How to program for device to be used with and without usb

[hyper]

Hi all,

I'm trying to create a device using PIC18F4550 which is able to :
- connect to the computer through USB to perform operations
- and also be used without a connection to the computer (as a stand-alone device)

I believe the the function 'usb_wait_for_enumeration()' runs infinitely until the PIC enumerates with the host. So, I cannot enter this function to the program, right?

I've tried using 'usb_enumerated()' to determine whether it has been enumerated or not. However, i have to add a certain delay before 'usb_enumerated()' or else it will always detect it as a stand-alone device. The rough structure of the program is:


//delay for the device to enumerate
delay_ms(3000);

// USB mode
if( usb_enumerated() == 1 )
{
//further codes here
}

// Stand-alone mode
else if( usb_enumerated() != 1 )
{
//further codes here
}

My question is, is there a more efficient solution other than inserting a delay?

[mskala]

I use what's in the CCS examples, a couple 100K resistors from USB power to ground, tapped in the middle and sent to a pin on the PIC. A routine checks that pin for a high to know that you've plugged into USB port.

[hyper]

I believe you're talking about USB_CONNECTION_SENSE_PIN ?? I have considered that but wouldn't that only determine whether the PIC's supply voltage is turned on? Because VBUS is pin1 of the USB connecter which is also VCC.

However, I have an external battery to power the PIC and if i'm not mistaken, would also cause a high on VBUS (VCC) although the USB is not plugged in? Please correct me if I'm wrong.

[mskala]

Yes, you'll need a diode to isolate USB power from VCC while operating from another power source. I don't have it right in front of me, but it's just a $0.20 Schotky diode.

[hyper]

Could you please further explain how that would solve the problem? I still don't really get it.

Thanks for the quick replies

[Ttelmah]

You have two potential voltage 'sources'. Vusb, and the local Vcc. There are two configurations to deal with:
1) You draw power from Vusb if it is available.
2) You only use local power.

In the former case, what you have to do, is connect the Vusb pin to your local power rail, using a low voltage drop diode (either a schottky diode, or a MOSFET gate).

Then in either case, you feed the voltage at the Vusb _pin_, to your 'sense' input via the resistors. The sense input, only sees a 'high', when this pin has voltage on it (doesn't care what the local rail is doing).

Then in your ''main', you have a loop, with tests like:

[hyper]

Hey,

Thanks for the explanation. I will be trying this out as soon as I can get my hands on the schottky diode. Hope it works.