your almost exelent and simple sollution. Almost exelent because it will cost me 1 I/O and I don't have one I/O left. I have made a mistake because after testing this code again it was working.

USBwriteStatus:
USBService
USBOUT 3, USBBUFFER ,CNT_UIT, exitusbwritestatus
exitusbwritestatus:
RETURN

However this sollution has a disadvantage because when the outgoing USBBuffer is full it will also goto "exitusbwritestatus" without sending the information via USB, so some status information from PIC to PC will be lost.

[Darrel Taylor]

You should be able to determine if it's plugged-in or not by monitoring the usb_device_state variable.

usb_device_state   VAR BYTE EXT
CONFIGURED_STATE   CON EXT

IF (usb_device_state = CONFIGURED_STATE) THEN
    ; OK to send
ELSE
    ; Detached or Enumerating, don't try to send.
ENDIF