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 interfacing 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.
If your target board has too much capacitance on VCC, then it could cause the programmer to brown-out or it could cause disruptions in the USB communication. The Pololu USB AVR Programmer v2 cannot handle more than a few µF of capacitance on VCC. The Pololu USB AVR Programmer v2.1 has an improved power switch with a controlled slew rate, so it can power target boards with up to about 33 µF on VCC. The Pololu USB AVR Programmer v2.1 still might experience a brown-out reset when you connect it to a target with more than a few µF of capacitance, but after browning out once it is often able to recover and supply power to the target.
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.
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 too much 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.