Alerting the ATMega32u4 that it's plugged into a computer (via USB)

Question

I need help in writing some code that will allow for the ATMega32u4 to be aware of when it's plugged into a USB port. I'm not really familiar with the ATMega32 or with it's function in Arduino, so I'm lost in the dark on this one.

Update: So in my circuit, the uC will be powered by Lithium Ion Batteries. On my PCB, their is a battery charging circuit that is powered through a USB C cable. My goal is to implement some type of firmware that can differentiate when the board is plugged into a wall outlet and a USB port on a computer. The reason for this is because I don't want the batteries to charge when the USB C cable is plugged into a laptop.

Second Update: Added pictures of the ATMega32 along with the charging circuit mentioned. I'll be the 5V voltage from USBC to charge the batteries. I'll have the data lines on the USB C cable connected so I can program the microcontroller through a USB cable. My goal here is for the microcontroller to recognize when it's connected to a computer. When this happens, I'll talk to the battery charging IC and tell it to not charge. enter image description here

enter image description here

• Jay: Updated my post, but the ATMega32 will be powered by Lithium Ion Batteries. They will be changed through a USB C connector that has the data pins connected.
• jsotola: you could try something like this ... if(Serial) { ledon() } else { ledoff() } ... see if the LED changes state when USB cable is plugged and unplugged
• chrisl: Here is the documentation link to jsotolas comment: arduino.cc/reference/en/language/functions/communication/serial/…
• timemage: I'm not sure you can get the best answer to this without details (schematic, etc) on the board itself.
• Jay: @timemage Added schematic part of the mC and charging IC

Answer 1

I imagine (haven't really tried it) this would work, with some minor adaptation to match your code structure.

// VBUS or counting frames
// Any frame counting?
u8 USBConnected()
{
 u8 f = UDFNUML;
 delay(3);
 return f != UDFNUML;
}

The method reads the UDFNUML - usb data frame number lower (byte) register, waits 3 milliseconds, which would be sufficient time get at least 1 more USB frame if the device was really connected to a USB host, reads the register again and checks if it some USB activity had happened.

Personally, I am not a big fan of busy wait delays like that because the CPU could be better utilized doing something useful in that period (3 milliseconds on a 16Mhz CPU = 48000 clocks ~= 48000 instructions) I'd probably use 1 millisecond timer to sample the register and keep track of the USB status

An alternative would be to use the bool USBDevice_::configured() method which is implemented like so

bool USBDevice_::configured()
{
 return _usbConfiguration;
}
// Endpoint 0 interrupt
ISR(USB_COM_vect)
{
 // ... snip ...
 else if (SET_CONFIGURATION == r)
 {
 if (REQUEST_DEVICE == (requestType & REQUEST_RECIPIENT))
 {
 InitEndpoints();
 _usbConfiguration = setup.wValueL;
 } else
 ok = false;
 // ... snip ....
}

Hopefully one of the two approaches work for you...