8. Using VCC or VBUS to supply power
Generally, when you are programming an AVR or using the programmer’s USB-to-TTL serial header, the device you are interface with should be powered from its own power supply. The programmer does not supply any power on its VCC pins by default. However, if you want to power a low-current and low-capacitance load, you can configure VCC to be an output using the progammer’s configuration software. When VCC is an output, its voltage will either be 3.3 V or 5 V, and that is determined by the programmer’s “Regulator mode” setting.
The VCC output should work fine if you just want to power a microcontroller and put a small capacitor (e.g. 0.1 μF) between its GND and VCC pins. You should not connect VCC to the output of any other device, since that could cause a short circuit. You should not draw more than 80 mA of current from VCC. The load you put on VCC should not have more than a few microfarads of capacitance. If you put too much capacitance on VCC, then it could cause the programmer to brown-out or it could cause disruptions in the USB communication.
Alternatively, you can use the programmer’s VBUS pin to power external devices. This pin is connected directly to the power line of the USB cable. As with the VCC output, you should not draw more than 80 mA, and you should not have more than a few microfarads of capacitance.
It is generally safer to use the VCC output than to use VBUS. The power supplied to the VCC output goes through a diode, which prevents current from flowing back into the USB port, and the programmer’s linear regulator, which has current limiting. The VBUS pin provides no protections, so there is a higher risk that you would accidentally damage something.